A network gateway is an internet working system capable of joining together two networks that use different base protocols. A network gateway can be implemented completely in software, completely in hardware, or as a combination of both. The key feature of a gateway is that it converts the format of the data, not the data itself.
Gateway functionality differs in many ways. For example, a router that can route data from an IPX network to an IP network is, technically, a gateway. The same can be said of a translational bridge that, converts from an Ethernet network to a Token Ring network and back again.
Software gateways can be found everywhere. Many companies use an email system such as Microsoft Exchange. These systems transmit mail internally in a certain format. When email needs to be sent across the Internet to users using a different email system, the email must be converted to another format, this conversion process is performed by a software gateway. In enterprises, the gateway often acts as a proxy server and a firewall. Gateways also associated with router-id switch.