How SSH Port Became 22
Personally I find how&#x2F;why FTP uses two ports, 20 and 21, to be more interesting:<p>So the first thing to know is the FTP is &#x27;old&#x27;, in that it predates TCP and originally ran on NCP:<p>&gt; <i>The original specification for the File Transfer Protocol was written by Abhay Bhushan and published as RFC 114 on 16 April 1971. Until 1980, FTP ran on NCP, the predecessor of TCP&#x2F;IP.[2] The protocol was later replaced by a TCP&#x2F;IP version, RFC 765 (June 1980) and RFC 959 (October 1985), the current specification.</i><p>* <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;File_Transfer_Protocol" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;File_Transfer_Protocol</a><p>The second thing to know is that, unlike TCP or UDP, NCP was not duplex:<p>&gt; <i>NCP preceded the Transmission Control Protocol (TCP) as a transport layer protocol used during the early ARPANET. NCP was a simplex protocol that utilized two port addresses, establishing two connections, for two-way communications. An odd and an even port were reserved for each application layer application or protocol. The standardization of TCP and UDP reduced the need for the use of two simplex ports for each application down to one duplex port.[1]</i><p>* <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Network_Control_Program" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Network_Control_Program</a><p>So the original transport layer that FTP relied on necessitated two ports, and when the move to a newer transport layer occurred the use of two ports was carried over for simplicity&#x27;s sake. And several decades later that design still exists.