hey i am sorry again but i have been thinking something for about 2 years. And it was end-to-end thing. for example i am using MSN Messenger, i want to transmit a file via MSN Messenger to my friend at his house who uses MSN Messenger. However, there is NAT implemented on my modem.This will change my IP address(source) by matching it with the public address as you know. And NAT is told to break the end-to-end rule. How can this become? since just source IP address changes not the ports at the transport layer. 2- About this end-to-end rule again. let's consider the MSN Messenger. Even NAT is applied on my modem, packets reach the destination(my friend) somehow. So what is the disadvantage about NAT?