. 17
( 19)


sponding to numeric values.
Network Operations Center
Network Control Program
See NOC.
See NCP.
network operator
network driver interface specification
Person who routinely monitors and controls a
network, performing such tasks as reviewing
network entity title and responding to traps, monitoring throughput,
See NET. configuring new circuits, and resolving prob-
Network File System
network processor card
See NFS.
See NP card.
Network Information Center
network service access point
See NIC.
Network Information Service
See NIS.
Connecting of any collection of computers,
network interface printers, routers, switches, and other devices for
Boundary between a carrier network and a pri- the purpose of communication over some trans-
vately-owned installation. mission medium.
370 CCNA Security Course Booklet, Version 1.0

Network-to-Network Interface Communication standards developed by the
ITU-T for baseband networks. Based on 64-
See NNI.
kbps B channels and 16- or 64-kbps D chan-
neutral wire nels. Contrast with BISDN.
Circuit wire that is connected to an earth
ground at the power plant and at the trans-
National Institute of Standards and Technology
Formerly the NBS, this U.S. government organ-
Next Hop Resolution Protocol ization supports and catalogs a variety of stan-
See NHRP. dards.
Network File System NetWare Loadable Module
As commonly used, a distributed file system Individual program that can be loaded into
protocol suite developed by Sun Microsystems memory and function as part of the NetWare
that allows remote file access across a network. NOS.
In actuality, NFS is simply one protocol in the
suite. NFS protocols include NFS, RPC, Exter-
Network Layer Reachability Information
nal Data Representation (XDR), and others.
BGP sends routing update messages containing
These protocols are part of a larger architecture
NLRI to describe a route and how to get there.
that Sun refers to as ONC.
In this context, an NLRI is a prefix. A BGP up-
NHRP date message carries one or more NLRI pre-
Next Hop Resolution Protocol. Protocol fixes and the attributes of a route for theNLRI
Used by routers to dynamically discover the prefixes; the route attributes include a BGP
MAC address of other routers and hosts con- next hop gateway address, community values,
nected to a NBMA network. These systems can and other information.
then directly communicate without requiring
traffic to use an intermediate hop, increasing
NetWare Link Services Protocol
performance in ATM, Frame Relay, SMDS, and
Link-state routing protocol based on IS-IS. The
X.25 environments.
Cisco implementation of NLSP also includes
NIC MIB variables and tools to redistribute routing
1) network interface card. Board that provides and SAP information between NLSP and other
network communication capabilities to and IPX routing protocols.
from a computer system. A network interface
card is also known as an adapter.
Network Management Processor
2) Network Information Center. Organization
Processor module on the Catalyst 5000 switch
whose functions have been assumed by the In-
used to control and monitor the switch.
terNIC. See interNIC.

network management system
Network Information Service
System responsible for managing at least part
Protocol developed by Sun Microsystems for
of a network. An NMS is generally a reason-
the administration of network-wide databases.
ably powerful and well-equipped computer
The service essentially uses two programs: one
such as an engineering workstation. NMSs
for finding a NIS server and one for accessing
communicate with agents to help keep track of
the NIS databases.
network statistics and resources.
Narrowband ISDN
Glossary 371

NMVT nonvolatile random-access memory
network management vector transport See NVRAM.
SNA message consisting of a series of vectors
normal mode
conveying network management specific infor-
Term used to describe problems between the
hot and neutral wires on a power line.
normal response mode
Network-to-Network Interface
See NRM.
ATM Forum standard that defines the interface
between two ATM switches that are both located NOS
in a private network or are both located in a public
network operating system
network. The interface between a public switch
Generic term used to refer to what are really
and private one is defined by the UNI standard.
distributed file systems. Examples of NOSs in-
Also, the standard interface between two Frame
clude LAN Manager, NetWare, NFS, and
Relay switches meeting the same criteria.
Novell IPX
Network Operations Center
See IPX.
Organization responsible for maintaining a net-
work. NP card
network processor card
Main computational and storage resource for
1) Endpoint of a network connection or a junc-
the LightStream 2020 ATM switch. Each Light-
tion common to two or more lines in a network.
Stream 2020 switch has one or two NPs. The
Nodes can be processors, controllers, or work-
second card, if present, serves as a backup for
stations. Nodes, which vary in routing and
the first. Each NP is associated with a floppy
other functional capabilities, can be intercon-
disk drive for loading software and a hard disk
nected by links, and serve as control points in
drive for storing software and configuration
the network. Node is sometimes used generi-
data. Each NP also has an access card that pro-
cally to refer to any entity that can access a net-
vides an Ethernet port.
work, and is frequently used interchangeably
with device. NP module
2) In SNA, the basic component of a network, On a LightStream 2020 ATM switch, the com-
and the point at which one or more functional bination of the NP card, the NP access card,
units connect channels or data circuits. and the disk assembly.
noise NP TCS monitoring module
Undesirable communications channel signals. See NPTMM.
nominal velocity of propagation npadmin account
See NVP. One of the four default user accounts that are
created in the factory on each LightStream
nonbroadcast multiaccess
2020 ATM switch. The npadmin account is for
privileged users. Its default interface is the CLI.
non-stub area
Resource-intensive OSPF area that carries a de-
network processor test and control system mon-
fault route, static routes, intra-area routes, inter-
itoring module
area routes, and external routes. Nonstub areas
Process that runs on the NP of every Light-
are the only OSPF areas that can have virtual
Stream 2020 ATM switch in an ATM network.
links configured across them, and are the only
NPTMM monitors the health of the system
areas that can contain an ASBR. Compare with
through the TCS and coordinates switch cu-
stub area.
tover when redundant switch cards are present.
372 CCNA Security Course Booklet, Version 1.0

normal response mode National Science Foundation
HDLC mode for use on links with one primary U.S. government agency that funds scientific
station and one or more secondary stations. In research in the United States. The now-defunct
this mode, secondary stations can transmit only NSFNET was funded by the NSF.
if they first receive a poll from the primary sta-
National Science Foundation Network
NSAP Large network that was controlled by the NSF
network service access point and provided networking services in support of
Conceptual point on the boundary between the education and research in the United States,
network and the transport layers. The NSAP is from 1986 to 1995. NSFnet is no longer in
the location at which OSI network services are service.
provided to the transport layer. Each transport
layer entity is assigned a single NSAP.
Network Time Protocol
NSAP Address Protocol built on top of TCP that assures accu-
network service access point address rate local time-keeping with reference to radio
Network-layer address for CLNS packets. An and atomic clocks located on the Internet. This
NSAP describes an attachment to a particular protocol is capable of synchronizing distributed
service at the network layer of a node, similar clocks within milliseconds over long time peri-
to the combination of IP destination address ods.
and IP protocol number in an IP packet. NSAP
null modem
encoding and format are specified by ISO
Small box or cable used to join computing de-
8348/Ad2. NSAP address has two major parts:
vices directly, rather than over a network.
the initial domain part (IDP) and the domain
specific part (DSP). The IDP consists of a 1- NVP
byte authority and format identifier (AFI) and a
nominal velocity of propagation
variable-length initial domain identifier (IDI),
Speed at which a signal moves through a cable,
and the DSP is a string of digits identifying a
expressed as a percentage or fraction of the
particular transport implementation of a speci-
speed of light in a vacuum. To calculate a cable
fied AFI authority. Everything to the left of the
length, a cable tester uses NVP together with
system ID can be thought of as the area address
the time a signal takes to return to the testing
of a network node.
network service access point selector
nonvolatile RAM
Part of the NSAP address field that identifies a
RAM that retains its contents when a unit is
process on the device. It is roughly equivalent
powered off. In Cisco products, NVRAM is
to a socket or a TCP port number in TCP/IP.
used to store configuration information.
The NSEL is not used in routing decisions. Do-
main-Specific Part (DSP): comprised of the NYSERNet
HODSP, the system ID, and the NSEL in bi- Network in New York (United States) with a T1
nary format. The last byte is the N-Selector backbone connecting NSF, many universities,
(NSEL) and must be specified as a single-byte and several commercial concerns.
length preceded by a ˜.™. A NET definition must
OAM cell
set the N-Selector to ˜00™.
Operation, Administration, and Maintenance
ATM Forum specification for cells used to
monitor virtual circuits. OAM cells provide a
Glossary 373

virtual circuit-level loopback in which a router OIR
responds to the cells, demonstrating that the online insertion and removal
circuit is up, and the router is operational. Feature that permits the addition, replacement,
or removal of interface processors in a Cisco
router without interrupting the system power,
optical carrier entering console commands, or causing other
Series of physical protocols (OC-1, OC-2, OC- software or interfaces to shut down.
3, and so on), defined for SONET optical signal Online insertion and removal is also known as
transmissions. OC signal levels put STS frames hot swapping.
onto multimode fiber-optic line at a variety of
speeds. The base rate is 51.84 Mbps (OC-1); omni-directional
each signal level thereafter operates at a speed This typically refers to a primarily circular an-
divisible by that number (thus, OC-3 runs at tenna radiation pattern.
155.52 Mbps).
octet Open Network Computing
8 bits. In networking, the term octet is often Distributed applications architecture designed
used (rather than byte) because some machine by Sun Microsystems, currently controlled by a
architectures employ bytes that are not 8 bits consortium led by Sun. The NFS protocols are
long. part of ONC.

ODA ones density
Open Document Architecture Scheme that allows a CSU/DSU to recover the
ISO standard that specifies how documents are data clock reliably. The CSU/DSU derives the
represented and transmitted electronically. data clock from the data that passes through it.
Open document Architecture was known as Of- In order to recover the clock, the CSU/DSU
fice Document Architecture. hardware must receive at least one 1 bit value
for every 8 bits of data that pass through it.
Ones density is also known as pulse density.
Open Data-Link Interface
Novell specification providing a standardized online insertion and removal
interface for NICs that allows multiple proto- See OIR.
cols to use a single NIC.
on-the-fly packet switching
OEMI channel See cut-through packet switching.
See block multiplexer channel.
open architecture
OFDM Architecture with which third-party developers
Orthogonal Frequency Division Multiplexing can legally develop products and for which
OFDM is a modulation technique used with public domain specifications exist.
IEEE 802.11g.
open circuit
Office Document Architecture Broken path along a transmission medium.
See ODA. Open circuits will usually prevent network
OSI Internet Management Open Data-Link Interface
Group tasked with specifying ways in which Open Data-Link Interface
OSI network management protocols can be Novell specification providing a standardized
used to manage TCP/IP networks. interface for NICs that allows multiple proto-
cols to use a single NIC.
374 CCNA Security Course Booklet, Version 1.0

Open Document Architecture OSI Internet Management
See ODA. See OIM.

Open Network Computing OSI Presentation Address
See ONC. Address used to locate an OSI Application en-
tity. It consists of an OSI Network Address and
Open Shortest Path First up to three selectors, one each for use by the
See OSPFv2 and OSPFv3. transport, session, and presentation entities.
Open System Interconnection OSI reference model
See OSI. Open System Interconnection reference model
Network architectural model developed by ISO
Open System Interconnection reference
and ITU-T. The model consists of seven layers,
each of which specifies particular network
See OSI reference model.
functions such as addressing, flow control,
oper account error control, encapsulation, and reliable mes-
sage transfer. The highest layer (the application
One of the four default user accounts that are
layer) is closest to the user; the lowest layer
created in the factory on each LightStream
(the physical layer) is closest to the media tech-
2020 ATM switch. The oper account is for gen-
nology. The next to lowest layer are imple-
eral users. Its default interface is the CLI.
mented in hardware and software, while the
Operation, Administration, and upper five layers are implemented only in soft-
Maintenance cell
ware. The OSI reference model is used univer-
See OAM cell. sally as a method for teaching and
understanding network functionality. Similar in
Optical Carrier
some respects to SNA.
See OC.
optical fiber
International association designed to promote
See fiber-optic cable.
OSI in vendor architectures.
Organizational Unique Identifier
See OUI.
Open Shortest Path First version 2
Orthogonal Frequency Division Multiplex OSPFv2 is an IPv4 link-state, hierarchical IGP
(OFDM) routing algorithm proposed as a successor to
A wireless modulation technique used by IEEE RIP in the Internet community. OSPF features
802.11a-compliant wireless LANs for transmis- include least-cost routing, multipath routing,
sion at 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. and load balancing. OSPF was derived from an
early version of the ISIS protocol.
Secondary signal on top of the 60-Hz wave-
form. It has a magnitude that ranges from 15 % Open Shortest Path First version 3
to 100 % of the normal voltage carried on the Protocol implementation for IPv6. It is based
power line. on OSPF version 2 (OSPFv2), with enhance-
Open System Interconnection
International standardization program created Organizational Unique Identifier
by ISO and ITU-T to develop standards for data The 3 octets assigned by the IEEE in a block of
networking that facilitate multivendor equip- 48-bit LAN addresses.
ment interoperability.
Glossary 375

outframe packet line card
Maximum number of outstanding frames al- See PLC.
lowed in an SNA PU 2 server at any time.
packet switch
out-of-band signaling WAN device that routes packets along the most
1) Transmission using frequencies or channels efficient path and allows a communications
outside the frequencies or channels normally channel to be shared by multiple connections.
used for information transfer. Out-of-band sig- A packet switch is also known as a packet
naling is often used for error reporting in situa- switch node (PSN), and was formerly known as
tions in which in-band signaling can be affected an interface message processor (IMP).
by whatever problems the network might be ex-
packet switch exchange
periencing. Contrast with in-band signaling.
See PSE.
2) Out-of-band management is the use of a ded-
icated management channel for device manage- packet switching
ment. This channel is isolated from the data
Networking method in which nodes share
channel and not vulnerable to network connec-
bandwidth with each other by sending packets.
tivity issues.
Compare with circuit switching and message
poll/final bit packet-switched data network
Bit in bit-synchronous data link layer protocols
See PSN.
that indicates the function of a frame. If the
frame is a command, a 1 in this bit indicates a packet-switched network
poll. If the frame is a response, a 1 in this bit See PSN.
indicates that the current frame is the last frame
packet-switching node
in the response.
See PSN.
See flow control.
packet assembler/disassembler
packet Device used to connect simple devices (like
Logical grouping of information that includes a character-mode terminals) that do not support
header containing control information and (usu- the full functionality of a particular protocol to
ally) user data. Packets are most often used to a network. PADs buffer data and assemble and
refer to network layer units of data. The terms disassemble packets sent to such end devices.
datagram, frame, message, and segment are
paddle card
also used to describe logical information group-
See access card.
ings at various layers of the OSI reference
model and in various technology circles.
Palo Alto Research Center
packet assembler/disassembler
See PAD.
pulse amplitude modulation
packet buffer
Modulation scheme where the modulating wave
See buffer.
is caused to modulate the amplitude of a pulse
packet Internet groper stream. Compare with AM and FM.
See ping.
Packet Level Protocol Password Authentication Protocol
See PLP. Authentication protocol that allows PPP peers
to authenticate one another. The remote router
attempting to connect to the local router is re-
376 CCNA Security Course Booklet, Version 1.0

quired to send an authentication request. Unlike not provide the level of redundancy of a full
CHAP, PAP passes the password and host name mesh topology, but is less expensive to imple-
or username in the clear (unencrypted). PAP ment. Partial mesh topologies are generally
does not itself prevent unauthorized access, but used in the peripheral networks that connect to
merely identifies the remote end. The router or a fully meshed backbone.
access server then determines if that user is al-
partial sequence number PDU
lowed access. PAP is supported only on PPP
lines. Compare with CHAP.
passive interface
parallel channel
A passive interface receives updates, but does
Channel that uses bus and tag cables as a trans-
not send them. It is used to control routing up-
mission medium. Compare with ESCON chan-
date. The passive-interface command can be
used with all IP interior gateway protocols.
parallel transmission That is that it can be use with RIP, IGRP,
Method of data transmission in which the bits EIGRP, OSPF, and IS-IS.
of a data character are transmitted simultane-
Password Authentication Protocol
ously over a number of channels. Compare with
See PAP.
serial transmission.
patch panel
An assembly of pin locations and ports which
Indicates that multiple paths exist between two
can be mounted on a rack or wall bracket in the
points in a network. These paths might be of
wiring closet. Patch panels act like switch-
equal or unequal cost. Parallelism is often a
boards that connect workstations cables to each
network design goal: if one path fails, there is
other and to the outside.
redundancy in the network to ensure that an al-
ternate path to the same point exists. path control layer
Layer 3 in the SNA architectural model. This
layer performs sequencing services related to
Palo Alto Research Center
proper data reassembly. The path control layer
Research and development center operated by
is also responsible for routing. Corresponds
XEROX. A number of widely-used technolo-
roughly with the network layer of the OSI
gies were originally conceived at PARC, in-
cluding the first personal computers and LANs.
path control network
PARC Universal Protocol
SNA concept that consists of lower-level com-
See PUP.
ponents that control the routing and data flow
parity check through an SNA network and handle physical
Process for checking the integrity of a charac- data transmission between SNA nodes. Com-
ter. A parity check involves appending a bit that pare with NAU.
makes the total number of binary 1 digits in a
path cost
character or word (excluding the parity bit) ei-
See cost.
ther odd (for odd parity) or even (for even par-
ity). path name
Full name of a UNIX, DOS, or LynxOS file or
partial mesh
directory, including all directory and subdirec-
Term describing a network in which devices are
tory names. Consecutive names in a path name
organized in a mesh topology, with some net-
are typically separated by a forward slash (/) or
work nodes organized in a full mesh, but with
a backslash (\), as in /usr/app/base/config.
others that are only connected to one or two
other nodes in the network. A partial mesh does
Glossary 377

payload PDU
Portion of a frame that contains upper-layer in- protocol data unit
formation (data). OSI term for packet. See also BPDU and
peak cell rate
private branch exchange
Digital or analog telephone switchboard located See PCR.
on the subscriber premises and used to connect
peak rate
private and public telephone networks.
Maximum rate, in kilobits per second, at which
PCI a virtual circuit can transmit.
protocol control information
peer-to-peer computing
Control information added to user data to com-
Peer-to-peer computing calls for each network
prise an OSI packet. The OSI equivalent of the
device to run both client and server portions of
term header.
an application. Also describes communication
PCM between implementations of the same OSI ref-
pulse code modulation erence model layer in two different network de-
Transmission of analog information in digital vices.
form through sampling and encoding the sam-
performance management
ples with a fixed number of bits.
One of five categories of network management
PCR defined by ISO for management of OSI net-
peak cell rate works. Performance management subsystems
Parameter defined by the ATM Forum for ATM are responsible for analyzing and controlling
traffic management. In CBR transmissions, network performance including network
PCR determines how often data samples are throughput and error rates.
sent. In ABR transmissions, PCR determines
peripheral node
the maximum value of the ACR.
In SNA, a node that uses local addresses and is
PDAs therefore not affected by changes to network
personal digital assistant addresses. Peripheral nodes require boundary
Handheld device. Depending on the model and function assistance from an adjacent subarea
version, they can offer a varying amount of fea- node.
tures including some of the following: personal
permanent virtual circuit
organizers, address book, calculators, clock and
See PVC.
calendar functions, computer games, Internet
access, e-mail, radio and MP3 playback, video permanent virtual connection
recording, GPS, mobile phones (smartphone),
See PVC.
web browsers or media players.
permanent virtual path
See PVP.
public data network
Network operated either by a government (as in permit processing
Europe) or by a private concern to provide See traffic policing.
computer communications to the public, usu-
Personal digital assistants
ally for a fee. PDNs enable small organizations
See PDAs.
to create a WAN without all the equipment
costs of long-distance circuits.
Pretty Good Privacy
Public-key encryption application that allows
secure file and message exchanges. There is
378 CCNA Security Course Booklet, Version 1.0

some controversy over the development and use physical sublayer
of this application, in part due to U.S. national See PHY.
security concerns.
physical unit
phase See PU.
Location of a position on an alternating wave
Physical Unit 2
See PU 2.
phase shift
Physical Unit 4
Situation in which the relative position in time
See PU 4.
between the clock and data signals of a trans-
mission becomes unsynchronized. In systems
Physical Unit 5
using long cables at higher transmission speeds,
See PU 5.
slight variances in cable construction, tempera-
ture, and other factors can cause a phase shift, Physics Network
resulting in high error rates. See PHYSNET.
physical sublayer Protocol Independent Multicast
One of two sublayers of the FDDI physical Multicast routing architecture that allows the
layer. See also PMD. addition of IP multicast routing on existing IP
networks. PIM is unicast routing protocol inde-
physical address
pendent and can be operated in two modes:
See MAC address.
dense mode and sparse mode.
physical control layer
PIM dense mode
Layer 1 in the SNA architectural model. This
One of the two PIM operational modes. PIM
layer is responsible for the physical specifica-
dense mode is data-driven and resembles typi-
tions for the physical links between end sys-
cal multicast routing protocols. Packets are for-
tems. Corresponds to the physical layer of the
warded on all outgoing interfaces until pruning
OSI model.
and truncation occurs. In dense mode, receivers
are densely populated, and it is assumed that
Physical layer
the downstream networks want to receive and
Layer 1 of the OSI reference model. The physi-
will probably use the datagrams that are for-
cal layer defines the electrical, mechanical, pro-
warded to them. The cost of using dense mode
cedural and functional specifications for
is its default flooding behavior. Contrast with
activating, maintaining, and deactivating the
PIM sparse mode.
physical link between end systems. Corre-
PIM dense mode is also known as dense mode
sponds with the physical control layer in the
SNA model.
physical layer convergence procedure
See PIM dense mode.
physical media
See PIM sparse mode.
See media.
PIM sparse mode
physical medium
One of the two PIM operational modes. PIM
See media.
sparse mode tries to constrain data distribution
physical medium dependent so that a minimal number of routers in the net-
See PMD. work receive it. Packets are sent only if they are
explicitly requested at the RP. In sparse mode,
Glossary 379

receivers are widely distributed, and the as- transmission, isochronous transmission, and
sumption is that downstream networks will not synchronous transmission.
necessarily use the datagrams that are sent to
them. The cost of using sparse mode is its re-
Packet Level Protocol
liance on the periodic refreshing of explicit join
Network layer protocol in the X.25 protocol
messages and its need for RPs. Sometimes
called sparse mode PIM or PIM SM. Contrast
Packet Level Protocol is also known as X.25
with PIM dense mode.
Level 3 or X.25 Protocol.
pin location
A color-coded slot on a patch panel. Cable
primary logical unit
wires are punched down using a punch tool to
The LU that is initiating a session with another
make an electrical connection that allows the
network to function.
physical medium dependent
packet Internet groper
Sublayer of the FDDI physical layer that inter-
Utility to determine whether a specific IP ad-
faces directly with the physical medium and
dress is accessible. It works by sending a
performs the most basic bit transmission func-
packet to the specified address and waiting for
tions of the network.
a reply. PING is used primarily to troubleshoot
Internet connections. PNNI
PING is also known as Packet Inter-network
Private Network-Network Interface
ATM Forum specification that describes an
ATM virtual circuit routing protocol, as well as
a signaling protocol between ATM switches.
picture element
Used to allow ATM switches within a private
The smallest element of a display image, corre-
network to interconnect.
sponding to a single displayed spot or color
Private Network-Network Interface is also
triad on a display, or to a single input spot from
known as Private Network Node Interface.
a camera.
plain old telephone service
Power over Ethernet
PoE is the powering of network devices over
PLC Ethernet cable. IEEE 802.3af and Cisco specify
packet line card two different PoE methods. Cisco power sourc-
Card on the LightStream 2020 ATM switch that ing equipment (PSE) and powered devices
can be configured only as an edge card. A PLC, (PDs) support both PoE methods.
in conjunction with an access card, supports up
point of presence
to eight Ethernet or two FDDI edge ports.
See POP.
point-to-multipoint connection
physical layer convergence procedure
One of two fundamental connection types. In
Specification that maps ATM cells into physical
ATM, a point-tomultipoint connection is a uni-
media, such as T3 or E3, and defines certain
directional connection in which a single source
management information.
end-system (known as a root node) connects to
plesiochronous transmission multiple destination end-systems (known as
Term describing digital signals that are sourced leaves). Compare point-to-point connection.
from different clocks of comparable accuracy
and stability. Compare with asynchronous
380 CCNA Security Course Booklet, Version 1.0

point-to-point connection by the telephone company and the building™s
main distribution facility.
One of two fundamental connection types. In
ATM, a point-topoint connection can be a uni-
directional or bidirectional connection between
1) Interface on an internetworking device (such
two ATM end-systems. Compare point-to-mul-
as a router).
tipoint connection.
2) In IP terminology, an upper-layer process
Point-to-Point Protocol that is receiving information from lower layers.
3) To rewrite software or microcode so that it
See PPP.
will run on a different hardware platform or in
poison reverse updates a different software environment than that for
Routing updates that explicitly indicate that a which it was originally designed.
network or subnet is unreachable, rather than 4) A female plug on a patch panel which ac-
implying that a network is unreachable by not cepts the same size plug as an RJ45 jack. Patch
including it in updates. Poison reverse updates cords are used in these ports to cross connect
are sent to defeat large routing loops. The Cisco computers wired to the patch panel. It is this
IGRP implementation uses poison reverse up- cross connection which allows the LAN to
dates. function.
polarization POS
Polarization is the physical orientation of the power-on servicing
element on the antenna that actually emits the Feature on the LightStream 2020 ATM switch
RF energy. All Cisco Aironet antennas are set that allows faulty components to be diagnosed,
for vertical polarization. A vertical dipole an- removed, and replaced while the rest of the
tenna is vertically polarized. switch continues to operate normally.
Power-on servicing is also known as hot swap-
policy routing
Routing scheme that forwards packets to spe-
cific interfaces based on user-configured poli- POST
cies. Such policies might specify that traffic power-on self test
sent from a particular network should be for- Set of hardware diagnostics that runs on a hard-
warded out one interface, while all other traffic ware device when that device is powered up.
should be forwarded out another interface. On a LightStream 2020 ATM switch, for exam-
ple, the NP, switch card, and line card all per-
policy-based routing
form the POST.
See policy routing.
Post, Telephone, and Telegraph
poll/final bit
See PTT.
See P/F.
plain old telephone service. See PSTN (Public
Access method in which a primary network de-
Switched Telephone Network).
vice inquires, in an orderly fashion, whether
secondaries have data to transmit. The inquiry power tray
occurs in the form of a message to each second- Power supply for a LightStream 2020 ATM
ary that gives the secondary the right to trans- switch. A LightStream 2020 switch can have
mit. one or two bulk power trays. In a redundant
system, the two power trays load share, but
each can power the entire system in the event
point of presence
that the other fails. The power tray can provide
Point of presence is the point of interconnection
either AC or DC power to the switch.
between the communication facilities provided
Glossary 381

power-on self test primary
See POST. See primary station.

power-on servicing Primary LU
See POS. See PLU.

PPK Primary Rate Interface
per-packet keying See PRI.
Method of overcoming the exploitation of en-
primary station
cryption keys with key hashing.
In bit-synchronous data link layer protocols
PPP such as HDLC and SDLC, a station that con-
Point-to-Point Protocol trols the transmission activity of secondary sta-
A successor to SLIP, PPP provides router-to- tions and performs other management functions
router and host-to-network connections over such as error control through polling or other
synchronous and asynchronous circuits. means. Primary stations send commands to sec-
ondary stations and receive responses.
PQ A primary station is also known as a primary.
Priority Queuing
print server
Routing feature in which frames in an interface
output queue are prioritized based on various Networked computer system that fields, man-
characteristics such as packet size and interface ages, and executes (or sends for execution)
type. print requests from other network devices.

presentation layer priority queuing
Layer 6 of the OSI reference model. This layer See PQ.
ensures that information sent by the application
private branch exchange
layer of one system will be readable by the ap-
See PBX.
plication layer of another. The presentation
layer is also concerned with the data structures Private Network Node Interface
used by programs and therefore negotiates data
transfer syntax for the application layer. Corre-
sponds roughly with the presentation services Private Network-Network Interface
layer of the SNA model. See PNNI.
presentation services layer process switching
Layer 6 of the SNA architectural model. This Operation that provides full route evaluation
layer provides network resource management, and per-packet load balancing across parallel
session presentation services, and some appli- WAN links. Involves the transmission of entire
cation management. Corresponds roughly with frames to the router CPU where they are
the presentation layer of the OSI model. repackaged for delivery to or from a WAN in-
terface, with the router making a route selection
Pretty Good Privacy
for each packet. Process switching is the most
See PGP.
resource-intensive switching operation that the
CPU can perform.
Primary Rate Interface programmable read-only memory
ISDN interface to primary rate access. Primary
rate access consists of a single 64-Kbps D
channel plus 23 (T1) or 30 (E1) B channels for
voice or data. Compare to BRI.
382 CCNA Security Course Booklet, Version 1.0

PROM proxy
programmable read-only memory Entity that, in the interest of efficiency, essen-
ROM that can be programmed using special tially stands in for another entity.
equipment. PROMs can be programmed only
proxy Address Resolution Protocol
once. Compare with EPROM.
See proxy ARP.
propagation delay
proxy ARP
Time required for data to travel over a network,
Proxy Address Resolution Protocol
from its source to its ultimate destination.
Variation of the ARP protocol in which an in-
protocol termediate device (for example, a router) sends
1) Formal description of a set of rules and con- an ARP response on behalf of an end node to
ventions that govern how devices on a network the requesting host. Proxy ARP can lessen
exchange information. bandwidth use on slow-speed WAN links.
2) Field within an IP datagram that indicates
proxy polling
the upper layer (Layer 4) protocol sending the
Technique that alleviates the load across an
SDLC network by allowing routers to act as
protocol address proxies for primary and secondary nodes, thus
See network address. keeping polling traffic off of the shared links.
Proxy polling has been replaced by SDLC
protocol analyzer Transport.
See network analyzer.
protocol control information packet-switched data network. See PSN
See PCI. (packet-switched network).
protocol converter PSE
Enables equipment with different data formats packet switch exchange
to communicate by translating the data trans- Essentially, a switch. The term PSE is generally
mission code of one device to the data trans- used in reference to a switch in an X.25 PSN.
mission code of another device.
protocol data unit 1) packet-switched network. Network that uti-
See PDU. lizes packet-switching technology for data
transfer. Packet-switched network is also
Protocol Independent Multicast
known as packet-switched data network
See PIM.
2) packet-switching node. Network node capa-
protocol stack
ble of performing packet switching functions.
Set of related communications protocols that
operate together and, as a group, address com-
munication at some or all of the seven layers of
partial sequence number protocol data unit
the OSI reference model. Not every protocol
PSNPs are used to request one or more LSPs
stack covers each layer of the model, and often
and acknowledge receipt of one or more LSPs.
a single protocol in the stack will address a
number of layers at once. TCP/IP is a typical PSTN
protocol stack. Public Switched Telephone Network
General term referring to the variety of tele-
protocol translator
phone networks and services in place world-
Network device or software that converts one
protocol into another, similar, protocol.
PSTN is also known as plain old telephone
service (POTS).
Glossary 383

PTT punch tool
Post, Telephone, and Telegraph Spring-loaded tool used for cutting and con-
Government agency that provides telephone necting wire in a jack or on a patch panel.
services. PTTs exist in most areas outside
North America and provide both local and
PARC Universal Protocol
long-distance telephone services.
Protocol similar to IP developed at PARC.
physical unit
permanent virtual circuit
SNA component that manages and monitors the
Virtual circuit that is permanently established.
resources of a node, as requested by an SSCP.
PVCs save bandwidth associated with circuit
There is one PU per node.
establishment and tear down in situations where
PU 2 certain virtual circuits must exist all the time.
Physical Unit 2 Compare with SVC.
SNA peripheral node that can support only Permanent virtual circuit is known as perma-
DLUs that require services from a VTAM host nent virtual connection in ATM terminology.
and that are only capable of performing the sec-
ondary LU role in SNA sessions.
permanent virtual path
PU 4 Virtual path that consists of PVCs. See also
Physical Unit 4 PVC and virtual path.
Component of an IBM FEP capable of full-du-
plex data transfer. Each such SNA device em-
ITU-T specifications for the ISDN UNI data
ploys a separate data and control path into the
link layer.
transmit and receive buffers of the control pro-
gram. Q.922A
ITU-T specification for Frame Relay encapsu-
PU 5
Physical Unit 5
Component of an IBM mainframe or host com- Q.931
puter that manages an SNA network. PU 5
ITU-T specification for signaling to establish,
nodes are involved in routing within the SNA
maintain, and clear ISDN network connections.
path control layer.
public data network
ITU-T specification signaling to establish,
See PDN.
maintain, and clear BISDN network connec-
tions. An evolution of ITU-T recommendation
Public Switched Telephone Network
pull string
Qualified Logical Link Control
Strong, heavy string used to pull cable in multi-
Data link layer protocol defined by IBM that al-
ple runs.
lows SNA data to be transported across X.25
pulse amplitude modulation networks.
See PAM.
pulse code modulation Quality of Service
See PCM. Measure of performance for a transmission sys-
tem that reflects its transmission quality and
pulse density service availability.
See ones density.
384 CCNA Security Course Booklet, Version 1.0

QoS parameters raceway
quality of service parameter Wall-mounted channel with a removable cover
Parameters that control the amount of traffic the used to support horizontal cabling.
source router in an ATM network sends over an
radio frequency
SVC. If any switch along the path cannot ac-
See RF.
commodate the requested QoS parameters, the
request is rejected, and a rejection message is radio frequency interference
forwarded back to the originator of the request.
See RFI.
Quadruple Phase Shift Keying
A modulation technique used by IEEE
random-access memory
802.11b-compliant wireless LANs for transmis-
Volatile memory that can be read and written
sion at 2 Mbps.
by a microprocessor.
Qualified Logical Link Control
random-access memory
See RAM.
quality of service
See QoS.
A linear measure of the distance that a wireless
transmitter can send a signal.
quartet signaling
Signaling technique used in 100VG-AnyLAN RARP
networks that allow data transmission at 100
Reverse Address Resolution Protocol
Mbps over four pairs of UTP cabling at the
Protocol in the TCP/IP stack that provides a
same frequencies used in 10BASE-T networks.
method for finding IP addresses based on MAC
addresses. Compare with ARP.
Message used to inquire about the value of rate enforcement
some variable or set of variables.
See traffic policing.
rate queue
1) Generally, an ordered list of elements wait-
Value that is associated with one or more vir-
ing to be processed.
tual circuits, and that defines the speed at which
2) In routing, a backlog of packets waiting to be
an individual virtual circuit will transmit data to
forwarded over a router interface.
the remote end. Each rate queue represents a
portion of the overall bandwidth available on an
queuing delay
ATM link. The combined bandwidth of all con-
Amount of time that data must wait before it
figured rate queues should not exceed the total
can be transmitted onto a statistically multi-
bandwidth available.
plexed physical circuit.
queuing theory
Regional Bell Holding Company
Scientific principles governing the formation or
One of seven telephone companies created by
lack of formation of congestion on a network or
the AT and T divestiture in 1984.
at an interface.
Regional Bell Operating Company
Research on Advanced Communications in Eu-
Local or regional telephone company that owns
and operates telephone lines and switches in
Project sponsored by the European Community
one of seven U.S. regions. The RBOCs were
(EC) for the development of broadband net-
created by the divestiture of AT and T.
working capabilities.
Regional Bell Operating Company is also
known as Bell Operating Company (BOC).
Glossary 385

rcp event of a failure, the redundant devices, serv-
ices, or connections can perform the work of
remote copy protocol
those that failed. See also redundant system.
Protocol that allows users to copy files to and
2) In telephony, the portion of the total infor-
from a file system residing on a remote host or
mation contained in a message that can be
server on the network. The rcp protocol uses
eliminated without loss of essential information
TCP to ensure the reliable delivery of data.
or meaning.
rcp server
redundant system
Router or other device that acts as a server for
Computer, router, switch, or other computer
system that contains two or more of each of the
read-only memory most important subsystems, such as two disk
See ROM. drives, two CPUs, or two power supplies. For
example, on a fully redundant LightStream
Ready To Send
2020 ATM switch, there are two NP cards with
See RTS.
disks, two switch cards, and two power trays. A
partially redundant LightStream 2020 switch
might have two NPs, one switch card, and one
The putting back together of an IP datagram at
power tray.
the destination after it has been fragmented ei-
ther at the source or at an intermediate node.
Physical phenomenon which occurs when radio
receiver sensitivity
frequency waves bounce off objects (for exam-
A measurement of the weakest wireless signal a
ple, metal or glass surfaces).
receiver can receive and still correctly translate
it into data.
The measure of how much a given material
bends light.
Part of the ICMP and ES-IS protocols that al-
lows a router to tell a host that using another
Regional Bell Holding Company
router would be more effective.
Regional Bell Operating Company
Software that intercepts requests for resources
within a computer and analyzes them for re-
mote access requirements. If remote access is registered jack connector
required to satisfy the request, the redirector See RJ connector.
forms an RPC and sends the RPC to lower-
layer protocol software for transmission
OSI terminology for a device that connects two
through the network to the node that can satisfy
or more networks or network systems. A data
the request.
link layer (Layer 2) relay is a bridge; a network
redistribution layer (Layer 3) relay is a router.
Allowing routing information discovered
through one routing protocol to be distributed
Ratio of expected to received keepalives from a
in the update messages of another routing pro-
link. If the ratio is high, the line is reliable.
Used as a routing metric.
Redistribution is also known as route redistrib-
remote bridge
redundancy Bridge that connects physically disparate net-
work segments via WAN links.
1) In internetworking, the duplication of de-
vices, services, or connections so that, in the
386 CCNA Security Course Booklet, Version 1.0

remote copy protocol RFC
See rcp. Request For Comments
Document series used as the primary means for
remote login communicating information about the Internet.
See rlogin. Some RFCs are designated by the IAB as Inter-
net standards. Most RFCs document protocol
Remote Monitoring
specifications such as Telnet and FTP, but some
are humorous or historical. RFCs are available
online from numerous sources.
Remote Operations Service Element
radio frequency interference
remote shell protocol
Radio frequencies that create noise that inter-
See rsh.
feres with information being transmitted across
remote source-route bridging unshielded copper cabling.
remote-procedure call Routing Information Field
See RPC. Field in the IEEE 802.5 header that is used by a
source-route bridge to determine through which
rendezvous point
Token Ring network segments a packet must
See RP.
transit. A RIF is made up of ring and bridge
numbers as well as other information.
Device that regenerates and propagates electri-
cal signals between two network segments.
Routing Information Identifier
Bit used by SRT bridges to distinguish between
Request For Comments
frames that should be transparently bridged and
See RFC.
frames that should be passed to the SRB mod-
request/response unit ule for handling.
See RU.
Research on Advanced Communications in Connection of two or more stations in a logically
Europe circular topology. Information is passed sequen-
See RACE. tially between active stations. Token Ring,
FDDI, and CDDI are based on this topology.
Set to zero. ring topology
Network topology that consists of a series of
Reverse Address Resolution Protocol
repeaters connected to one another by unidirec-
tional transmission links to form a single closed
loop. Each station on the network connects to
Reverse Path Multicasting
the network at a repeater. While logically a
See RPM.
ring, ring topologies are most often organized
RF in a closed-loop star. Compare with bus topol-
radio frequency ogy, star topology, and tree topology.
Generic term referring to frequencies that cor-
respond to radio transmissions. Cable TV and
Routing Information Protocol
broadband networks use RF technology.
IGP supplied with UNIX BSD systems. The
most common IGP in the Internet. RIP uses hop
count as a routing metric.
Glossary 387

RIPng 2020 ATM switch. The root account is for use
by the system or network administrator only. Its
Routing Information Protocol next generation
default interface is the bash shell. See also
Distance vector routing protocol with a limit of
15 hops that uses split-horizon and poison re-
verse to prevent routing loops. It is based on
root bridge
IPv4 RIP v2 and similar to RIPv2, but uses
Exchanges topology information with desig-
IPv6 for transport. The multicast group address
nated bridges in a spanning-tree implementa-
FF02::9 identifies all RIPng enabled
tion in order to notify all other bridges in the
routers.(RIPng, RFC 2080)
network when topology changes are required.
RIPv2 This prevents loops and provides a measure of
defense against link failure.
Routing Information Protocol version 2
Defined in RFC 1723 and is supported in IOS
versions 11.1 and later. RIPv2 is not a new pro-
Remote Operations Service Element
tocol, just RIPv1 with some extensions to bring
OSI RPC mechanism used by various OSI net-
it up-to-date with modern routing environ-
work application protocols.
ments. RIPv2 has be updated to supports
VLSM, authentication, and multicast updates. round-trip time
See RTT.
RJ connector
registered jack connector route
Standard connectors originally used to connect Path through an internetwork.
telephone lines. RJ connectors are now used for
route extension
telephone connections and for 10BASE-T and
other types of network connections. RJ-11, RJ- In SNA, a path from the destination subarea
12, and RJ-45 are popular types of RJ connec- node through peripheral equipment to a NAU.
route map
rlogin Method of controlling the redistribution of
remote login routes between routing domains.
Terminal emulation program, similar to Telnet,
Route Processor
offered in most UNIX implementations.
See RP.
route redistribution
Remote Monitoring
See redistribution.
MIB agent specification described in RFC 1271
that defines functions for the remote monitoring route summarization
of networked devices. The RMON specification
Consolidation of advertised addresses in OSPF
provides numerous monitoring, problem detec-
and IS-IS. In OSPF, this causes a single sum-
tion, and reporting capabilities.
mary route to be advertised to other areas by an
area border router.
read-only memory Route/Switch Processor
Nonvolatile memory that can be read, but not
See RSP.
written, by the microprocessor.
routed protocol
root account
Protocol that can be routed by a router. A router
1) Privileged account on UNIX systems used
must be able to interpret the logical internet-
exclusively by network or system administra-
work as specified by that routed protocol. Ex-
amples of routed protocols include AppleTalk,
2) One of the four default user accounts that are
DECnet, and IP.
created in the factory on each LightStream
388 CCNA Security Course Booklet, Version 1.0

router Routing Table Protocol
Network layer device that uses one or more See RTP.
metrics to determine the optimal path along
routing update
which network traffic should be forwarded.
Message sent from a router to indicate network
Routers forward packets from one network to
reachability and associated cost information.
another based on network layer information.
Routing updates are typically sent at regular in-
Occasionally called a gateway (although this
tervals and after a change in network topology.
definition of gateway is becoming increasingly
Compare with flash update.


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