Here I’m compiling a list of important terms, acronyms, buzzwords, and jargon for quick referencing. Things should be alphabetized, but the list is quite long and still growing, so you’ll probably want to use your browser’s find/search function (ctrl + f) to quickly find the term you’re looking for.

Work In Progress

This page is very much still being worked on - I’m completely changing how I manage the glossary (it’s gotten a little out of hand) Apologies for broken links or missing definitions (to say nothing of missing terms!)

Index

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A

Access Control Entry

A single entry in an Access Control List (ACL). Essentially serves as a rule, or a filter, for how the interface will (or won’t) forward traffic.

ACEs are typically processed in the order they are added to the ACL, and when a given packet is matched by the an ACE it is either forwarded or dropped, per the ACE. Any subsequent ACEs are ignored/not processed.

Access Control Lists

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Access Control List (ACL)

A list of Access Control Entries (ACEs) which serve as a set of rules for filtering/allowing traffic to be forwarded on a router’s interface. All interfaces can have up to two ACLs, one for Inbound traffic, and one for Outbound traffic.

Access Control Lists

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Address Resolution Protocol (ARP)

A communication protocol used for discovering the MAC address of unknown devices. When a device needs to forward traffic to an IP for which it has no associated MAC address in its address table, it will broadcast an ARP Request message containing it’s IP and the IP of the device whose MAC it is trying to find. Only the correct recipient device will unicast an ARP Reply to the source device.

Address Resolution Protocol (ARP),

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AdministrativeDistance

In the event that more than one Dynamic Routing protocol is being used in the same network, AD indicates which protocol is more ‘trustworthy’ (more likely to select a good route) so the router can determine which protocol’s metric should be used to select a route.

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Area (OSPF)

An area is a collection of routers and links that share an LSDB.

OSPF uses areas to divide a large network into more easily managed sizes, to help reduce the size of LSDBs and the number of LSAs being transmitted.

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Autonomous System (AS)

A single organization (i.e. a company)

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B

Backup Designated Router (BDR)

The backup to an OSPF network’s Designated Router (DR).

A router in an OSPF Broadcast-type network is elected Designated Router (DR) and another is elected Backup Designated Router (BDR). These two routers serve as the central points for exchanging OSPF routing information. All other OSPF-enabled routers will synchronize their LSDBs from the DR and BDR, and will only form full adjacencies with the DR and BDR.

The BDR will become the DR if the DR is disabled or otherwise becomes unreachable.

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Border Gateway Protocol (BGP)

The only EGP Dynamic Routing Protocol presently in use in modern networks.

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Bridge

Typically, this term refers to a network switch in modern parlance.

Technically, bridges are predecessors to modern switches; they are Layer 2 devices that forward traffic within a LAN. Actual bridges are very uncommon in modern networking, though they are still referenced in a lot of networking technology and documentation, thus they are commonly conflated with switches.

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Broadcast

Broadcast messages are delivered to all devices in a local network. Compare to Multicast and Unicast

Broadcast messages are sent to a networks Broadcast Address.

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Broadcast Address

The IP Address a network uses for forwarding Broadcast Messages. Typically, though not necessarily, it is the last usable host address of the network/subnetwork’s address space.

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Broadcast Domain

A network segment whose constituent nodes can all be reached with a broadcast message

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Broadcast Storm

When a group of switches loop broadcast messages to the point that the network suffers from congestion.

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C

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

A system for preventing collisions in network traffic when multiple devices are sending traffic in the same def_CollisionDomain.

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Collision Domain

A network segment where simultaneous data transmissions collide with one another, resulting in lost or improperly organized PDUs.

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Cyclic Redundancy Check (CRC)

An error-detecting code commonly used in computer networking. In particular, used by the Ethernet header FCS.

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D

Datagram

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Designated Router (DR)

A router in an OSPF Broadcast-type network is elected Designated Router (DR) and another is elected Backup Designated Router (BDR). These two routers serve as the central points for exchanging OSPF routing information. All other OSPF-enabled routers will synchronize their LSDBs from the DR and BDR, and will only form full adjacencies with the DR and BDR.

The BDR will become the DR if the DR is disabled or otherwise becomes unreachable.

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Dijkstra’s Algorithm

An algorithm for finding the shortest path between to nodes in a weighted graph (a network).

OSPF, Dynamic Routing

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Domain Name System (DNS)

A system for resolving IP addresses from host names, usually URLs/Web Domain Names, by requesting them from DNS Servers. Plain-English (or whatever language) host names are easier to read, write, and remember than IPv4/v6 addresses, so it is useful to be able to enter a website name into a browser (for instance) and have the computer automatically determine the correct IP address to forward/receive traffic from.

Domain Name System (DNS)

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Drop Eligible Indicator (DEI)

A field in the 802.1Q header. It is only a single bit long, and that bit signals if the packet may be dropped if the network is overly congested.

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Dynamic Routing

Any process by which routers dynamically (automatically) discover routes to network destinations, and advertise routes to connected addresses to neighbor routers.

There are a number of Dynamic Routing protocols including RIP, EIGRP, and OSPF.

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E

Equal Cost Multi-Path (ECMP)

In Dynamic Routing, when two or more routes to the same destination have an equal def_Metric cost, traffic will be load-balanced across them.

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Exterior Gateway Protocol (EGP)

A type of Dynamic Routing protocol which is used to share routes between different autonomous systems.

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Enhanced Interior Gateway Routing Protocol (EIGRP)

An advanced distance vector Dynamic Routing protocol. Originally Cisco proprietary, much of the protocol has been released for all manufacturers to use, though most vendors have not implemented it. Considered a more advanced routing protocol than RIP, another distance vector protocol.

EIGRP is the only IGP capable of unequal-cost load-balancing.

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EtherChannel

EtherChannel allows a switch to treat multiple physical interfaces as a single logical interface. This allows two switches to be connected together by multiple interfaces and use all of the bandwidth available to all connected interfaces while still being able to use STP to prevent Layer 2 loops/broadcast storms.

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Ethernet

A collection of computer networking technologies first standardized by the IEEE as IEEE 802.3.

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Extended Unique Identifier (EUI)

Typically called EUI-64 or modified EUI-64. EUI-64 is a method of converting a MAC address (48 bits) into a 64-bit interface identifier. This identifier can then be used as the host portion of a /64 IPv6 address.

IPv6, EUI-64

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F

Fiber Distributed Data Interfaces (FDDI)

A standard for fiber-optic based LANs. Later included specifications for copper cabling. Effectively made obsolete by FastEthernet.

While the standard fell out of fashion due to its comparatively lack-luster network speeds and high costs, it notably had very long transmission distances compared to copper UTP based networks. Cable runs could allegedly extend up to 200km, though it seems doubtful that most networks would ever come close to that.

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First Hop Redundancy Protocol (FHRP)

A computer networking protocol which is designed to protect the default gateway used on a subnetwork by allowing two or more routers to provide backup for that address; in the event of failure of an active router, the backup router will take over the address, usually within a few seconds. Credit: Wikipedia

”First Hop” refers to the fact that the default gateway in any network is the first hop, i.e. the first router to whatever destination outside the network a given host is sending traffic to.

First Hop Redundancy Protocols (FHRPs),

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Floating Static Route

A statically configured route whose Administrative Distance has been configured to be higher than those of dynamically learned routes to the same destination. These routes are inactive unless the route learned by the dynamic protocol is removed, due to hardware failure or any other reason.

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Frame

The Layer 2 PDU. It’s header contains source and destination MAC Addresses, among other data.

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Frame Check Sequence (FCS)

A 4-byte data field at the end of an Ethernet frame used for detecting errors in the frame using a cyclic redundancy check (CRC).

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G

Gateway Load Balancing Protocol (GLBP)

A Cisco-proprietary redundant router protocol that has basic load balancing functionality. While some other protocols can load balance across two or more VLANs, GLBP can load balance traffic from the same VLAN across up to four routers.

GLBP

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Gratuitous ARP

ARP replies sent without being requested (no ARP request message was received).

Address Resolution Protocol (ARP), First Hop Redundancy Protocols (FHRPs)

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H

Host

An endpoint/edge node on a network. Typically either a PC or a server, but could be devices like printers, phones, etc.

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Host File

On most Operating Systems, this is a file that contains mappings of IP addresses to host names. These names are used to manually send/receive traffic to/from network nodes without the user needing to memorize and type out the destination’s IP address repeatedly. This is used as an alternative to, and along side with, DNS, not as a replacement for it.

See Verifying & Configuring IP Parameters On Clients for OS-specific info.

Verifying & Configuring IP Parameters On Clients, DNS

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Host Route

An IP route to a specific host. I.e. has a /32 netmask.

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Hot Standby Router Protocol (HSRP)

A Cisco proprietary redundancy protocol for gateway routers.

HSRP

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Hub

A Layer 1 device that receives traffic on any interface and floods it out all other interfaces. Hubs are predecessors to modern switches, and do nothing to break up the def_CollisionDomain. Therefore, all devices connected to them must run in def_HalfDuplex mode using CD.

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I

The Institute of Electrical and Electronics Engineers (IEEE)

A standards organization, responsible for creating and maintaining many of the most common Computer Networking standards used today, including the Ethernet suite (802.3). See Industry Standards

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Interior Gateway Protocol (IGP)

A type of Dynamic Routing Protocol which shares routes within a single autonomous system.

See Types of Dynamic Routing Protocol

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Internet Protocol (IP)

The network layer communications protocol in the Internet Protocol Suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially established the Internet. Credit: Wikipedia

Internet Protocol Suite (TCP_IP)

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Internet Protocol Suite (TCP/IP)

Commonly referred to as TCP/IP, this is the collection of protocols and general rules/framework for how the Internet, and communication across it, should work. The three foundational protocols are the Transmission Control Protocol (TCP), the User Datagram Protocol (UDP), and the Internet Protocol (IP).

The first versions of the modern IP Suite were developed by DARPA with funding from the US Department of Defense.

IP

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Intermediate System to Intermediate System (IS-IS)

A Link State based Interior Gateway Dynamic Routing protocol.

See Dynamic Routing

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Inter-Switch Link (ISL)

A Cisco-proprietary trunking protocol.

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L

Link Aggregation Control Protocol (LACP)

IEEE 802.3ad; An industry-standard EtherChannel protocol. Dynamically negotiates the creation & maintenance of an EtherChannel.

EtherChannel, Industry Standards

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Local Area Network (LAN)

A LAN is a collection of devices connected together in one physical location.

A more precise techinical definition is A LAN is a group of connected devices that are all within the same Broadcast Domain.

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Link State

A type of Dynamic Routing protocol in which all connected routers develop identical ‘connectivity maps’ (i.e. network maps).

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Link State Advertisement (LSA)

A basic OSPF message. They are used to exchange routing information between routers in the same area.

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Link State Database (LSDB)

A kind of ‘connectivity map’ stored by an OSPF-activated router. Contains information on the available routes of the router’s OSPF neighbors in the form of LSAs.

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M

Maximum Transmission Unit (MTU)

The size of the largest PDU that can be communicated in a single transaction.

In Ethernet the maximum frame size is 1518 bytes. 18 bytes are consumed by the header and FCS, leaving an MTU of 1500 bytes.

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Metric

In Dynamic Routing, a metric is the standard by which any particular protocol ranks the superiority of available routes to a network destination i.e. the ‘cost’ of using a given route where lower ‘costs’ are preferable.

Different protocols use different metrics, and thus are not meaningfully comparable. For comparisons between different routes from different protocols see def_AdministrativeDistance.

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N

Network Management Station (NMS)

AKA Network Management System In a SNMP network, the NMS is the device or devices that manage the other devices on a network; The SNMP ‘server.‘

Simple Network Management Protocol (SNMP)

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Non-Preemptive

(Regarding a console command) does not interrupt the running process.

E.g. when configuring the OSPF priority of an interface to prioritize a particular router to be DR, the actual reselection of DR/BDR won’t take place until OSPF is reset, even if the interface’s priority is changed. This means the ip ospf priority command is non-preemptive.

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O

Open Shortest Path First (OSPF)

A link state interior gateway dynamic routing protocol.

Determines routes using Dijkstra’s Algorithm with information from the LSDB, a kind of network connectivity map. Routers share information about their known connections with each other via LSAs.

OSPF, Dynamic Routing

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Open Systems Interconnection Model (OSI Model)

A reference model, created by the International Organization for Standardization (ISO) that serves as an example for how computer networks should be structured. While it is not directly implemented in any major networks today, it is primarily useful as a reference for discussing the theory of Networking.

The OSI Model splits computer networking into seven layers:

  1. Physical
  2. Data Link
  3. Network
  4. Transport
  5. Session
  6. Presentation
  7. Application

OSI Model

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P

Packet

The Layer 3 PDU. Typically refers to IP Packets in particular.

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Protocol Data Unit (PDU)

A single unit of information used by a computer networking protocol.

Common examples are Segments (Layer 4), Packets (Layer 3), and Frames (Layer 2)

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R

Resolution (Address Resolution)

  1. The process of obtaining an internet address from a host name. See DNS
  2. (Address Resolution) Determining the link-layer address (e.g. MAC address) of a neighbor. See ARP, NDP

Domain Name System (DNS), Address Resolution Protocol (ARP), Neighbor Discovery Protocol (NDP)

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Request For Comments (RFC)

A publication from various standards-setting organizations, most notably the Internet Engineering Task Force (IETF). An RFC is submitted by an individual or a group, usually in the form of a memorandum describing protocols, procedures, research, et cetera that are relevant to the Internet and its supporting systems. While not all RFCs are standards (particularly the purely informational ones), RFCs serve as the official documentation for Internet Standards.

Typically, these publications are informational in nature, though RFCs have been written purely for comedic value.

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S

Segment

  1. The Layer 4 PDU. Also called a Datagram.
  2. A portion of a network.
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def_StatelessAddressAutoConfiguration

Stateless Address Auto-Configuration (SLAAC)

A protocol that allows network hosts to automatically learn the IPv6 prefix of the local link (i.e. 2001:db8: :/64 ), and then automatically generate an IPv6 address. The address will be generated either using EUI-64 or pseudo-randomly, depending on the device & manufacturer.

IPv6, SLAAC, EUI-64

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Static Routing

Manually configuring a network route on a router. As the route cannot change on its own it is ‘static’. This is as opposed to Dynamic Routing, where network routes are automatically created and updated by the routers of a network.

Static Routing, Static Routing, Dynamic Routing

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T

Transmission Control Protocol (TCP)

One of the main protocols of the Internet Protocol Suite. Compliments the Internet Protocol (IP) by providing reliable, ordered, and error-checked data streams between hosts.

IP

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TCP/IP

See IP);

I

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U

Unshielded Twisted Pair (UTP)

Broadly refers to cabling that uses pairs of copper conductors, twisted together to reduce electro-magnetic interference. Most commonly, this refers to typical RJ45-terminated network cabling, e.g. Cat-5, Cat-5e, Cat-6, etc.

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User Datagram Protocol (UDP)

One of the primary protocols that makes up the Internet Protocol Suite. UDP plays a similar role to TCP but lacks many of the latter’s features, such as sequencing and error correction. However, it is capable of more data throughput due to its lower operation overhead. It is commonly used for applications where transfer speeds are more important that accuracy or reliability, for example VoIP or live video broadcasting.

UDP

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V

Variable Length Subnet Masking (VLSM)

A system in which network prefixes have variable length. Typically, classful netmask for an IPv4 address would have a /8, /16, or /24 bit long prefix, depending on the class. In VLSM, a network prefix may be an arbitrary number between 1 and 32, depending on the number of host addresses needed for the subnet.

VLSM, and CIDR (which is based on VLSM) were created to slow the exhaustion of IPv4 addresses by reducing the allocating larger subnets than needed.

Subnetting

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Virtual Router Redundancy Protocol (VRRP)

Provides automatic assignment of available IP routers to participating hosts.

VRRP

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