Terms in this set (255)
The 10-Mbps baseband Ethernet specification using two pairs of twisted-pair cabling (Categories 3, 4, or 5):
One pair transmits data and the other receives data. 10BASE-T, which is part of the IEEE 802.3 specification, has a distance limit of approximately 100 m (328 feet) per segment.
A name for the IEEE Fast Ethernet standard that uses two-pair copper cabling, a speed of 100 Mbps, and a maximum cable length of 100 meters.
A name for the IEEE Gigabit Ethernet standard that uses four-pair copper cabling, a speed of 1000 Mbps (1 Gbps), and a maximum cable length of 100 meters.
The IEEE standardized protocol for
The IEEE standard for wireless
LANs using the U-NII spectrum, OFDM encoding, at speeds of up to 54 Mbps.
The IEEE standard for wireless
LANs using the ISM spectrum, DSSS encoding, and speeds of up to 11 Mbps.
The IEEE standard for wireless
LANs using the ISM spectrum, OFDM or
DSSS encoding, and speeds of up to 54
The IEEE standard for wireless
LANs using the ISM spectrum, OFDM
encoding, and multiple antennas for single stream speeds up to 150 Mbps.
Authentication, authorization, and
accounting. Authentication confirms the
identity of the user or device. Authorization
determines what the user or device is allowed to do. Accounting records information about access attempts, including inappropriate requests.
A LAN network design
term that refers to a switch interface connected to end-user devices, configured so that it does not use VLAN trunking.
In Frame Relay, the physical
serial link that connects a Frame Relay DTE
device, usually a router, to a Frame Relay
switch. The access link uses the same physical layer standards as do point-to-point leased lines.
A wireless LAN device that
provides a means for wireless clients to send data to each other and to the rest of a wired network, with the AP connecting to both the wireless LAN and the wired Ethernet LAN.
In security, the recording of access attempts. See AAA
In both IPv4 and IPv6, a set of consecutive addresses. This term is typically used for public addresses, assigned by some numbering authority (IANA/ICANN, an RIR, or an ISP).
The general topic of how on one computer, two adjacent layers in a networking architectural model work together, with the lower layer providing services to the higher layer.
administrative distance In Cisco routers,
a means for one router to choose between
multiple routes to reach the same subnet
when those routes were learned by different
routing protocols. The lower the administrative distance, the better the source of the routing information.
Asymmetric digital subscriber line.
One of many DSL technologies, ADSL is
designed to deliver more bandwidth downstream (from the central office to the customer site) than upstream.
All-nodes multicast address
All-nodes multicast address A specific
IPv6 multicast address, FF02::1, with linklocal scope, used to send packets to all
devices on the link that support IPv6
All-routers multicast address
A specific IPv6 multicast address, FF02::2, with linklocal scope, used to send packets to all devices that act as IPv6 routers on the local link.
The term used by Cisco to refer to
a variety of security tools that help prevent
various attacks, including antivirus, antiphishing, and antispam.
Area Border Router (ABR)
A router using OSPF in which the router has interfaces in multiple OSPF areas.
Address Resolution Protocol. An
Internet protocol used to map an IP address
to a MAC address. Defined in RFC 826.
A list of IP addresses of neighbors
on the same VLAN, along with their
MAC addresses, as kept in memory by hosts
The first packet-switched network,
first created around 1970, which served
as the predecessor to the Internet
A feature of many Internet access
technologies, including DSL, cable, and
modems, in which the downstream transmission rate is higher than the upstream transmission rate.
The lack of an imposed time ordering on a bit stream. Practically, both sides agree to the same speed, but there is no check or adjustment of the rates if they are slightly different. However, because only 1 byte per transfer is sent, slight differences in clock speed are not an issue.
Asynchronous Transfer Mode. The
international standard for cell relay in which
multiple service types (such as voice, video,
and data) are conveyed in fixed-length (53-
byte) cells. Fixed-length cells allow cell
processing to occur in hardware, thereby reducing transit delays.
In security, the verification of the identity of a person or a process. See AAA.
In security, the determination
of the rights allowed for a particular user
or device. See AAA.
An IEEE standard mechanism
(802.3u) with which two nodes can exchange messages for the purpose of choosing to use the same Ethernet standards on both ends of the link, ensuring that the link functions and functions well.
An internetwork in the administrative control of one organization, company, or governmental agency, inside which that organization typically runs an interior
gateway protocol (IGP).
A physical connector on a router that is designed to be used to allow a remote terminal, or PC with a terminal emulator,
to access a router using an analog modem.
A serial link between two routers, created without CSU/DSUs, by connecting a DTE cable to one router and a DCE cable to the other. Typically used in labs to build serial links without the expense of an actual leased line from the telco.
A term that, over the years, has been used to refer to the logic behind the EIGRP routing protocol. More commonly today, this logic is referred to as advanced distance vector logic.
A reference to the speed of a networking link. Its origins come from earlier communications technology in which the
range, or width, of the frequency band dictated how fast communications could occur.
basic service set (BSS)
In wireless LANs, a WLAN with a single access point.
bitwise Boolean AND
A Boolean AND between two numbers of the same length in which the first bit in each number is ANDed, and then the second bit in each number, andthen the third, and so on.
math operation performed on a pair of one-digit binary numbers. The result is another one-digit binary number. 1 AND 1 yields 1; all other combinations
yield a 0.
The low-order 4 bits of the configuration
register in a Cisco router. The value
in the boot field in part tells the router where
to look for a Cisco IOS image to load.
Generally, any address that represents all devices, and can be used to send one message to all devices. In Ethernet, the MAC address of all binary 1s, or FFFF.FFFF.FFFF in hex. For IPv4, see subnet broadcast address.
A set of all devices that receive broadcast frames originating from any device within the set. Devices in the same VLAN are in the same broadcast domain.
An Ethernet frame sent to destination address FFFF.FFFF.FFFF, meaning that the frame should be delivered to all hosts on that LAN.
When subnetting a Class A, B, or C network, the one subnet in each classful network for which all subnet bits
have a value of binary 1. The subnet broadcast address in this subnet has the same numeric value as the classful network's network-wide broadcast address.
A common physical signal path composed
of wires or other media across which
signals can be sent from one part of a computer to another.
An Internet access technology that uses a cable TV (CATV) cable, normally used for video, to send and receive data.
Cisco Discovery Protocol. A media-
and protocol-independent devicediscovery
protocol that runs on most
Cisco-manufactured equipment, including
routers, access servers, and switches. Using
CDP, a device can advertise its existence to
other devices and receive information about
other devices on the same LAN or on the remote side of a WAN.
A device on the other end
of some communications cable that is advertising CDP updates.
Classless interdomain routing. An
RFC-standard tool for global IP address range assignment. CIDR reduces the size of Internet routers' IP routing tables, helping deal with the rapid growth of the Internet. The term classless refers to the fact that the summarized groups of networks represent a group of addresses that do not conform to IPv4 classful (Class A, B, and C) grouping rules.
console port A physical socket on a router
or switch to which a cable can be connected
between a computer and the router/switch,
for the purpose of allowing the computer to
use a terminal emulator and use the CLI to
configure, verify, and troubleshoot the router/
The time required for routing
protocols to react to changes in the network,
removing bad routes and adding new, better
routes so that the current best routes are in all the routers' routing tables.
Customer premises equipment. Any
equipment related to communications that is
located at the customer site, as opposed to
inside the telephone company's network.
An Ethernet cable that swaps the pair used for transmission on one device to a pair used for receiving on the device on the opposite end of the cable. In 10BASE-T and 100BASE-TX networks, this cable swaps the pair at pins 1,2 to pins 3,6 on the other end of the cable, and the pair at pins 3,6 to pins 1,2 as well.
Carrier sense multiple access
with collision detection. A media-access
mechanism in which devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. If two devices transmit at once, a collision occurs and is detected by all colliding devices. This collision subsequently delays retransmissions from those devices for some random length of time.
Channel service unit/data service
unit. A device that understands the Layer
1 details of serial links installed by a telco
and how to use a serial cable to communicate with networking equipment such as routers.
One of three options
for internal processing on some models
of Cisco LAN switches in which the frame is
forwarded as soon as enough of the Ethernet header has been received for the switch to
make a forwarding decision, including forwarding
the first bits of the frame before the
whole frame is received.
DCE Data communications equipment.
From a physical layer perspective, the device providing the clocking on a WAN link, typically a CSU/DSU, is the DCE. From a packet switching perspective, the service provider's switch, to which a router might connect, is considered the DCE.
On a computer that
receives data over a network, the process in
which the device interprets the lower-layer
headers and, when finished with each header, removes the header, revealing the next-higherlayer PDU.
default gateway/default router
On an IP host, the IP address of some router to which the host sends packets when the packet's destination
address is on a subnet other than the
The mask used in a Class
A, B, or C network that does not create any
subnets; specifically, mask 255.0.0.0 for Class A networks, 255.255.0.0 for Class B networks, 255.255.255.0 for Class C networks.
On a router, the route that is
considered to match all packets that are not
otherwise matched by some more specific
The legal term for the demarcation
or separation point between the telco's equipment and the customer's equipment.
denial of service (DoS)
A type of attack whose goal is to cause problems by preventing legitimate users from being able to access services, thereby preventing the normal operation
of computers and networks.
Dynamic Host Configuration
Protocol. A protocol used by hosts to dynamically discover and lease an IP address, and learn the correct subnet mask, default gateway, and DNS server IP addresses.
Any device that uses DHCP
protocols to ask to lease an IP address from a DHCP server, or to learn any IP settings from that server.
The name of the router IOS
feature that forwards DHCP messages from
client to servers by changing the destination
IP address from 255.255.255.255 to the IP
address of the DHCP server.
Software that waits for
DHCP clients to request to lease IP addresses, with the server assigning a lease of an IP address as well as listing other important IP settings
for the client.
directed broadcast address
The logic behind the behavior of some interior routing protocols, such as RIP. Distance vector routing algorithms
call for each router to send its entire
routing table in each update, but only to its
neighbors. Distance vector routing algorithms can be prone to routing loops but are computationally simpler than link-state routing algorithms.
Domain Name System. An application
layer protocol used throughout the Internet
for translating host names into their associated IP addresses.
dotted-decimal notation (DDN)
The format used for IP version 4 addresses, in which four decimal values are used, separated by periods (dots).
Digital signal level 0. A 64-kbps line,
or channel of a faster line inside a telco,
whose origins are to support a single voice
call using the original voice (PCM) codecs.
Digital signal level 1. A 1.544-Mbps
line from the telco, with 24 DS0 channels of
64 kbps each, plus an 8-kbps management
and framing channel. Also called a T1.
Digital signal level 3. A 44.736-Mbps
line from the telco, with 28 DS1 channels plus overhead. Also called a T3.
Digital subscriber line. Public network
technology that delivers high bandwidth over
conventional telco local-loop copper wiring
at limited distances. Typically used as an
Internet access technology, connecting a user to an ISP.
A device that connects to a
telephone line, using DSL standards, to transmit and receive data to/from a telco using DSL.
Data terminal equipment. From a Layer
1 perspective, the DTE synchronizes its clock based on the clock sent by the DCE. From a packet-switching perspective, the DTE is the device outside the service provider's network, typically a router.
A mode of operation in which a
host or router runs both IPv4 and IPv6.
On opposite ends of
any Ethernet link, the condition in which one
of the two devices uses full-duplex logic and
the other uses half-duplex logic, resulting in
unnecessary frame discards and retransmissions on the link.
Duplicate Address Detection (DAD)
A term used in IPv6 to refer to how hosts first
check whether another host is using a unicast address before the first host uses that address.
Similar to a T1, but used in Europe. It
uses a rate of 2.048 Mbps and 32 64-kbps
channels, with one channel reserved for framing and other overhead
The common name for all
the IEEE standards that send data at 100
megabits per second.
Generally, a process or a device that
screens network traffic for certain characteristics, such as source address, destination address, or protocol, and determines whether to forward or discard that traffic based on the established criteria.
A device that forwards packets between
the less secure and more secure parts
of the network, applying rules that determine
which packets are allowed to pass, and which are not.
A type of read/write permanent
memory that retains its contents even with
no power applied to the memory, and uses no moving parts, making the memory less likely to fail over time.
The result of the LAN switch forwarding
process for broadcasts and unknown
unicast frames. Switches forward these frames out all interfaces, except the interface in which the frame arrived. Switches also flood multicasts by default, although this behavior can be changed.
The process of regulating the
amount of data sent by a sending computer
toward a receiving computer. Several flow
control mechanisms exist, including TCP flow control, which uses windowing.
To send a frame received in one
interface out another interface, toward its ultimate destination.
A process used by protocols that do error recovery, in which the number that acknowledges data lists the next data that should be sent, not the last data that was successfully received.
A line from the telco with four wires, composed of two twisted-pair
wires. Each pair is used to send in one direction, so a four-wire circuit allows full-duplex communication.
One of three internal processing options on some Cisco LAN switches in which the first bits of the frame can be forwarded before the entire frame is received, but not until the first 64 bytes of the frame are received, in which case, in a well-designed LAN, collision fragments should not occur as a result of this forwarding logic.
frame A term referring to a data link header
and trailer, plus the data encapsulated between the header and trailer.
Frame Check Sequence
A field in many data link trailers used as part of the error detection process.
An international standard data link protocol that defines the capabilities
to create a frame-switched (packet-switched)
service, allowing DTE devices (typically routers) to send data to many other devices using a single physical connection to the Frame Relay service.
Frequency Hopping Spread
A method of encoding data on a
wireless LAN in which consecutive transmissions occur on different nearby frequency bands as compared with the prior transmission. Not used in modern WLAN standards.
Generically, any communication
in which two communicating devices
can concurrently send and receive data. In
Ethernet LANs, the allowance for both devices to send and receive at the same time, allowed when both devices disable their CSMA/CD logic.
A network topology in which
more than two devices can physically communicate and, by choice, all pairs of devices are allowed to communicate directly.
The common name for all the IEEE standards that send data at 1 gigabit per second.
global routing prefix
An IPv6 prefix that defines an IPv6 address block made up of global unicast addresses, assigned to one organization, so that the organization has a block of globally unique IPv6 addresses to use in its network.
global unicast address
A type of unicast IPv6 address that has been allocated from a range of public globally unique IP addresses,
as registered through IANA/ICANN, its member agencies, and other registries or ISPs.
Generically, any communication
in which only one device at a time can
send data. In Ethernet LANs, the normal result of the CSMA/CD algorithm that enforces the rule that only one device should send at any point in time.
High-Level Data Link Control. A bitoriented
synchronous data link layer protocol
developed by the International Organization
for Standardization (ISO).
The upstream, transmit end of a
cable TV (CATV) installation.
In computer networking, a set of
bytes placed in front of some other data,
encapsulating that data, as defined by a particular protocol.
Any device that uses an IP address.
The IP address assigned to a
network card on a computer.
The alphameric name of an IP host.
A term used to describe a part
of an IPv4 address that is used to uniquely
identify a host inside a subnet. The host part
is identified by the bits of value 0 in the subnet mask.
A route with a /32 mask, which
by virtue of this mask represents a route to a
single host IP address.
Hypertext Markup Language. A
simple document-formatting language that
uses tags to indicate how a given part of a
document should be interpreted by a viewing
application, such as a web browser.
Hypertext Transfer Protocol. The
protocol used by web browsers and web servers to transfer files, such as text and graphic files.
A LAN device that provides a centralized
connection point for LAN cabling,
repeating any received electrical signal out
all other ports, thereby creating a logical bus.
Hubs do not interpret the electrical signals as
a frame of bits, so hubs are considered to be
Layer 1 devices.
The Internet Assigned Numbers
Authority (IANA). An organization that owns
the rights to assign many operating numbers
and facts about how the global Internet
works, including public IPv4 and IPv6 addresses. See also ICANN.
The Internet Corporation for
Assigned Names and Numbers. An organization appointed by IANA to oversee the distributed process of assigning public IPv4 and IPv6 addresses across the globe.
Subdivisions of a Class A, B, or
C network, as configured by a network administrator. Subnets allow a single Class A, B, or C network to be used instead of multiple networks, and still allow for a large number of groups of IP addresses, as is required for efficient IP routing.
IP version 4
Literally, the version of the
Internet Protocol defined in an old RFC 791,
standardized in 1980, and used as the basis of TCP/IP networks and the Internet for over 30 years.
IP version 6
A newer version of the
Internet Protocol defined in RFC 2460, as
well as many other RFCs, whose creation was motivated by the need to avoid the IPv4 address exhaustion problem.
IPv4 address exhaustion
The process by which the public IPv4 addresses, available to create the Internet, were consumed through the 1980s until today, with the expectation that eventually he world would run out of available IPv4 addresses.
IPv6 neighbor table
The IPv6 equivalent of the ARP table. A table that lists IPv6 addresses of other hosts on the same link, along with their matching MAC addresses, as typically learned using Neighbor Discovery Protocol (NDP).
Integrated Services Digital Network.
A service offered by telephone companies
that permits telephone networks to carry
data, voice, and other traffic. Often used as
an Internet access technology, as well as dial when routers lose their normal WAN
Inter-Switch Link. A Cisco-proprietary
protocol that maintains VLAN information as
traffic flows between switches and routers
International Organization for
Standardization. An international organization that is responsible for a wide range of standards, including many standards relevant to networking. The ISO developed the OSI reference model, a popular networking reference
A proprietary feature of Cisco
routers in which the router sends messages
on a periodic basis as a means of letting the
neighboring router know that the first router
is still alive and well.
Layer 4 protocol data unit. The
data compiled by a Layer 4 protocol, including Layer 4 headers and encapsulated
high-layer data, but not including lower-layer
headers and trailers.
Layer 3 protocol
A protocol that has
characteristics like OSI Layer 3, which defines logical addressing and routing. IPv4 and IPv6 are Layer 3 protocols.
Layer 3 switch
The process used by switches for
discovering MAC addresses, and their relative location, by looking at the source MAC address of all frames received by a bridge or switch.
A serial communications circuit
between two points, provided by some
service provider, typically a telephone company (telco). Because the telco does not sell a physical cable between the two endpoints, instead charging a monthly fee for the ability to send bits between the two sites, the service is considered to be a leased service.
A type of unicast IPv6 address that represents an interface on a single
data link. Packets sent to a link-local address
cross only that particular link and are never
forwarded to other subnets by a router. Used
for communications that do not need to leave
the local link.
With IPv6 multicasts, a term that refers to the parts (scope) of the network to which a multicast packet can flow, with link-local referring to the fact that the packet stays on the subnet in which it originated.
A classification of the underlying
algorithm used in some routing protocols.
Link-state protocols build a detailed database that lists links (subnets) and their state (up, down), from which the best routes can then be calculated.
link-state advertisement (LSA)
In OSPF, name of the data structure that resides inside the LSDB and describes in detail the various components in a network, including routers and links (subnets).
link-state database (LSDB)
In OSPF, the data structure in RAM of a router that holds the various LSAs, with the collective LSAs representing the entire topology of the network.
Logical Link Control. The higher of
the two data link layer sublayers defined by
the IEEE. Synonymous with IEEE 802.2.
A line from the premises of a
telephone subscriber to the telephone company CO.
local subnet broadcast address
IPv4 address 255.255.255.255. A packet sent to this address is sent as a data link broadcast, but only flows to hosts in the subnet into which it was originally sent. Routers do not forward these packets.
A username (with matching
password), configured on a router or
switch. It is considered local because it exists on the router or switch, and not on a remote server.
A generic reference to addresses
as defined by Layer 3 protocols that
do not have to be concerned with the physical details of the underlying physical media. Used mainly to contrast these addresses with
data link addresses, which are generically
considered to be physical addresses because
they differ based on the type of physical
Media Access Control. The lower of
the two sublayers of the data link layer defined by the IEEE. Synonymous with IEEE
802.3 for Ethernet LANs.
A standardized data link
layer address that is required for every device that connects to a LAN. Ethernet MAC addresses are 6 bytes long and are controlled by the IEEE. Also known as a hardware address, a MAC layer address, and a physical address.
A unit of measure used by routing
protocol algorithms to determine the best
route for traffic to use to reach a particular
The process in LAN design by which every switch port connects to a single device, with no hubs connected to the switch ports, creating a separate collision domain per interface. The term's origin relates
to the fact that one definition for the word
"segment" is "collision domain," with a switch
separating each switch port into a separate
collision domain or segment.
Modulator-demodulator. A device
that converts between digital and analog
signals so that a computer can send data to
another computer using analog telephone
lines. At the source, a modem converts digital signals to a form suitable for transmission over analog communication facilities. At the destination, the analog signals are returned to their digital form.
A LAN switch that can also perform Layer 3 routing functions. The name comes from the fact that this device makes forwarding decisions based on logic from multiple OSI layers (Layers 2 and 3).
A type of fiber-optic cabling
with a larger core than single-mode cabling,
allowing light to enter at multiple angles. Such cabling has lower bandwidth than single mode fiber but requires a typically cheaper light source, such as an LED rather than a laser.
A server connected to a network
that resolves network names into network
named access list
An ACL that identifies
the various statements in the ACL based on a name, rather than a number.
Network Address Translation. A
mechanism for reducing the need for globally
unique IP addresses. NAT allows an organization with addresses that are not globally unique to connect to the Internet, by translating those addresses into public addresses in the globally routable address space.
In routing protocols, another
router with which a router decides to exchange routing information.
Neighbor Advertisement (NA)
A message defined by the IPv6 Neighbor Discovery Protocol (NDP), used to declare to other neighbors a host's MAC address. Sometimes sent in response to a previously received NDP Neighbor Solicitation (NS) message
Neighbor Discovery Protocol (NDP)
A protocol that is part of the IPv6 protocol
suite, used to discover and exchange information about devices on the same subnet (neighbors). In particular, it replaces the IPv4 ARP protocol.
Neighbor Solicitation (NS)
A message defined by the IPv6 Neighbor Discovery Protocol (NDP), used to ask a neighbor to reply with a Neighbor Advertisement, which lists the neighbor's MAC address.
A collection of computers, printers,
routers, switches, and other devices that
can communicate with each other over some
network broadcast address
network broadcast address In IPv4, a
special address in each classful network that
can be used to broadcast a packet to all hosts in that same classful network. Numerically, the address has the same value as the network number in the network part of the address and all 255s in the host octets—for example, 10.255.255.255 is the network broadcast address for classful network 10.0.0.0.
network interface card (NIC)
A computer card, sometimes an expansion card and sometimes integrated into the motherboard of the computer, that provides the electronics and other functions to connect to a computer network. Today, most NICs are specifically Ethernet NICs, and most have an RJ-45 port, the most common type of Ethernet port.
A number that uses dotted-decimal notation like IP addresses, but the number itself represents all hosts in a single Class A, B, or C IP network.
The portion of an IPv4 address
that is either 1, 2, or 3 octets/bytes long,
based on whether the address is in a Class A, B, or C network.
Network Time Protocol (NTP)
Network Time Protocol (NTP) A protocol
used to synchronize time-of-day clocks
so that multiple devices use the same time of
day, which allows log messages to be more
easily matched based on their timestamps.
A generic term referring
to any set of protocols and standards
collected into a comprehensive grouping
that, when followed by the devices in a network, allows all the devices to communicate. Examples include TCP/IP and OSI.
In an IP route in a routing
table, part of a routing table entry that refers
to the next IP router (by IP address) that
should receive packets that match the route.
network interface card.
ordered data transfer
A networking function,
included in TCP, in which the protocol
defines how the sending host should number
the data transmitted, defines how the receiving device should attempt to reorder the data if it arrives out of order, and specifies to discard the data if it cannot be delivered in order.
Open System Interconnection reference
model. A network architectural model developed by the ISO. The model consists of seven layers, each of which specifies particular network functions, such as addressing, flow control, error control, encapsulation, and reliable message transfer.
Open Shortest Path First. A popular
link-state IGP that uses a link-state database
and the Shortest Path First (SPF) algorithm to calculate the best routes to reach each known subnet.
OSPF version 2
The version of the OSPF
routing protocol that supports IPv4, and not
IPv6, and has been commonly used for over
OSPF version 3
The version of the OSPF
routing protocol that supports IPv6, and not
outgoing interface In an IP route in a routing
table, part of a routing table entry that
refers to the local interface out which the local router should forward packets that match the route.
An (incorrect) IP subnet design condition in which one subnet's range of addresses includes addresses in the range of another subnet.
A logical grouping of bytes that
includes the network layer header and encapsulated data, but specifically does not include any headers and trailers below the network layer.
A generic reference to network services, typically WAN services, in which the service examines the contents of the transmitted data to make some type of forwarding decision. This term is mainly used to contrast with the WAN term circuit
switching, in which the provider sets up a
(Layer 1) circuit between two devices and the provider makes no attempt to interpret the meaning of the bits.
network topology in which
more than two devices could physically communicate but, by choice, only a subset of the pairs of devices connected to the network is allowed to communicate directly.
Post, telephone, and telegraph. A
government agency that provides telephone
services. PTTs exist in some areas outside of North America and provide both local and
long-distance telephone services.
public IP address
An IP address that is
part of a registered network number, as assigned by an Internet Assigned Numbers
Authority (IANA) member agency, so that
only the organization to which the address
is registered is allowed to use the address.
Routers in the Internet should have routes
allowing them to forward packets to all the
publicly registered IP addresses.
quartet A term used in this book, but not
in other references, to refer to a set of four
hex digits in an IPv6 address
Random-access memory. A type of
volatile memory that can be read and written
by a microprocessor.
Request For Comments. A document
used as the primary means for communicating information about the TCP/IP protocols. Some RFCs are designated by the Internet
Architecture Board (IAB) as Internet standards, and others are informational. RFCs are available online from numerous sources, including www.rfc-editor.org.
Routing Information Protocol. An interior
gateway protocol (IGP) that uses distance
vector logic and router hop count as the metric. RIP version 2 (RIP-2) replaced the older RIP version 1 (RIP-1), with RIP-2 providing more features, including support for VLSM.
A popular type of cabling connector
used for Ethernet cabling. It is similar to the
RJ-11 connector used for telephone wiring in
homes in the United States. RJ-45 allows the
connection of eight wires.
Read-only memory. A type of nonvolatile
memory that can be read but not written
to by the microprocessor.
A troubleshooting term that
refers to the reason why a problem exists, specifically a reason for which, if changed, the problem would either be solved or changed to a different problem.
A protocol that defines packets that can be routed by a router. Examples of routed protocols include IPv4 and IPv6.
Router Advertisement (RA)
A message defined by the IPv6 Neighbor Discovery Protocol (NDP), used by routers to announce their willingness to act as an IPv6 router on a link. These can be sent in response to a previously received NDP Router Solicitation (RS) message.
router ID (RID)
In OSPF, a 32-bit number,
written in dotted-decimal notation, that
uniquely identifies each router.
Router Solicitation (RS)
A message defined by the IPv6 Neighbor Discovery Protocol (NDP), used to ask any routers on the link to reply, identifying the router, plus other configuration settings (prefixes and prefix lengths).
A set of messages and
processes with which routers can exchange
information about routes to reach subnets in a particular network. Examples of routing protocols include the Enhanced Interior Gateway Routing Protocol (EIGRP), the Open Shortest Path First (OSPF) protocol, and the Routing Information Protocol (RIP).
A list of routes in a router, with each route listing the destination subnet and mask, the router interface out which to forward packets destined to that subnet, and as needed, the next-hop router's IP address.
A generic reference to any
routing protocol's messages in which it sends routing information to a neighbor.
In Cisco IOS switches
and routers, the name of the file that resides
in RAM memory, holding the device's currently used configuration.
The communication between two networking devices for the purposes of the functions defined at a particular layer of a networking model, with that communication happening by using a header defined by that layer of the model. The two devices set values in the header, send the header and encapsulated data, with the receiving device(s) interpreting the header to decide what action to take.
Secure Shell (SSH)
A TCP/IP application
layer protocol that supports terminal emulation between a client and server, using dynamic key exchange and encryption to keep the communications private.
In TCP, a term used to describe
a TCP header and its encapsulated data (also called an L4PDU). Also in TCP, the process of accepting a large chunk of data from the application layer and breaking it into smaller pieces that fit into TCP segments. In Ethernet, a segment is either a single Ethernet cable or a single collision domain (no matter how many cables are used).
The process of breaking a
large piece of data from an application into
pieces appropriate in size to be sent through
A type of cable with many
different styles of connectors used to connect a router to an external CSU/DSU on a leased line installation.
A type of interface on a router, used to connect to some types of WAN links, particularly leased lines and Frame Relay access links.
An option on Cisco IOS
switches and routers that prompts the user for basic configuration information, resulting in new running-config and startup-config files.
An Ethernet that uses a
hub, or even the original coaxial cabling, that
results in the devices having to take turns
sending data, sharing the available bandwidth.
Shortest Path First (SPF) algorithm
The name of the algorithm used by link-state routing protocols to analyze the LSDB and find the least-cost routes from that router to each subnet.
A type of fiber-optic cabling
with a narrow core that allows light to enter
only at a single angle. Such cabling has a
higher bandwidth than multimode fiber but
requires a light source with a narrow spectral
width (such as a laser).
For protocols such as
TCP that allow the receiving device to dictate
the amount of data the sender can send
before receiving an acknowledgment—a concept called a window—a reference to the fact that the mechanism to grant future windows is typically just a number that grows upward slowly after each acknowledgment, sliding upward.
solicited-node multicast address
A type of IPv6 multicast address, with link-local scope, used to send packets to all hosts in the subnet that share the same value in the last six hex digits of their unicast IPv6 addresses. Begins with FF02::1:FF00:0/104.
Synchronous Optical Network. A
standard format for transporting a wide range of digital telecommunications services over optical fiber.
In a subnetted IPv4 address,
interpreted with classful addressing rules, one of three parts of the structure of an IP address, with the subnet part uniquely identifying different subnets of a classful IP network.
The process of subdividing a
Class A, B, or C network into smaller groups
A network device that filters, forwards,
and floods Ethernet frames based on
the destination address of each frame.
An Ethernet that uses
a switch, and particularly not a hub, so that
the devices connected to one switch port do
not have to contend to use the bandwidth
available on another port. This term contrasts with shared Ethernet, in which the devices must share bandwidth, whereas switched Ethernet provides much more capacity, as the devices do not have to share the available bandwidth.
A feature of many Internet access
technologies in which the downstream
transmission rate is the same as the upstream transmission rate.
The imposition of time ordering on a bit stream. Practically, a device will try to use the same speed as another device on the other end of a serial link. However, by examining transitions between voltage states on the link, the device can notice
slight variations in the speed on each end
and can adjust its speed accordingly.
A line from the telco that allows transmission
of data at 1.544 Mbps, with the ability
to treat the line as 24 different 64-kbps
DS0 channels (plus 8 kbps of overhead).
Transmission Control Protocol. A connection-oriented transport layer TCP/IP protocol
that provides reliable data transmission
Transmission Control Protocol/
Internet Protocol. A common name for the
suite of protocols developed by the U.S.
Department of Defense in the 1970s to support the construction of worldwide internetworks. TCP and IP are the two best-known protocols in the suite.
A common abbreviation for telephone
The standard terminal-emulation
application layer protocol in the TCP/IP protocol stack. Telnet is used for remote terminal connection, enabling users to log in to remote systems and use resources as if they were connected to a local system. Telnet is defined in RFC 854.
Short for traceroute. A program available
on many systems that traces the path
that a packet takes to a destination. It is used mostly to troubleshoot routing problems between hosts.
In computer networking, a set of
bytes placed behind some other data, encapsulating that data, as defined by a particular protocol. Typically, only data link layer protocols define trailer
The name of a networking device that was a precursor to modern LAN switches. Bridges forward frames between LAN segments based on the destination MAC address. Transparent bridging is so named because the presence of bridges is ransparent to network end nodes.
In campus LANs, an Ethernet segment
over which the devices add a VLAN
header that identifies the VLAN in which the
A switch interface configured
so that it operates using VLAN trunking
(either 802.1Q or ISL).
Also called VLAN trunking. A
method (using either the Cisco ISL protocol
or the IEEE 802.1Q protocol) to support
multiple VLANs, allowing traffic from those
VLANs to cross a single link.
trunking administrative mode
The configured trunking setting on a Cisco switch interface, as configured with the switchport mode command.
trunking operational mode
The current behavior of a Cisco switch interface for VLAN trunking.
Transmission medium consisting
of two insulated wires, with the wires
twisted around each other in a spiral. An electrical circuit flows over the wire pair, with the current in opposite directions on each wire, which significantly reduces the interference between the two wires.
User Datagram Protocol.
Connectionless transport layer protocol in
the TCP/IP protocol stack. UDP is a simple
protocol that exchanges datagrams without
acknowledgments or guaranteed delivery.
Generally, any address in
networking that represents a single device or
interface, instead of a group of addresses (as would be represented by a multicast or broadcast address).
unicast IP address
An IP address that
represents a single interface. In IPv4, these
addresses come from the Class A, B, and C
unique local address
A type of IPv6 unicast
address meant as a replacement for IPv4
unknown unicast frame
An Ethernet frame whose destination MAC address is not listed in a switch's MAC address table, so the switch must flood the frame.
up and up
Jargon referring to the two interface
states on a Cisco IOS router or switch
(line status and protocol status), with the first
"up" referring to the line status and the second "up" referring to the protocol status. An interface in this state should be able to pass data link frames.
A timer used by a router to
indicate when to send the next routing update.
Uniform Resource Locator. A standard
for how to refer to any piece of information
retrievable via a TCP/IP network, most notably used to identify web pages. For example, http://www.certskills.com/blog is a URL that identifies the protocol (HTTP), host name (www.certskills.com), and web page (blog).
A mode of the user interface to
a router or switch in which the user can type
only nondisruptive EXEC commands, generally just to look at the current status, but not to change any operational settings.
Unshielded twisted-pair. A
type of cabling, standardized by the
Telecommunications Industry Association
(TIA), that holds twisted pairs of copper wires (typically four pair) and does not contain any shielding from outside interference.
variable-length subnet mask
The capability to specify a different
subnet mask for the same Class A, B, or C
network number on different subnets. VLSM
can help optimize available address space.
virtual circuit (VC)
In packet-switched services like Frame Relay, VC refers to the ability of two DTE devices (typically routers) to send and receive data directly to each other, which supplies the same function as a physical leased line (leased circuit), but doing
so without a physical circuit. This term
is meant as a contrast with a leased line or
virtual LAN (VLAN)
A group of devices,
connected to one or more switches, with the
devices grouped into a single broadcast domain through switch configuration. VLANs
allow switch administrators to separate the
devices connected to the switches into separate VLANs without requiring separate physical switches, gaining design advantages of separating the traffic without the expense of buying additional hardware.
Virtual Private Network (VPN)
The process of securing communication between two devices whose packets pass over some public and unsecured network, typically the Internet. VPNs encrypt packets so that the communication is private, and authenticate the identity of the endpoints.
VLAN configuration database
The name of the collective configuration of VLAN IDs and names on a Cisco switch.
A configuration concept inside Cisco switches, used as an interface between IOS running on the switch and a VLAN supported inside the switch, so that
the switch can assign an IP address and send IP packets into that VLAN.
VLAN Trunking Protocol (VTP)
A Cisco proprietary messaging protocol used between Cisco switches to communicate configuration information about the existence of VLANs, including the VLAN ID and VLAN name.
The default file used to store a
Cisco switch's VLAN configuration database.
Voice over IP. The transport of voice
traffic inside IP packets over an IP network.
VLAN Trunking Protocol.
VTP client mode
One of three VTP operational
modes for a switch with which switches
learn about VLAN numbers and names from
other switches, but which does not allow the
switch to be directly configured with VLAN
VTP server mode
One of three VTP operational
modes. Switches in server mode can
configure VLANs, tell other switches about
the changes, and learn about VLAN changes
from other switches.
VTP transparent mode
One of three VTP operational modes. Switches in transparent mode can configure VLANs, but they do not tell other switches about the changes, and they do not learn about VLAN changes from other switches.
Software, running on a computer,
that stores web pages and sends those
web pages to web clients (web browsers) that request the web pages.
A TCP or UDP port number reserved for use by a particular application.The use of well-known ports allows a client to send a TCP or UDP segment to a server, to the correct destination port for that application.
An organization formed by many companies in the wireless industry (an industry association) for the purpose of getting multivendor certified-compatible wireless products to market in a more timely fashion than would be possible by simply relying
on standardization processes.
wide-area network (WAN)
A part of a larger network that implements mostly OSI Layer 1 and 2 technology, connects sites that typically sit far apart, and uses a business model in which a consumer (individual or business) must lease the WAN from a service provider (often a telco).
The mask used in Cisco
IOS ACL commands and OSPF and EIGRP
Represents the number of bytes
that can be sent without receiving an
A local-area network (LAN)
that physically transmits bits using radio
waves. The name "wireless" compares these
LANs to more traditional "wired" LANs,
which are LANs that use cables (which often
have copper wires inside).
A wireless device that wants
to gain access to a wireless access point for
the purpose of communicating with other
wireless devices or other devices connected
to the wired internetwork.
For every classful IPv4 network
that is subnetted, the one subnet whose
subnet number has all binary 0s in the subnet part of the number. In decimal, the zero subnet can be easily identified because it is the same number as the classful network number.
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