TCP/IP Part III
Bridges, routes and gateways
Bridge
Interconnects LAN segments at the network interface layer level and forwards frames between them.
A bridge performs the function of a MAC relay, and is independent of any higher layer protocol
(including the logical link protocol). It provides MAC layer protocol conversion, if required.
A bridge is said to be transparent to IP. That is, when an IP host sends an IP datagram to another host on
a network connected by a bridge, it sends the datagram directly to the host and the datafram "crosses" the
bridge whithout sending IP host being aware of it.
Router
Interconnects networks at the internetwork layer level and routes packets between them.
The router must understand the addressing structure associated whith the networking protocols it supports
and take decisions on whether, or how, to forward packets. Routers are able to select the best transmission
paths and optimal packet sizes. The basic routing function is implemented in the IP layer of the TCP/IP
protocol stack, so any host or workstation running TCP/IP over more than one interface could, in theory and
also with most of today's TCP/IP implementations, forward IP datagrams. However, dedicated routers provide
much more sophisticated routing than the minimum functions implemented by IP.
Because IP provides this basic routing function, the term "IP router" is often used. Other, older terms for
router are "IP gateway" "Internet gateway" and "gateway." The term gateway is now normally used for connections
at a higher layer than the internetwork layer.
A router is said to be visible to IP. That is, when a host sends an IP datagram to another host on a network
connected by a router, it sends the datagram to the router so that it can forward it to the target host.
Gateway
Interconnects networks at higher layers than bridges and routers. A gateway usually supports address mapping
from one network to another, and might also provide transformation of the data between the environments to
support end-to-end application connectivity. Gateways typically limit the interconnectivity of two networks
to a subset of the application protocols supported on either one.
A gateway is said to be opaque to IP. That is, a host cannot send an IP datagram through a gateway, it can
only send it to a gateway. The higher level protocol information carried by the datagrams is then passed on
by the gateway using whatever networking architecture is used on the other side of the gateway.