TCP Ports and Sockets

Because several application programs can be running on one machine using a single network interface, TCP needs to keep track of which data goes to which program. This tracking is done by assigning a port number to every TCP connection. The TCP port number does not need to be the same on the local and remote processes. When a TCP segment is received, TCP knows which process to pass it to by looking at the port number in the TCP header. For example, a user web browsing and polling the e-mail server at the same time is not aware that TCP is playing traffic director to the received data; all data from TCP port 80 is pointed to the web browser, and all data from TCP port 25 is pointed to the e-mail client.

A TCP socket is defined as the combination of the local IP address and the TCP port number. An application wanting to use the facilities of TCP must request a unique socket from the TCP. This is known as opening a socket. The local socket and the remote socket define a connection. An application must know these two things in order to successfully communicate to a remote application through TCP.

Routers and firewalls use the TCP port number to block or allow traffic to pass through a network. A complete list of TCP port numbers can be found in RFC 1700 (www.ietf.org/rfc/rfc1700.txt?number=1700). The following table lists some of the more common TCP port numbers.

Table 3-2. Common TCP Port Numbers

Port Number	Process Name (Abbreviation)	Description
20	FTP-Data	FTP (File Transfer Protocol) Data
21	FTP	FTP Control
23	TelNet	Telnet
25	SMTP	Simple Mail Transfer Protocol
53	DNS	Domain Name System
69	TFTP	Trivial File Transfer Protocol
80	HTTP	HTTP (HyperText Transfer Protocol) (WWW)
110	POP3	Post Office Protocol version 3
115	SFTP	Simple File Transfer Protocol
161	SNMP	SNMP (Simple Network Management Protocol)
179	BGP	Border Gateway Protocol
443	HTTPS	Secure HTTP