In this section I provide a description of the most common routing (or gateway) protocols used in TCP/IP. I begin with an overview of various concepts that are important to know in order to understand how routing protocols work. I then describe the TCP/IP routing protocols themselves in two subsections. The first covers interior routing protocols, which are used between routers in an autonomous system, and the second looks at exterior routing protocols, used between autonomous systems.

An Interior Gateway Protocol (IGP) calculates routes within a single AS. The IGP enables nodes on different networks within an AS to send data to one another. The IGP also enables data to be forwarded across an AS from ingress to egress, when the AS is providing transit services.