airportdnsNetworktime-capsule
3TB Airport Time Capsule (FW 7.7.3) DHCP sends the correct DNS address to hosts for the Primary network, but sends the Gateway address for the Guest network. If I correct the DNS manually on a host, Internet works fine. What's the problem?
Any LAN Host:
DNS 172.16.42.1
Should be 8.8.8.8
If the old Airport Express was connected directly to your cable/DSL modem and the new Time Capsule replaced it (again, directly connected), you may need to reboot the modem (generally by unplugging it and plugging it back in, maybe holding down the reset button) to reset its DHCP. You may also need to do this in a particular order, generally 1) shutdown Time Capsule, 2) reboot modem, and 3) restart Time Capsule.
Reason: some broadband modems seem to only assign one internal IP at a time and only check for a device on startup.
The feature you are looking for is called "DHCP". This does exactly what you want, assign an IP address constantly given a specific MAC Address.
Open up Airport Utility.app, click your Time Capsule, then "Manual Setup". When the management dialog loads, click "Internet" up at the top, then "DHCP" in the category/tab bar.
From there, you can click the plus in the bottom-left hand side of "DHCP Reservations". This will open up a dialog that allows you to create a reservation via MAC Address, or "DHCP Client ID".
Fill in a system name (does not necessarily have to be the specific hostname), click MAC Address, then Continue.
Fill in the MAC address, choose an IP address, then click Done, and then Update the Time Capsule!
When the client computer next refreshes it's IP address (most computer's do this once every two hours or so), it should pick up the new reservation, now and forever.
Alternatively you can just shut the interface on the computer and turn it back one. Or go into Advanced and manually renew the lease. etc. etc.
Best Answer
I got this hint from https://discussions.apple.com/thread/2735366?start=0&tstart=0
Check out the last post from Paul Verity.