Airports fail to serve DHCP addresses

Question

I am having trouble reconfiguring an existing network with 2 Airport devices -- switching to having the Airports serve addresses via DHCP is failing, and would appreciate help determining what I'm doing wrong here.
Details:
I have a network with a 16-IP public address space x.x.x.208-223.  My DSL router/firewall is at x.209, and I have various devices with manually assigned statics IPs in the x.210-x.218 range.  The router is configured to serve up the last 4 IP addresses x.219-x.222 via DHCP.  Within the static IP range, I have a Time Capsule at x.210 and an Airport Express at x.214, located at far ends of the house, and running differently named wireless networks.  They have so far been configured with [Connection sharing: Off (Bridge Mode)]  Under Internet > Internet Connection and appropriate manual settings under Internet > TCP/IP.
All has worked fine with this configuration to-date.  Our few devices without a static IP were assigned DHCP addresses from the 219-222 range, whether connected via our hard-wired ethernet network, or over either wireless network -- the addresses picked up from the router at 209.
While the 4 addresses available by DHCP were previously sufficient, with the proliferation of mobile devices in my family, it's not enough anymore, and I am trying to reconfigure the Airports to serve up private network blocks 10.0.x.x and 10.0.y.y via DHCP [Connection sharing: Share a public IP address].  This is not working.  When I reconfigure either Airport in this manner, network devices connecting via wireless no longer can connect to the internet.  Connection to the wireless network itself is fine, but the device is unable to get a DHCP address from the Airport.  Instead, after a short delay it gets a self-assigned address in the 169.254.x.x range.  This happens with either Airport base station, and both with my iPhone 4 as well as my MacBook Pro.
Thinking that having the router serve DHCP within the public space might be interfering with the Airport DHCP in its own private block, I tried disabling DHCP at the router.  Even so, I cannot get the Airports to start serving IP addresses to their wireless clients.
Any suggestions ?

Answer 1

Hi Linc,
I appreciate the discussion.  Let me reply in-line
Linc Davis wrote:
I'm having a lot of trouble understanding your questions. Rather than trying to figure it out, let me tell you how it should work.
You have a DSL modem that is also a router. It gets an address for its WAN interface from your ISP's DHCP server, or it has a static address -- doesn't matter which.
My router has a static WAN IP address, which is in my ISP's subnet.  The LAN is my own public subnet 209.128.72.208/28, and the router's LAN IP address is 209.128.72.209.
It had better not be serving DHCP on the WAN, or you may get kicked off by your ISP. I'd be surprised if you were even able to configure it that way.
No, it's not serving DHCP on the WAN.  It is serving DHCP only on the LAN, in the 209.128.72.219-222 range.
The router's LAN interface has a fixed private IP address that you configure. It serves DHCP to other devices on the LAN.
Yes, that is how it is.
The AE can get an address from the router for its Ethernet port from the router, or it can have a fixed address, whichever you want.
Yes, it is configured with its own static, manually configured address, which is 209.128.72.214, outside of the DHCP range served by the router.
Set it to operate in bridge mode, which means that the wireless network will be like an extension of the wired network, with the same address pool. Wireless devices will get their addresses from the router.
Yes.  To-date I have operated it in bridge mode, exactly that way.  And it's worked great up until now.  Devices with manually assigned IP addresses in the 209.128.72.210-218 range have worked fine either when connected by ethernet or wireless, and those without (iPhones, etc) have picked up DHCP addresses from the router just fine.
The AE does not need to be a DHCP server.
Unless your needs are very unusual, that setup will work for you.
And so it has worked that way for a while.  However, my needs have changed recently.  The range of available IPs for DHCP from the router is limited to 4 devices (209.128.72.219-222).  That worked fine when we had only 4 devices (iPhone, iPad, Roku, and Blackberry).  Now, the kids have wireless devices (iPod touch and an old iPhone).  The available range is not sufficient for 6 devices.  So, rather than serve wireless devices with addresses from the limited remaining IPs in the public subnet, I want to have the AE serve DHCP in a private network 10.1.x.0/24 , which obviously has alot more room. 
I'm willing to try turning off DHCP on the router completely.  That doesn't seem to help -- I've tried it twice.