NAT means Network Address Translation and is another option you may choose.

It sounds complicated but its just a way of giving devices in your home their own addresses so they can all access the internet.

If every single device in the world had its own address, there wouldn't be enough numbers to go round.

The advantage of NAT is that the extension numbers are harder for hackers to find and so can be more secure. The disadvantage is that some software has problems dealing with them.

Unless you have a good reason to use one of the other, just use the default settings that come with your broadband service.

SOURCE  http://www.bbc.co.uk