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| 2.5 |  | Private Addressing and NAT |
| 2.5.2 |  | Discontiguous subnets |

Mixing private addresses with globally unique addresses can create discontiguous subnets. Discontiguous subnets 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 network, 207.21.24.0. They are discontiguous because the 10.0.0.4/30 network separates them. Classful routing protocols, notably RIP v1 and IGRP, cannot support discontiguous subnets because the subnet mask is not included in routing updates. If Site A and Site B are running RIP v1, Site A will receive updates about network 207.21.24.0/24 and not about 207.21.24.32/27. This is due to the fact that 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 the Site B route.

Even some classless routing protocols require additional configuration to solve the problem of discontiguous subnets. RIP v2 and EIGRP automatically summarize on classful boundaries unless explicitly told not to. Usually, this type of summarization is desirable. However, in the case of discontiguous subnets, the following command must be entered for both RIP v2 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, packets and routing updates should be filtered. This is done to avoid leaking any RFC 1918 addresses between autonomous systems. If both the LAN and the provider use addresses from the 192.168.0.0/16 block, the routers could get confused if confronted with updates from both systems.