The bind System Call
Once the socket( ) system call completes, a new socket is created and initialized. It represents a new communication channel that can be identified by the following five elements: protocol, local IP address, local port number, remote IP address, and remote port number.
Only the "protocol" element has been set so far. Hence, the next action of the User Mode process consists of setting the "local IP address" and the "local port number." These two elements identify the process that is sending packets onto the socket so the receiving process on the remote machine can determine who is talking and where the answers should be sent. 
Continue reading here: Writing Packets to a Socket
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