Data transmitted over the Internet carries a 32-bit IP address that identifies its destination computer and a 16-bit number that identifies a port on that computer. The combination of the IP address and the port number is called a "socket". A pair of sockets, one on the sending computer and one on the receiving computer uniquely identifies a specific connection on the Internet.
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Computer Networking Ports and Sockets

A specific computer connected to the Internet is identified by it's unique IP address, however a computer may have several different Internet applications running simultaneously, so how does the computer know which application to send the data to? By it's "port" number. Applications running on a computer communicate through ports.

Data transmitted over the Internet carries a 32-bit IP address that identifies its destination computer and a 16-bit number that identifies a port on that computer. The combination of the IP address and the port number is called a "socket". A pair of sockets, one on the sending computer and one on the receiving computer uniquely identifies a specific connection on the Internet.

For example, the standard port number for Telnet is 23. You can access Telnet on a host whose IP address is 67.199.40.115 with the socket address 67.199.40.115:23. Port numbers range from 0 to 65535. The IANA (Internet Assigned Numbers Authority) has divided port numbers into three groups, Well Known, Registered, and Dynamic (or "Private") Ports.

Ports in the range from 0 to 1023 are called Well Known ports and are reserved by the IANA for use by common TCP/IP applications like HTTP, FTP, and Telnet. Some common well known port numbers are listed below.

Port  Process  Description
20FTPFile Transfer Protocol - Data
21FTPFile Transfer Protocol - Control
22SSHSecure Shell
23TelnetTerminal Emulation Protocol
25SMTPSimple Mail Transfer Protocol
53DNSDomain Name System
69TFTPTrivial File Transfer Protocol
80HTTPHypertext Transfer Protocol
110POP3Post Office Protocol
443HTTPSHypertext Transfer Protocol - Secure

Registered ports have numbers in the range from 1024 to 49151. The Registered ports are NOT reserved by the IANA and on most systems can be assigned to processes by system administrators or programmers.

Dynamic (or Private) ports have numbers in the range from 49152 to 65535. Dynamic port numbers are assigned randomly by the computers operating system, so if a system administrator or programmer chooses to use a port in this range, there is a risk of a port conflict.

System administrators can change the port number associated with common internet services like FTP, and Telnet, and sometimes they reconfigure a processes default port number for security reasons. A hacker probing their network will find they cannot connect through the conventional ports.

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