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IP Routing
There are two levels of complexity involved in IP routing:
- Local delivery
- when the IP software is presented with a datagram for delivery,
it first checks the network part of the destination address to see
if matches its own network number. If it does, then the datagram
can be delivered locally, and is simply handed to the physical
network delivery system (typically an Ethernet device driver) for
direct delivery over the local network.
- Internet delivery
- if the network numbers are different, then the datagram must be
delivered to a local router (or IP gateway), using direct delivery
as above, for "on-delivery" across the Internet.
- Datagrams are forwarded from router to router across each of
the intervening networks until they reach a router connected to a
network where they can be locally delivered.
- The IP software in each router must keep a routing
table to know the IP address of an appropriate next-hop
router which brings the datagram closer to its destination. For
hosts, this is commonly done by specifying a default
route.
- Note that routers must have (at least) two IP addresses!
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