Cisco Systems 78-11741-02 manual 1.127, IPC-44

Configuring IP Addressing

Configuring Network Address Translation

addresses from a rotary pool. Allocation is done on a round-robin basis, and only when a new connection is opened from the outside to the inside. Non-TCP traffic is passed untranslated (unless other translations are in effect). Figure 7 illustrates this feature.

Figure 7 NAT TCP Load Distribution

1

Inside

1.1.1.1

B	DA
	1.1.1.127

9.6.7.3

C

6.5.4.7

Intranet

5

SA 1.1.1.127

2

DA

1.1.1.1

3

4

SA

1.1.1.1

Real

hosts

1.1.1.2

1.1.1.3

Virtual

host

NAT table

Protocol	Inside Local IP	Inside Global IP	Outside Global
	address:port	address:port	IP address:port
TCP	1.1.1.1:23	1.1.1.127:23	9.6.7.5:3058
TCP	1.1.1.2:23	1.1.1.127:23	6.5.4.7:4371
TCP	1.1.1.3:23	1.1.1.127:23	9.6.7.3:3062

1.1.1.127

S4804

The router performs the following process when translating rotary addresses:

1.The user on host B (9.6.7.3) opens a connection to the virtual host at 1.1.1.127.

2.The router receives the connection request and creates a new translation, allocating the next real host (1.1.1.1) for the inside local IP address.

3.The router replaces the destination address with the selected real host address and forwards the packet.

4.Host 1.1.1.1 receives the packet and responds.

5.The router receives the packet, performs a NAT table lookup using the inside local address and port number, and the outside address and port number as the key. The router then translates the source address to the address of the virtual host and forwards the packet.

The next connection request will cause the router to allocate 1.1.1.2 for the inside local address.

To configure destination address rotary translation, use the following commands beginning in global configuration mode. These commands allow you to map one virtual host to many real hosts. Each new TCP session opened with the virtual host will be translated into a session with a different real host.

Cisco IOS IP Configuration Guide

IPC-44