The Most Popular TCP and UDP Port Numbers
Most TCP ports are general-purpose channels that can be called into service as needed but otherwise sit idle. Some lower-numbered ports, however, are dedicated to specific applications. While many TCP ports belong to applications that no longer exist, certain ones are very popular. Jason Gillikin TCP Ports 20 and 21 FTP servers use TCPport 21to manage their side of FTP sessions. The server listens for FTP commands arriving at this port and responds accordingly....