Tài liệu Connecting Networks Together

Router
Figure 5.9 is a functional diagram of a router. A database of routes is stored and
maintained by all routers. Called a routing table, it contains information concerning
routes between the node owning the table and the potential destination nodes. At a
minimum it includes the destination ID, intermediate interface ID(s) and forwarding
address(es), and information to distinguish the best route to use when multiple
routes are possible. It is significantly more complex than the table maintained by
bridging devices. However, its extent is limited to the immediately reachable nodes
that surround it, so that it is significantly smaller. Searching a routing table is a relatively
simple task. For each route, a typical routing table will include the following
fields:
ã Destination address: The IP address of the node to which the source directs the
packet to be delivered. For direct deliveries, the destination IP address carries
the same network ID as the router. For indirect deliveries, the destination
address does not carry the same network ID as the router, and the datagram is
sent to the forwarding address contained in the table entry.
ã Network mask: A bit mask is used to determine the network ID of the destination
IP address. An IP datagram with a destination IP address that contains the
specific network ID for this route will be forwarded over it.
ã Forwarding IP address: For indirect deliveries, the IP address of a directly
reachable router to which the IP datagram is forwarded for eventual delivery
to the destination IP address. The IP address to which the IP datagram is to be
forwarded on its next hop