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A routing domain is a term used to identify a lower level of a computer network hierarchy with respect to network traffic routing. All of the computers and routers contained under the domain must be administered by a single source, such as a company or organization, and adhere to a single routing protocol. Additional sub-networks may exist within a given domain in order to detail a network topology, so long as the subnets follow the same routing protocol. Furthermore, a particular domain may exist as part of a larger network.
The way a routing domain fits into the overall hierarchy depends somewhat on the construction of the network and the routing protocols used. It is often a subnetwork of what's known of as an administrative domain, which may have any number of routing domains within it. This way, two different routing domains may be operating under different routing protocols within a single administrative domain, but still be administered by a single source. Two or more administrative domains can also be connected in the case a third routing protocol need be implemented, but will be kept separate from the other two.
Stepping up in the hierarchy, an administrative domain may exist in what's known of as an autonomous system. An autonomous system can essentially be viewed as any collection of routing domains that have an established route to the Internet. In most cases, a single administrative domain and its routing domain will be an autonomous system, also sometimes termed a congruent domain. This is because in order for network traffic to get to another administrative domain, it usually has to traverse the Internet to reach the second administrative domain.
The way a routing domain works is through the use of an explicitly established routing protocol. Inside, there are any number of computers, referred to as end systems (ES). Connecting them into groups are typically routers, or other networking devices, that are termed intermediate systems (IS). These groupings, or subnets, are referred to as end system to intermediate system (ES-IS) protocols. The protocols that group up the intermediate systems which share the common routing protocol are referred to as an intra-domain intermediate system to intermediate system (IS-IS) protocol.
While the rules for routing domains specify that a single routing protocol be used throughout the domain, there are occasional exceptions. For example, a single ES can have a direct path to an IS. Technically, this can be viewed as another routing protocol, since the route is established between the ES and IS, though it doesn't interfere with the primary intra-domain IS-IS protocol. In general, an intra-domain IS-IS protocol is often referred to as an interior gateway protocol (IGP).
Since it is possible to have multiple routing domains within a single administrative domain, there are methods for interconnecting those instances where multiple administrative domains need to connect. In this case, a protocol different from the one used by the routing domain may be used to connect the two administrative domains. This is known of as an inter-domain IS-IS protocol. While two administrative domains managed by a single administrative source may exist, for the sake of security, they are typically connected via what's referred to as a border gateway protocol (BGP).