Ports
Both TCP and UDP use port numbers for communications between hosts.
Port numbers are similar to phone numbers in that Transport layer services can be called by their port number.
When a computer wants to transfer a file over FTP, it uses TCP port 21 to establish and control the connection and TCP port 20 to transfer the data.
TCP port 20 and 21 are called Well-Known Port Numbers because applications expect to find FTP services on TCP port 21 and to transfer their data on TCP port 20.