Got myself a second hand Apple Aiport Extreme and during configuration it stopped working. After A LOT of trial and error, discovered it was due to a setting, which is not properly explained.
On the back of the device are 4 ethernet ports, clearly marked differently. 3 with the usual <--> for ethernet and one with a kind O, which is for the internet, if you have a cable modem, or maybe a half bridge DSL router.
So we have a very clear paradigm of internet and LAN.
Right, here's the INTERNET tab of the Airport Utility. So I'm thinking this is where you configure INTERNET access. Right? Wrong! In fact this configures the LAN. So, turn off the ethernet WAN port at your peril, because if you do you'll fall off the LAN and you'll have to do a reset.