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I have a question about DHCP and NAT.

I know NAT translates local IP addresses and ports into a single IP address and specific ports that can be used to communicate with the Internet.

I also know DHCP is used to assign or re-use an unused IP address to a computer that connected to the network.

How does NAT obtain its single IP address? Does it use DHCP?

  • I'm curious as to why people are voting to close this question ? – davidgo Jan 5 '16 at 23:47
  • I have no idea too. Happens to me a lot in stack. I don't understand if I did something wrong or the community doesn't like my questions. – Pedro Barros Jan 6 '16 at 0:19
  • The question was difficult to understand before it was edited. – Kevin Panko Jan 12 '16 at 16:02
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NAT and DHCP are totally independent technologies, which do not rely on each other at all.

The first concept to understand is one of network segments - simplifying slightly, all IP addresses in the same IP range are in the same segment. A DHCP server listens on a given segment for requests for an IP address and offers it when asked. A DHCP server needs to listen on the same interface as the segment is on - thus it can run on a computer with a single network card, or on a router (in a home network it would typically run on the router, serving addresses on the LAN network).

On a typical home router NAT is run on the WAN interface, and relables packets leaving the network with the external IP address. It holds a translation table so it knows which internal machine requested what. In this way its possible for a single real-world IP address to be shared between multiple workstations.

Your question "when I connect two different computers they both get different external IP's. How can this happen? Shouldn't my router be NAT since it protects my local IP Address?" presents something of a curveball. The answer is yes, in a home network, any devices behind NAT would typically share a single external IP address.

There are a few possibilities why you are getting different IP addresses, including -

  1. Your provider is using Carrier Grade NAT, and and is, in effect doing double-nat. Carrier Grade NAT requires the use of more then 1 IP address, so it could be that different TCP sessions are being allocated different IP's

  2. You may not be using NAT at all, and your provider is providing you with a subnet of IP addresses which are real-world routable. This would seem somewhat unlikely in a home environment (and indeed there are security implications to it), but not uncommon if requested on business accounts.

  3. Its conceivable your router has more then 1 real-world IP address on it and is doing more complex NATTING (similar to carrier grade nat) to overcome some limitations of NAT.

  • Clear and concise. You helped a lot of my friends with the same question. Thanks. – Pedro Barros Jan 6 '16 at 0:22

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