Routing is the process, performed at each site of a communication network upon receipt of a message, that decides to which neighbor the message is to be sent or that the message is to be received locally. We present a general model of routing based on a decision tree representation of routing information. We describe several classes of optimally routable networks, i.e., networks for which the maximum size of a routing tree over all sites of the network is ⌈log2 n⌉.