RIP (Routing Information Protocol)

Routing protocol is used by one routing device to advertise to other routing devices networks attached to its ports. Hmm... actually all routing protocols are about that.

At the time when RIP was created idea was that each routing device had multiple ports and each of those ports represents different network. A table of local networks was created in memory of those early routers. That information needed to be forwarded to other similar routing devices. That way systems connected to the other routers networks were able to send data to each other. They (withe-coats of that age) addressed that information exchange in simplest possible way. It just broadcasts information about networks it knows to all its interfaces for others to listen. This simple idea was getting more and more complicated as networks grew. Then some extras have started to be applied to coupe with challenges. All that can be read here.