HP MSR2000/3000/4000 Router Series Layer 3 - IP Services Configuration Guide

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Configuring NAT
Network Address Translation (NAT) translates an IP address in the IP packet header to another IP address.
Typically, NAT is configured on gateways to enable private users to access an external network and to
enable external users to access private network resources such as a Web server.
Figure 48 sh
ows how NAT works.
Figure 48 NAT operation
1. The internal host at 192.168.1.3 sends an IP packet to the external server at 1.1.1.2.
2. Upon receiving the packet, the NAT device then translates the private address 192.168.1.3 to the
public address 20.1.1.1 and forwards the packet to the server on the external network.
Meanwhile, the NAT device adds the mapping of the two addresses to its NAT table.
3. The external server receives the packet and responds.
4. The NAT device receives the reply and performs a NAT table lookup by using the source IP
address as the key. The device then translates the destination to the address of the internal host and
forwards the packet.
The NAT operation is transparent to the terminals. NAT hides the private network from the external users
and shows that the IP address of the internal PC is 20.1.1.1.
Terminology
NAT device
A device where NAT is configured. Typically, a gateway functions as a NAT device.
NAT interface
An interface with NAT enabled on a NAT device.
192.168.1.3
Src : 192.168.1.3
Dst : 1.1.1.2
Src : 20.1.1.1
Dst : 1.1.1.2
192.168.1.1 20.1.1.1
Src : 1.1.1.2
Dst : 20.1.1.1
Src : 1.1.1.2
Dst : 192.168.1.3
1.1.1.2
Server
Host
NAT
Intranet
Internet
Before NAT
192.168.1.3
After NAT
20.1.1.1
Direction
Outbound
20.1.1.1 192.168.1.3Inbound