Port (computer networking)

Always associated with an IP address of the host and the protocol type of the communication. A port is an endpoint of communication in an OS.

• Port is identified by a 16 bit number (0 to 65535).
• Ports are ncessary for the client-server model of applications.

Common ports

• 21: FTP
• 22: SSH
• 53: DNS service
• 80: HTTP
• 110: Post Office Protocol (POP3, email)
• 443: HTTP Secure
• $ less /etc/services/ shows a list of well-known TCP and UDP port numbers

Opening TCP ports

• nc is netcat utility, used for anything TCP/UDP
• opens TCP connections, send UDP packets, listen on arbitrary TCP or UDP ports, port scanning, deal with IPv4 and IPv6
• nc -l 5000 opens port 5000

Port Forwarding

An application of NAT (essentially IP remapping), that redirects a communication request from one address and port number combination to another, as packets pass through a router.

• most commonly used to make a service on a host that resides on a private network (behind a router) available to hosts on the opposite side of the gateway
• done by remapping the destination IP address and port num to an internal host