However routers often have an 'allow UDP' checkbox. UDP can be globally disabled, or enabled only for certain ports. uPNP can mitigate this, but most of us have that turned off to prevent Trojan horses from opening the gates entirely.
Bigger question is, why in the world does your router disable UDP by default!?
Things like games and video streams are almost universally UDP, because it's better to forget about the data than stop everything, and go get your lost packet.
Not to mention DNS. Presumably the router would have a DNS caching server that could get around the block, but you wouldn't be able to have a computer use any other DNS server...
