When the networks that must be connected are using completely different protocols from each other, a powerful and intelligent device called as gateway is used. A gateway is a device that can interpret and translate the different protocols that are used on two distinct networks.
Gateway comprise of software, dedicated hardware or a combination of both. Gateway operate through all the seven layers of the OSI Model.
A gateway can actually convert data so that it works with an application on a computer on the other side of the gateway. For example a gateway can receive e-mail message in one format and convert them into another format.
Gateway can connect systems with different communication protocols, languages and architecture. Fir example IBM networks using System Network Architecture (SNA) can be connected to LANs using a gateway.