Port numbers work as network addresses. Port numbers work like telephone extensions. Just as a office telephone system can use a main phone number and give each employee an extension number (like x300, x502, etc.), a computer has a main address and a set of port numbers to handle incoming and outgoing connections. With TCP and UDP, port numbers start at 0 and go up to 65535. Lower range numbers are dedicated to common Internet protocols like 21 for FTP, 80 for HTTP.