Why 22 was available is also interesting. Network Control Protocol was the precursor to TCP and used different ports for inbound and outbound traffic, typically even for outbound and odd for inbound.
If you look in /etc/services you'll notice that all the older protocols listen on odd numbered ports.
Some of this still survives today. In active mode FTP servers listen on 21, ACK the inbound request and then connect to the client from 20.
If you look in /etc/services you'll notice that all the older protocols listen on odd numbered ports.
Some of this still survives today. In active mode FTP servers listen on 21, ACK the inbound request and then connect to the client from 20.