For example, some commercial file sharing sites disallow simultaneous downloads from the same user using a free account.
Some people said the server does this by checking the source IP address, so tricks like using socks proxy can trick the servers.
My question is, for many people using the same ISP behind NAT, won't the server see all these people having the same source IP address? Won't blocking simulaneous downloads by identical IP would block all people behind NAT, even they are using different machines?
If that's true, is there any true method for server to tell TCP connections are originating from same machine?