How do applications coexist over TCP and UDP?
Each application running over TCP or UDP distinguishes itself from other applications using the service by reserving and using a 16-bit port number. Destination and source port numbers are placed in the UDP and TCP headers by the originator of the packet before it is given to IP, and the destination port number allows the packet to be delivered to the intended recipient at the destination system. So, a system may have a Telnet server listening for packets on TCP port 23 while an FTP server listens for packets on TCP port 21 and a DNS server listens for packets on port 53. TCP examines the port number in each received frame and uses it to figure out which server gets the data. UDP has its own similar set of port numbers. Many servers, like the ones in this example, always listen on the same well-known port number. The actual port number is arbitrary, but is fixed by tradition and by an official allocation or “assignment” of the number by the Internet Assigned Numbers Authority (IANA). •
