Is Port Forwarding only neccessary if someone tries to connect to you first?

Question

I got thinking about how webcams over MSN Messenger worked. After some digging on Google, I found that the camera stream on a port in the 6000's.

It then occured to me - I don't allow that reem of ports to be open on my router - yet the webcam stream works when I chat with a contact?

How is this? If I initiate a connection to a remote service on my client machine, does that somehow by-pass any Port Forwarding on my router and connect anyway?

Is Port Forwarding only for when someone tries to establish a connection to you first?

Answer 1

MSN, skype, google talk, sip and various such services use techniques known as nat-traversal to bypass port forward rules (or add their own at will).

There are various of them, so just laying down acronyms you can include UPnP (this is what is probably used by msn), STUN, TURN, hole punching, ALG configurations on your router and others.

You probably want to check out this wikipedia article and the associated links http://en.wikipedia.org/wiki/NAT_traversal

Answer 2

The only reason you need port forwarding in the first place is because your router uses NAT. Once you understand how NAT works, you will understand why you need port forwarding and why it is indeed only necessary if someone else tries to connect to you. So why don't you need port forwarding to share your webcam with someone? Well, you do. But there are protocols such as UPNP to automatically configure those port forwardings.