Identifiers assigned to routers in order to limit traffic to routers within a specified area. These resemble IP addresses. For instance, a common practice is to give each router belonging to one group the ID 0.0.0.0.
Autonomous System (AS)
One or more networks that are governed by a single protocol within-do not use IP addresses, but use a special globally unique mu,bers assigned by the IANA.
backup designated router (BDR)
A router within an autonomous system (AS) that is assigned the role of backup router to the AS's designated router. This takes over whenever the DR is not available.
Border Gateway Protocol (BGP-4)
The current version of the external routing protocol used on the Internet. Used for communications between Autonomous Systems
A state in which all routers within an area are up-to-date, and their routing tables all match.
designated router (DR)
A router that has the function of relaying information to all the other routers in the area.
A type of Network Address Translation (NAT) in which the destination IP addresses is translated by the NAT-capable router.
A fixed predetermined number assigned to a packet by the sending computer; it defines the function or session type. Commonly in the range of 0 to 1023.
A routing protocol that calculates the total cost to get to a particular network ID and compares that cost to the total cost of all the other routes to get to that same network ID. The router then chooses the route with the lowest cost.
A type of network address translation (NAT) in which many computers can share a pool of routable (non-public) IP addresses that number fewer than the computers.
A process by which routers in an internetwork automatically exchange information with all other routers, enabling them to build their own list of routers to various networks.
In TCP/Ip communication, an arbitrary number generated by sending a computer that the receiving computer uses as a destination address when sending a return packet.
A router that acts as the default gateway for a number of client computers.
One trip through a router by a packet.
Intermediate System to Intermediate System (IS-IS)
A link-state dynamic routing protocol (one that announces and forwards individual route changes as they appear), using the concept of areas. Its major advantage over OSPF is that it has supported IPv6 from the start. It is not nearly as popular as OSPF.
A dynamic routing protocol that is more efficient than RIP, announcing and forwarding individual route changes as they appear, rather than sending the entire routing table. There are two of these dynamic routing protocols: OSPF and IS-IS.
A relative value that defines the "cost" of using a route.
NAT translation table
The table used by a router in conjunction with Network Address Translation (NAT) to track the IP address of a computer on a private network using a source ephemeral port so that it can direct the appropriate incoming traffic to that computer.
Network Address Translation (NAT)
A means of translating a systems IP address into another IP address before sending it out to a larger network, in order to hide the IP addresses on a private network.
Open Shortest Path First (OSPF)
Uses a link-state algorithm and shares more detailed information - called link-state advertisements.
In TCP/IP, a 16-bit number, between 0 and 65,535, assigned to a particular TCP/IP session. All TCP/IP packets (except for some really low-level maintenance packets) contain these that two communicating computers use to determine the kind of session to use, and how to get the packet or response back to the sending computer.
Port Address Translation (PAT)
A term used by manufactures to refer to either overloaded NAT or port forwarding.
A NAT function that hides a port number from outside networks, enabling public servers to work behind a NAT router. It gives a server on a private network the protection of NAT, while allowing access to that server from outside the private network.
A distance-vector routing protocol, dating from the 1980s, that has a maximum hop count of 15. These routers sent out updates every 30 seconds, causing huge network overloads. It was also limited to only working with Classful address, not recognizing CIDR subnets. Plus, it had no authorization mechanism, leaving these routers open to hackers sending false routing table information.
The current version of this distance vector routing protocol that was adopted in 1994. It fixes many of the problems with earlier versions. It has support for CIDR addressing, updates at random intervals, and has a built-in authentication protocol. It still has the 15-hop limit and time-to-convergence problems on large WANs, so it is considered obsolete for all but small, private WANs.
A device connecting separate networks, which forwards a packet from one network to another based only on the network address of the protocol being used.
Routing Information Protocol (RIP)
A distance-vector routing protocol that, to date, has had two version. Both versions have a maximum hop count of 15.
A table used by a router to determine where to send each packet, based on its destination network ID address.
A type of Network Address Translation (NAT) in which the source IP addresses are translated by the NAT-capable router.
Static NAT (SNAT)
A type of network address translation (NAT) in which the router maps a single routable (non-private) IP address to a single machine, enabling you to access that machine from outside the network.
Network routes that are entered into a router's routing table manually.
A special serial cable used to connect to a Cisco-brand router in order to perform the initial configuration on the router.