kair: How to Disable DHCP while Maintaining NA

There's a User Contributed Answer about using WDS without DHCP, but my situation is a little different. In case someone else can benefit, here's how I got it working.

Problem: Using Airport Extreme for Internet connection, I want to disable the DHCP functionality (so my computers can get their addressing information from a local DHCP server) but retain NAT functionality (so everyone can access the Internet). Although that doesn't appear possible from the Airport Administration tools, there's a trick that makes it work.

0. Power off as many systems as you can to keep things simple, though you'll have to keep at least one computer up to run Airport Administration.

1. Using Airport Administration, configure the DHCP lease time to be essentially forever. (I used 8000 days, or 20+ years, which ought to be good enough).

2. On the DHCP address pool, select other and then specify a pool size of 2. In my case I used 172.16.1.1 - 172.16.1.2. (It seems that you have to do step 1 first.) Now the Airport Extreme will only have one address (e.g. 172.16.1.2) to give out since it uses the other address (e.g. 172.16.1.1) for itself.

3. Release and renew the IP address on the one computer you're using. (In Windows, that's IPCONFIG /RELEASE and then IPCONFIG /RENEW from the command line; in Mac OS X uncheck and then recheck the appropriate network port in System Preferences -> Network -> Network Port Configurations). At this point your computer has grabbed the one and only IP address the Airport can hand out, and as far as Airport is concerned, the lease is practically permanent.

4. Configure your local DHCP server. Be sure to specify the Airport Extreme (e.g. 172.16.1.1) as the default gateway or default router. Also, the IP address range should be in the same subnet as the Airport Extreme, but without overlapping its 2 addresses. I used 172.16.1.100 - 172.16.1.199.

5. Disconnect the Airport Extreme from the local network.

6. Release and renew the IP address of the local system again. This time it will get an IP address from the local server instead of the Airport Extreme.

7. Reconnect the Airport Extreme.

Voila, the Airport Extreme is happily functioning as a NAT device for all your computers, but they're getting their IP addresses and other information from a different server.

Aside: In case some folks are wondering, there are legitimate reasons for wanting to use a local DHCP server. In my case, I wanted to run a local DNS service on my home network (so I don't have to remember IP addresses all the time), and I needed the DHCP server to specify the local DNS server. Airport always identifies itself as the DNS server, and there doesn't seem to be a way to change that.

Aside 2: The difference between this case and the User Contributed Answer that's already posted is that in the other post the user does not want Airport to function as a NAT (since s/he has an existing broadband router to do that). In my case, I do want NAT functionality from the Airport.


Very nicely done.