RIP Routing Information Protocol
The first widely used routing protocol on the internet. Distance vector with hop count as its only metric, and a maximum distance of 15 hops — anything beyond counts as unreachable.
It's built on Bellman-Ford: every router sends its entire routing table to its neighbours every 30 seconds, and the neighbours add one hop. Simple to implement, but slow to converge and prone to "count to infinity", where two routers take turns raising the cost by one until the limit is hit. The countermeasures are split horizon, route poisoning and hold-down timers. The metric ignores bandwidth, so a 10-gigabit link two hops away loses to an ancient serial line one hop away. RIPv2 (RFC 2453) added classless addressing and authentication, RIPng covers IPv6. Today it survives mainly in lab setups and textbooks — OSPF won.