IGRP, or Interior Gateway Routing Protocol, is a distance-vector routing protocol developed by Cisco for use in larger networks. It helps routers communicate with each other to determine the best paths for data packets. IGRP uses metrics like bandwidth, delay, load, and reliability to calculate the most efficient route. IGRP is designed to support classful routing, meaning it does not send subnet mask information in its updates. This protocol has been largely replaced by EIGRP (Enhanced Interior Gateway Routing Protocol) and other more advanced protocols, but it laid the groundwork for modern routing techniques.