The protocols used in a computer network are the mechanisms by which the hosts exchange information over physical distances. In the OSI model, the network layer is the primary layer for host-to-host communication. The network layer makes use of services from the link and physical layers; it also provides an end-to-end service to the transport layer. The network layer can utilise a number of physical networks to provide its services. The two services provided by the network layer are the connectionless service, and the connection-orientated service. This chapter begins by looking at the connection orientated service provided by the X.25 protocol. This protocol is important as it is being used on all public packet-switched networks operated by the PTTs. The X.25 protocol defines the necessary operation of layers one, two and three.