So I want to host a server and I have to forward an UDP port. I'm familiar with networking but I have one question.
I'm behind a modem and router. The modem has a subnet A IP address and the router has a subnet C IP address.
Do I have to forward the port on my modem to my router and vice versa?
More aboutquestion forwarding
Port forwarding is made necessary by NAT. So your inbound connection would not be able to reach the server which is NATed, unless you tell the router how to forward that traffic. The (cable?) modem shouldn't block any ports, I don't think you can or need to set up forwarding on that device.
My DSL modem has a "default server" setting which supposedly will accept every inbound connection to the specified IP address (my router).
If I enter my routers IP all the inbound connection will be forwarded to my router. And I assume that I must forward every inbound connection from my router to my computer.