How does the packets go out even behind Firewall or NAT with some application?

Such as Skype/Team viewer/Logmein etc application, which send audio/video behind NAT (behind firewall). But when i make a small tiny application which send text to another NAT location it failed to do the same.

Example: 

Sender: 
-> Public ip: 91.1.2.3 My lan ip is: 192.168.1.2 with port 14446 udp
-------> Data format: RTP packets
Receiver:
<------- Data received: 0 packets
-> Public ip: 92.1.2.3 Friend lan ip is: 10.0.0.2 with port 14446 udp

* same in both way

How others does this? What is the way of doing peer 2 peer application development to overcome NAT issues? Always we have public ip's and mostly it has NAT issues.

But how does then Skype works in such cases too? Do we have a audio/video port range for UDP or always UDP is open from anything? But mine does not work above range ports for UDP i also tried. What is the secret? that is making me curious!!.

Note: My goal is audio packets handling where i believe too much filtering or firewall cause latency and delay and other issues gets involved relatively too. So i would like to know very clearly for my application that some of the ports (which port ranges?) can be used for such purposes, where it really not blocking develo开发者_运维百科pment stress.

There are a number of types of NATs, which vary in what traffic they'll allow in.

See the Wikipedia article on NATs

For most NATs, STUN will let you open ports AND find out what port you opened (may be different than the port you sent from). In SIP and RTSP you'd typically provide the external IP and port determined by STUN to the other end.

A fully-symmetric NAT means that STUN won't let you use a 3rd-party server to prop ports via STUN, so you'll have to use UPnP (if enabled) or map ports in the router (or set up triggers), or you'll have to play evil games to make both sides think they initiated the connection. (Not easy and not guaranteed.)

See the ICE & TURN specs (RFCs) from the IETF for detailed mechanisms to traverse NATs - though note that in some cases you must use an external proxy to forward packets.

One common solution is that the client program connects outward to the server and thus establishes a connection. Most firewalls allow outward connections - the assumption being that you are trusted and can always connect to the outside. When the server then wishes to send a message to you, it responds on the open connection.

I believe the port that you use is what is usually used to determine if it should be allowed or not. Certain ports are always let through. I'm not sure of the exact ports, but that will be different for all NATs and firewalls.