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

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Discovering computer networks: hands on in the Open Networking Lab

5.6 Implications of NAT

In this part you will consider the implications of NAT, in particular how the combination of NAT and private addresses has enabled home networking to become so common.

Now watch the video below, which is about 2 minutes long.

Implications of NAT

Download this video clip.Video player: 33_implications_of_nat.mp4
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So just to take stock. We’ve been talking about NAT, network address translation. This is done systematically so that devices with private IP addresses in a local network can communicate with the internet.

Private addresses are changed at the gateway router for the router’s own address, and then passed on to the internet; replies come back to the router, which changes the address back again so it can be delivered to the correct destination inside the private network. Port numbers are used to keep traffic from different hosts separate.

So NAT allows you set up a private network and still allow devices in that private network to have access to the internet through the gateway router.

On domestic gateways, NAT is all automated and requires no configuration. (On a larger managed network in a commercial or corporate setting, some configuration may be needed).

This combination of NAT and private addresses has allowed the problem of a lack of global IPv4 addresses to be sidestepped. Private addresses can be reused in LANs, millions of times across the globe; each device doesn’t need a unique IP address. But with private addresses alone, we wouldn’t be able to communicate with the internet; and it’s NAT that makes this possible.

The combination of DHCP, private addresses and NAT also allows home networks to be largely self-configuring. Without them, every device on the internet would require manual setup – that was fine in the very early days of the internet, but completely impossible at the scale of the internet nowadays. So without NAT, internet use from home could never have happened.

End transcript
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Network address translation (NAT) is the systematic replacement of IP addresses and port numbers by the router. This can be done to allow devices with IP addresses in a private address range to communicate with the internet. A translation is made as traffic leaves the router and reversed when replies reach the router.

Private addresses are ideal for home networks because devices can be added to the network without needing to obtain new public IP addresses from a registration authority. However, private addresses were intended for use within local networks only. NAT in the gateway router makes it possible for devices with private addresses to communicate with the internet. All devices in the private network effectively share a single address on the internet which reduces the demand for IPv4 addresses.

The combination of private addresses, DHCP and NAT makes it possible for small home networks to be set up easily, without having to allocate public IP addresses. This combination also underpins the rapid rise of home networks and the very large scale of the internet.

Activity 8 Test yourself

5 minutes

Identify the one correct statement in the list below.


NAT never changes private addresses.


NAT and private address ranges mean that not all devices need to have unique IP addresses to use the internet.


You need permission from a registration authority to set up NAT.


NAT is only applied to packets as they leave a LAN for the internet.

The correct answer is b.


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