This shows how to set up a D-link router for the game "Call of Duty"
and will walk you through the basics of opening ports. Of course, the
"Call of Duy" ports are not going to help you in Flight Simulator: this
is just to understand the principles.
The specific ports that you need to open for multiplayer on Flight
UDP UDP 6073
Once you get that sorted out, you can use the Microsoft "Zone" servers,
or you can get on board with one of the many FSHost servers. Get
FSHostSpy and install it to learn what's out there http://www.chocolatesoftware.com/fshostspy/
I haven't tried IVAO myself, but with the new SquawkBox 3 software that
VATSIM uses, it's actually easier to get online with VATSIM than it is
to set up the Microsoft multiplayer features, and voice is built-in.
Before you jump in, you need to know that VATSIM and IVAO both try to
simulate real air traffic control systems, so if you are not already
familiar with these from the real world, you will need to spend some
time studying the system before you jump in. If that appeals to you,
I'd encourage you to skip the whole multiplayer setup and go straight
to one of these networks. If you just want to fly around with other
pilots, go with multiplayer.