A device that interconnects two networks, and whose presence is usually visible to network users (as distinct from a bridge, whose presence is generally not visible). A gateway may be required to deal with one or more of the following differences between networks it connects:

1. (a) change of addressing domain—where the networks have addressing domains managed by separate groups, a gateway may be used to handle address transformations for messages traversing the gateway;

2. (b) control of charging—where the networks have different approaches to charging (e.g. a local area network that imposes no charges connecting to a wide area network that charges on a per-packet basis), a gateway may be used to handle user authorization and usage accounting;

3. (c) change of protocol—where the networks use different protocols, a gateway may be used to carry out necessary protocol conversion (if practicable) or to intercept attempts by a user on one network to use functions not available on the other and to supply suitable responses.

The terms bridge, gateway, and relay are among those whose meanings vary between different communities of users at a given time, and within a given community of users at different times.