When large enterprise and regional networks use centralized routing protocols to manage their routers, an intradomain routing policy is followed. In instances where multiple routing protocols are being used to cross-pollinate routing information, an interdomain routing policy is followed because more than one routing domain is being used to manage the route discovery across the entire network.
The distinctions between intradomain and interdomain make more sense when you examine how large and extremely large IP networks are structured. Intradomain routing is generally used in intranetworks. Most company-wide networks can be considered intranetworks. An intranetwork can be a single LAN or a collection of LANs connected by private data links. What defines an intranetwork is its usage. Intranets have an exclusive set of users, and access is restricted only to that user community. Most intranets pursue an intradomain routing policy. The term "intranetwork" is relatively new. The distinction arose as more and more office LANs and company-wide private data networks began installing access points to the global Internet.
Intranetworks use Regional Internet Service Providers (RISPs) and/or National Internet Service Providers (NISPs) to gain access to the Internet. RISPs and NISPs operate internetworks, which are networks made up of multiple distinct networks that operate as separate routing domains. These networks connect to a common internetwork backbone that exists as a routing domain apart from the distinct connected networks.
This tip is excerpted from Understanding the Network, a Practical Guide to Internetworking, by Michael J. Martin, published by New Riders.