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Discovering computer networks: hands on in the Open Networking Lab
Discovering computer networks: hands on in the Open Networking Lab

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5.3 Changing addresses

In this part you will see how the gateway router’s external public IP address can be substituted for the private IP address of traffic leaving a private network. This translation of addresses is at the heart of network address translation (NAT).

Now watch the video below, which is about 3 minutes long.

Changing addresses

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The IP address on the external network interface of the gateway router is able to communicate with the internet. The gateway router will replace private source addresses with its own public address as it forwards traffic from the private LAN to the internet. The traffic now appears to come from the gateway itself and does not contain a private address; it can therefore be routed successfully over the internet. When traffic returns to the gateway router, the gateway will reverse the swap, replacing its own address with the private destination address, and then switch the packets on the LAN.

This is network address translation (NAT). NAT allows devices with addresses in the private range to communicate with the internet. It also means that all devices on the private LAN effectively share a single IP address to connect to the internet.

Activity 5 Test yourself

5 minutes

Identify the one correct statement in the list below.

a. 

NAT is an acronym for network address transmission.


b. 

A typical home gateway router can carry out network address translation (NAT).


c. 

If you want to use NAT on a typical home network, you will need to buy an additional computer to carry out network address translation.


The correct answer is b.