Upper-Layer Devices
There are only a few upper-layer devices, none of which operate at any specific layer. Because they perform a range of functions for the network, they fall into the class of devices known as
gateways. A gateway translates one type of network data into another. Gateways can be either hardware or software, but the most popular way to run a gateway is as a software program on
a dedicated computer.
There are many, many types of gateways, but the one most people think of is an e-mail gateway. E-mail gateways translate e-mail messages from one type of e-mail system so that they can
be transmitted on another (for example, from GroupWise e-mail to SMTP mail for the Internet).
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