Gateways interconnect networks at higher layers than bridges and routers.
A gateway usually supports address mapping from one network to another, and may also provide transformation of the data between the environments to support end-to-end application connectivity.

Gateways typically limit the interconnectivity of two networks to a subset of the application protocols supported on either one. For example, a VM host running TCP/IP may be used as an SMTP/RSCS mail gateway.

The term "gateway," when used in this sense, is not synonymous with "IP gateway."

Note: A gateway is said to be opaque to IP. That is, a host cannot send an IP datagram through a gateway; it can only send it to a gateway. The higher-level protocol information carried by the datagrams is then passed on by the gateway using whatever networking architecture is used on the other side of the gateway.

Closely related to routers and gateways is the concept of a firewall, or firewall gateway, which is used to restrict access from the Internet or some untrusted network to a network or group of networks controlled by an organisation for security reasons.