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Mixing private addresses with globally
unique addresses can create discontiguous subnets, which are subnets from the same major network that are separated
by a completely different major network or subnet.
In the figure, Site A and Site B both have LANs that are addressed using
subnets from
the same major net (207.21.24.0). They are discontiguous because the
10.0.0.4/30
network separates them. Classful routing protocols, notably RIPv1 and IGRP,
cannot
support discontiguous subnets because the subnet mask is not included in
routing updates. If Site A and Site B are running RIPv1, Site A will receive
updates about network 207.21.24.0/24 and not about 207.21.24.32/27 because
the subnet mask is not included in the update. Because Site A has an
interface directly
connected to that network (in this case, E0), Site A will reject Site B's
route.
Even some classless routing protocols require additional configuration to
solve the
problem of discontiguous subnets. RIPv2 and EIGRP automatically summarize
on
classful boundaries unless explicitly told not to. Usually, this type of
summarization is
desirable, but in the case of discontiguous subnets, the following command
must be
entered for both RIPv2 and EIGRP to disable automatic summarization:
Router(config-router)#no auto-summary
Finally, when using private addresses on a network that is connected to
the Internet, you
should filter packets and routing updates to avoid "leaking" any
RFC 1918 addresses
between autonomous systems. For example, if both you and your provider use
addresses
from the 192.168.0.0 /16 block, your routers could get confused if
confronted with
updates from both systems.
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