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Just a quick, somewhat newbie question...

I noticed that my home router has the option to disable NAT. How will the router behave with NAT turned off? Will it simply broadcast all incoming traffic to the subnet?

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migrated from serverfault.com Jan 4 '10 at 4:04

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3 Answers 3

With NAT enabled, the router will modify the IP header in outgoing packets so that the source address matches your Internet public address (and vice versa for incoming packets).

If you disable NAT, it won't do that anymore. So, basically, you will be sending IP packets with private source IP address (e.g. 192.168.x.y) on the Internet, which of course will automatically get rejected by your ISP.

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Right, forgot it also did outbound translation. Thanks, I know what I need to do in my setup now. –  BSmith Jan 2 '10 at 22:00
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Private addresses don't get automatically rejected by all ISPs because they may be using them themselves internally. What will almost definitely happen is that the internet at large won't have any routes back to the private addresses, so even if packets get out, the replies will never get back to you. Functionally the same result, just semantically different. –  David Mackintosh Jan 3 '10 at 3:25

NAT is what lets you use internal-only IP subnets, e.g., 192.168.0.*. While routing, the router will substitute its address for the internal-only address. Unless you have a subnet of real IP addresses available to you, don't turn off NAT.

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If you turn off NAT it will also turn off DHCP allocation, the router will expect the upstream to be providing IP addresses. In most cases this will only work for one IP address, although a lot of ISPs let you buy more IP addresses off them. Turning it off is only useful in that situation, or in the case you have another router downstream which will allocate DHCP addresses to your wider network. Or if you just own one computer.

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