End Systems (ES) and Intermediate Systems (IS)

A communications network is a collection of systems (nodes) which transfer information between users attached to the networks. Two types of systems are defined within a communications network. These are:

• End Systems. These are equipment (usually computers) which users (either human or processes executing on a computer) may use to access the information at a remote site or equipment which provides "services" at a remote site which may be accessed via the network.
• Intermediate Systems. These are network equipment (e.g. routers) that do not directly support users, but forward received data onward towards the intended recipient. Intermediate systems do not need to understand the information being sent between the users, but do understand and may modify the information added by the network to provide the communication.

End Systems may be directly connected, but more normally rely on the service provided by one or more Intermediate Systems. Examples of intermediate systems are bridges or LAN switches (which operate at the data link layer) and router or network switches (which operate at the network layer).

The communications process between end systems and intermediate systems is usually defined in terms of a reference stack. Intermediate systems handle only protocol information at and below the network layer, whereas end systems use protocols at all the layers of the reference model.

Two End Systems connected by an Intermediate System (in this case a router) The figure shows various protocol layers