I posted a thread the other day about problems I was having with the XBox Live Party Chat system and my Vigor router. After more investigation I discovered that the problem was down to routing.

I've got a dynamic IP from VirginMedia and a static IP from O2.
I use my VM as my primary connection (and default gateway). O2 is WAN2 and back up.
I use a Load-Balancing feature of the Vigor to force all of my XBox traffic onto WAN1.

The problem I have is that several of my friends also use BE and O2 as their ISPs. This means that all of us have public IPs in the 78.105.x.x subnet. I'm 78.105.120.x whilst a friend (on BE) is 78.105.127.x The subnet mask I was given to use with my IP is 255.255.248.0. Therefore the Vigor is routing all traffic destined to the 78.105.x.x network out via WAN2, ignoring my Load-Balancing rule and this is breaking XBox Live (to those people on O2/Be).

To fix the problem I've made my o2 subnet more restrictive (255.255.255.0). The thing is, I'm not sure what impact that might have on other things ? :dunno:

The biggest problem with messing about with subnets is getting the correct pair of broadcast addresses (all host bits true and false).
As long as you are subnetting your allocation, you should be OK, but don't try to supernet as it probably will not work at all.