Cisco Systems 78-11741-02 manual Overloading an Inside Global Address, IPC-39

Configuring IP Addressing

Configuring Network Address Translation

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.

Configuring Dynamic Translation with a Route Map

To configure dynamic inside source address translation with a route map, use the following commands in global configuration mode:

	Command			Purpose
Step 1
	Router(config)# ip	nat pool name start-ip end-ip		Defines a pool of global addresses to be allocated as
	{netmask netmask	prefix-length prefix-length}		needed.
Step 2
	Router(config)# route-map		name permit sequence	Defines a route map permitting those addresses that
				are to be translated.
Step 3
	Router(config)# ip	nat inside source route-map name		Establishes dynamic source translation, specifying
	pool name			the route map defined in the prior step.
Step 4
	Router(config)# interface		type number	Specifies the inside interface and enters interface
				configuration mode.
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 and enters interface
				configuration mode.
Step 7
	Router(config-if)#	ip nat	outside	Marks the interface as connected to the outside.

See the “Dynamic Inside Source Translation Example” section at the end of this chapter for examples of dynamic inside source translation.

Overloading an Inside Global Address

You can conserve addresses in the inside global address pool by allowing the router to use one global address for many local addresses. When this overloading is configured, the router maintains enough information from higher-level protocols (for example, TCP or UDP port numbers) to translate the global address back to the correct local address. When multiple local addresses map to one global address, the TCP or UDP port numbers of each inside host distinguish between the local addresses.

Figure 5 illustrates NAT operation when one inside global address represents multiple inside local addresses. The TCP port numbers act as differentiators.

Cisco IOS IP Configuration Guide

IPC-39