Airport blocking access to website

I've got a very strange problem with my Airport Extreme base station which has me scratching my head...

For some reason, a couple of hours ago, I lost access to a server on which I host several websites. I can't connect via HTTP, FTP, or download email.

I have confirmed that the server is fine - if I plug my MacBook directly into the cable modem it all works OK. If I connect using my mobile phone over bluetooth, it all works OK.

I (and 5 colleagues in my office connected both wirelessly and via ethernet to the airport) are unable to connect to the server or any of the websites on it. All other internet access seems OK. It's almost as if Airport is blocking access to this one IP address.

Using Terminal, a ping to the domain name resolves to the correct IP address but experiences 100% packet loss.

I've performed a hard reset on the base station and set it all up again fresh, but have exactly the same problem.

As I understand it, the Airport has a firewall built in to the WAN port (which is connected to my cable modem), which could be blocking access to this site. Is there any way to manage that firewall?

I've run out of ideas - any help would be greatly appreciated!

 

I don't see how it could be my ISP. As I said, I'm able to connect OK when plugged directly into my cable modem. I know from previous experience that my ISP will only provide support to the modem - any LAN beyond that is beyond their control.