Cisco Systems 78-11741-02 manual IPC-42, NAT Translating Overlapping Addresses

Configuring IP Addressing

Configuring Network Address Translation

Figure 6 NAT Translating Overlapping Addresses

1.1.1.1

DNS request for host C address

SA=1.1.1.1 DA=x.x.x.x

DNS response from x.x.x.x

SA=x.x.x.x DA=1.1.1.1 C=3.3.3.3

1.1.1.1message to host C SA=1.1.1.1 DA=3.3.3.3

DNS request for host C address

SA=2.2.2.2 DA=x.x.x.x

DNS server

x.x.x.x

Internet

Host C 1.1.1.3

DNS response from x.x.x.x

SA=x.x.x.x DA=2.2.2.2 C=1.1.1.3

1.1.1.1 message to host C

SA=2.2.2.2 DA=1.1.1.3

	NAT table
Inside Local	Inside Global	Outside Global	Outside Local
IP Address	IP Address	IP Address	IP Address
				S4792
1.1.1.1	2.2.2.2	1.1.1.3	3.3.3.3

The router performs the following process when translating overlapping addresses:

1.The user at host 1.1.1.1 opens a connection to host C by name, requesting a name-to-address lookup from a DNS server.

2.The router intercepts the DNS reply and translates the returned address if there is an overlap (that is, the resulting legal address resides illegally in the inside network). To translate the return address, the router creates a simple translation entry mapping the overlapping address 1.1.1.3 to an address from a separately configured, outside local address pool.

The router examines every DNS reply from everywhere, ensuring that the IP address is not in the stub network. If it is, the router translates the address.

3.Host 1.1.1.1 opens a connection to 3.3.3.3.

4.The router sets up translations mapping inside local and global addresses to each other, and outside global and local addresses to each other.

5.The router replaces the SA with the inside global address and replaces the DA with the outside global address.

6.Host C receives the packet and continues the conversation.

7.The router does a lookup, replaces the DA with the inside local address, and replaces the SA with the outside local address.

8.Host 1.1.1.1 receives the packet and the conversation continues, using this translation process.

Cisco IOS IP Configuration Guide

IPC-42