In larger, more complex computer networks, data must be directed more
carefully. In almost all cases, large networks are actually composed of a
collection of LANs that are interconnected or “internetworked”. This is where
routers come in.
Routers take network data messages from a LAN and convert them into packets
suitable for transmission beyond the LAN on a wide area network (WAN). The goal
is almost always to get these packets to another LAN and ultimately to the
correct host on that LAN. Part of the “conversion” process is to add a packet
header. Other routers will generally only look at a packet’s header information,
not at the contents or data in the packet.
Routers also make decisions about where to send these packets, based on: the
addresses contained within the packet headers and a table of routes maintained
within the router. Updating these routing tables and forwarding data packets
between portions of a network are one of the primary purposes of a router.
Building packets and unwrapping packets are additional router functions
performed by the first and last routers, respectively, that a message passes
through.
In addition to directing packets, a router may be responsible for filtering
traffic, allowing some packets to pass through and rejecting others. Filtering
can be a very important function of routers; it allows them to help protect
computers and other network components. It is also possible that at the
destination end a router may have to break large packets up to accommodate the
size limits of the destination LAN.
There is no reason that routers cannot be used to send messages between hosts
but more typically routers are used to connect LANs to each other or to connect
a LAN to a WAN. Most large computer networks use the TCP/IP protocol suite. In
some sense this is the lingua franca of the Internet.
As mentioned, one of tasks of a router is to maintain routing tables which
are used to decide where a packet is to go and thus which interface it should be
sent out. In the past these tables were built and updated by hand and this is
referred to as static routing. In dynamic routing, the router learns about where
various addresses are relative to itself and builds up routing tables based on
this information.
