Based on what Joe Lucia has said, this is quite doable and the result is quite elegant. In the instructions below, I have assumed that you want to run a DHCP service on a Mac server. However, the concept would apply to any DHCP server which will run in parallel with your Airport.
- Use aiport utility and set the airport to "Share a public IP address" mode.
- Using the aiport utility go to the DHCP tab and set the IP address range to distribute just one address which will become the server's address, eg: start address 192.168.1.2 and end address 192.168.1.2. Add a "DHCP Reservation" and set it to the address that you entered in the range, eg: 192.168.1.2. This will become the IP that will be distributed to the Mac server itself.
- On the Mac server, using "Network preferences..." set your local network interface to DHCP so it will get the address that you chose in step 2 from the aiport. Also configure the Mac server's DNS in "Advanced..." to point to itself, following the example address above, it should be 192.168.1.2.
- Configure the DHCP service on the Mac server, specifying the regular range for your clients. In this case, with the example addresses above, Aiport will have the address 192.168.1.1 and your Mac server will get 192.168.1.2 from the airport. Thus, your range should start at 192.168.1.3 and go to whatever end address you choose. Additionally, while configuring the DHCP service, make sure you set the DNS / LDAP and so on to, following this example, 192.168.1.2 which will be the address of the Mac server. Lastly, while configuring the DHCP service, set the router IP to the IP of the aiport station, which, following this example, will be 192.168.1.1.
It is pretty elegant because the Airport will dish out the address to your server when it comes up and will not interfere after that because it has only IP within its range. When your clients will broadcast for an address, the Aiport will not pick it up since its address is already taken and in case the server is down, it will not pick it up either because it has only one IP which is reserved. Then, the only DHCP server on your network will be your Mac server that will pick-up the requests and dish out addresses to clients on the network.
It is true that if your Mac server goes down, there will be no DHCP server on the network to answer the requests. However, that's how DHCP servers are and, of course, there's nothing stopping you from adding another Mac server (or any other DHCP server) to Airport and extending the range of distributed addresses to two so that you have some load balancing.
