Gateways are devices that act as translators between systems that
do not use the same communication protocols, data formatting,
structures, languages, or architectures. Gateways repackage the
data packets and change the syntax to match that of the destination
system. When networks are divided into subnets, gateways are
required to connect one subnet to another.
The Default Gateway is the address of the gateway or router that
you use to move packets between subnets. If multiple gateways or
routers exist, then the default gateway is the address of the first
gateway or router that you use to move packets between subnets.
TCP/IP Overview 91