Overview
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 hosts to access external networks and external hosts to access private network resources such as a Web server.
Figure 49: NAT operation
As shown in Figure 49:
Upon receiving a request from the host to the server, NAT translates the private source address 192.168.1.3 to the public address 20.1.1.1 and forwards the NATed packet. NAT adds a mapping for the two addresses to its NAT table.
Upon receiving a response from the server, NAT translates the destination public address to the private address, and forwards the packet to the host.
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 host is 20.1.1.1.