Some better known well-known ports

  ftp-data 20 TCP - file transfer protocol
  ftp      21 TCP
  * Becoming obsolete because of lack of security.
  * Replaced with sftp (secure file transfer protocol)
  * Provide many of the ftp features using ssh functions.

  ssh      22 TCP - secure terminal

  telnet   23 TCP - terminal
   * becoming obsolete because of lack of security.

  smtp     25 TCP - simple mail transfer protocol (SMTP) - sending.

  pop     110 TCP - post office protocol - receiving.
                    Mail retrieved and deleted from server.
                    * Neither smtp or pop provide encryption or other security.
  * POP - Post Office Protocol, one connection per user at a time. When mail
    is read, it is moved from system's inbox area to user's directory. Older
    protocol.


  IMAP    143 TCP - Internet Message Access Protocol - insecure method
                    for fetching email, leaves copy on server.
  * IMAP, Internet Message Access Protocol, allows multiple connections to
    a user's email Inbox and email is left in user's inbox on server unless
    moved.


  domain   53 TCP/UDP - associates an IP@ with a Domain Name.

  bootps   67 UDP -  BOOTP/DHCP server
  bootpc   68 UDP -  BOOTP/DHCP client

  web service  80 - unencrypted web services.

  secure web   443 - HTTPS, web service over TLS/SSL.

  secure SMTP  587 - securely (TLS/SSL) sending email from client.
          also 465  # older version. Most emails servers accept both.

  imaps        993 - for fetching email from server over SSL.

  pop3s        995 - for fetching email from server over SSL.

  It is common for a service to have a different set of reserved ports for
    secure connections.