an interior gateway protocol

Think of the Internet as a world of independent companies, each running its own internal mail system however it likes, plus a separate set of agreements for shipping mail between companies. An interior gateway protocol, or IGP, is the internal mail system: the routing protocol a single organization runs to compute paths among its own routers. It governs routing inside one administrative domain, not between domains.

More precisely, the Internet is divided into autonomous systems (ASes) — networks each under one organization's control, like a university, company, or ISP. Within an AS, routers run an IGP to discover and compute least-cost paths to every internal destination; RIP (distance-vector) and OSPF (link-state) are the classic IGPs, with IS-IS being another widely used link-state IGP. Because all routers in an AS are under one authority, an IGP can optimize purely for technical goals like lowest delay or highest bandwidth, trusting that every router cooperates.

This matters because it draws the central boundary of routing on the Internet. Inside your AS, an IGP picks efficient paths and converges quickly on failures. Between ASes, a different kind of protocol — an exterior gateway protocol, namely BGP — takes over, because there the concern is business policy and trust between strangers, not raw efficiency. Understanding 'this is intra-domain, that is inter-domain' is the key organizing idea of large-scale routing.

An honest caveat: 'interior' and 'exterior' describe scope, not a single algorithm. An IGP can be distance-vector or link-state; what unites IGPs is that they all run within one trust domain and optimize technical metrics. They are not designed to enforce inter-organization policy or scale to the entire Internet's routing table — that is deliberately left to BGP.