I want to make a peer to peer communication tool. If I have this tool I can contact another guy having the same tool. Like we communicate with messengers. The problem is that if Iam using socket communication and these two machines are not having global IP. These two are in different networks and connected through different gateways through the intrnet. So I have a confusion that how a packet send from one machine will reach the other end. Bcoz the other machines IP is not exactly global IP. If you can provide some good links please tell me.

Could you please make it clearer: do both machines have a local IP addresses?

It is impossible to communicate between the two programs if they are both behind a firewall and having an address that is only accessable by other pc's behind the same firewall. One of the two pc's must have a "real" ip address or an internet ip address, not a subnet ip address.

You can go around this by having a third pc somewhere with an internet ip address (e.g. a linux shell where you are permission to run network programs [maybe you have a friend with a linux box somewhere]) to which both clients connect. This way this third program can route the communication between your messenger clients.