Gateways are application-specific interfaces that link all seven layers of the OSI model when they are dissimilar at any or all levels. For instance, if you need to connect a network that uses one of the OSI networking models to one using IBM’s Systems Network Architecture (SNA) model, use a gateway. Gateways can also translate from Ethernet to Token Ring, although simpler solutions than gateways exist if you need such a translation. Because gateways must translate so much, they tend to be slower than other solutions, particularly under heavy loads.

The primary use for gateways today is for handling e-mail. POP3 and SMTP are two examples of protocols that are handled by gateways. Most e-mail systems that can connect to disparate systems either use a computer set up as a gateway for that chore or let the e-mail server handle the gateway chores itself.