168.100.0. The client
is on the network 192.168.10.0. In order for the packet to get to the server, it needs to go
to Router A on its 192.168.10.1/24 interface. It then sends the packet to Router B through
its 192.168.15.1 interface. Router B then receives that data on its 192.168.15.5 interface and
then forwards it over to Server A through its 192.168.100.1 interface.
That all seems logical enough; but the real question is how did Router A know to
send the packet over to Router B and how did Client A know to send the packet to Router
A in the first place? The answer is routing tables, the rules that dictate how packets should
be sent based on the destination address. Workstations also have routing tables but are
more generally defined. Workstations are usually configured to send any traffic not destined
to the local subnet to its default gateway. If you look at Figure 11-1 again, that??™s
typically how Client A would know to send packets outside its segment to the default
gateway, which we will assume is configured to Router A??™s 192.168.10.1 interface.
When Router A receives the packet destined for 192.168.100.10/24, it determines
that the destination is not a network that is on either of its interfaces (192.168.10.1 or
192.168.15.1). There are essentially three ways Router A can be configured to send the
packet over to Router B. The first way is a default route, which is really the same as a
default gateway.
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