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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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5.4 NAT in detail

In this part you will examine the IP packets in detail to see network address translation happening at the router.

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

NAT in detail

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Network address translation happens in the router. The router inspects each outgoing IP packet: if the source address is a private address, the router will replace this with its own public IP address. It will also record which translations it has made. Returning traffic will arrive with the router’s own IP address as the apparent destination, but the router will inspect each packet and replace the destination address, using the information it stored earlier to reverse the translation back to a private address. The packet can then be switched over the LAN in the normal way.

Activity 6 Test yourself

5 minutes

Identify the one correct statement in the list below.


A router will not look at a packet containing a private address; it will just pass it on unchanged.


NAT changes private addresses in IP packets to a random IP address.


NAT changes private addresses in IP packets to the router’s own public address.


Computers and other devices in a private network always substitute the router’s address for their own before they send IP packets over the LAN.

The correct answer is c.