Cisco Systems 78-11741-02 manual Translating Overlapping Addresses, IPC-41

Configuring IP Addressing

Configuring Network Address Translation

	Command			Purpose
Step 3
	Router(config)# ip	nat inside source list		Establishes dynamic source translation, specifying
	access-list-number	pool name overload		the access list defined in the prior step.
Step 4
	Router(config)# interface		type number	Specifies the inside interface.
Step 5
	Router(config-if)#	ip nat	inside	Marks the interface as connected to the inside.
Step 6
	Router(config)# interface		type number	Specifies the outside interface.
Step 7
	Router(config-if)#	ip nat	outside	Marks the interface as connected to the outside.

Note The access list must permit only those addresses that are to be translated. (Remember that there is an implicit “deny all” at the end of each access list.) An access list that is too permissive can lead to unpredictable results.

Packets that enter the router through the inside interface and packets sourced from the router are checked against the access list for possible NAT candidates. The access list is used to specify which traffic is to be translated.

See the “Overloading Inside Global Addresses Example” section at the end of this chapter for an example of overloading inside global addresses.

Translating Overlapping Addresses

The NAT overview discusses translating IP addresses, which could occur because your IP addresses are not legal, officially assigned IP addresses. Perhaps you chose IP addresses that officially belong to another network. The case of an address used both illegally and legally is called overlapping. You can use NAT to translate inside addresses that overlap with outside addresses. Use this feature if your IP addresses in the stub network are legitimate IP addresses belonging to another network, and you want to communicate with those hosts or routers.

Figure 6 shows how NAT translates overlapping networks.

Cisco IOS IP Configuration Guide

IPC-41