Common protocols and ports: FTP 20/21, SFTP/SSH 22, TFTP 69, HTTP 80, HTTPS 443, DNS 53, DHCP 67/68, NTP 123

A port number is a 16-bit value (0 to 65535) in the TCP or UDP header that tells the receiving host which application or service should get the data. Servers listen on well-known ports (0 to 1023) so clients know where to connect, while the client picks a temporary high-numbered source port for each conversation. An IP address plus a port, such as 192.168.1.10:443, is called a socket. Firewalls and troubleshooting both depend on knowing which ports a service uses, so memorize this list.