So i've been having a jolly time getting a server (a Dell computer) to talk to DynDNS and the problem lay with the forwarding of ports.

The network at my parents house consists of a generic ISP router plugged into an Airport Express to make it wireless (yey!). I also use a bunch of these wonderous white boxes to extend the wireless signal around the house.

So the error i made was trying to forward ports from the router directly to the Dell computer. This didn't work. I finally realised that:

1. The router assigns the Airport Express an IP address
2. In turn, that particular AE is responsible for assigning each computer on the WAN an IP address which is different from the IP that the AE is assigned by the router (confusing!).

So to get this to work you need to forward the ports twice:

1. Forward the ports from the router to the AE (using the IP addresses assigned by the router).
2. Forward the ports from the AE to the Dell computer (using the IP addresses assigned by the AE).

It works. Thankfully.