To connect totally dissimilar network, a device called gateways is used. Gateways perform protocol conversion for all seven layers of OSI model (operating system interface). A common use for a gateway is to connect a LAN and a SNA main frame computer, changing protocol and transmitting packets between two entirely different systems.
Gateways handle massages, addresses and protocol conversion necessary to deliver a massage from one network to another. Gateways offer the greatest flexibility in internetworking communication.
A gateway can receive E-mail massage in one format and convert them into another format.
Conversion between routers and gateways
Routers and gateways can menu plate the packet being transmitted. In the case of a router, the manipulation may be simply some determined of where the packet comes from and where the packet is to go. In a gateway, the decision may be a little more function. It not only performs routers function but also convert the massage from one packet format to one another.
A gateway is slower that a router.