Chapter 10 Network Address Translation (NAT)

10.6.3 How NAT Works

Each packet has two addresses – a source address and a destination address. For outgoing packets, the inside local address is the source address on the LAN, and the inside public address is the source address on the WAN. For incoming packets, the inside local address is the destination address on the LAN, and the inside public address is the destination address on the WAN. NAT maps private (local) IP addresses to globally unique ones required for communication with hosts on other networks. It replaces the original IP source address (and TCP or UDP source port numbers for Many-to-One and Many-to-Many Overload NAT mapping) in each packet and then forwards it to the Internet. The P-660RU-Tx keeps track of the original addresses and port numbers so incoming reply packets can have their original values restored. The following figure illustrates this.

Figure 39 How NAT Works

NAT Table

LAN

 

 

 

 

 

 

 

 

 

 

 

 

Inside Local

 

Inside Public

 

 

 

 

 

 

 

IP Address

 

IP Address

192.168.1.13

192.168.1.10

 

1

 

192.168.1.11

 

2

 

 

 

 

192.168.1.12

 

3

 

 

 

 

192.168.1.13

 

4

 

192.168.1.12

 

 

 

 

 

 

 

 

 

 

SA

 

 

 

 

 

SA

 

 

 

 

 

 

 

 

 

 

 

 

 

192.168.1.10

 

 

 

 

 

1

 

WAN

 

 

 

 

Inside Local

Inside

Public

Address

Address

192.168.1.11 192.168.1.10

 

99

P-660RU-Tx User’s Guide