Networking —

TCP/UDP Port Functions

Each TCP/IP protocol or application has a port associated with it. When a communication is received, the target port number is checked to determine which protocol or service it is destined for. The request is then forwarded to that protocol or service. Take, for example, HTTP, whose assigned port number is 80. When a Web browser forms a request for a web page, the request is sent to port 80 on the target system. When the target system receives the request, it examines the port number and when it sees that the port is 80, it forwards the request to the Web server application.

TCP/IP has 65,535 ports available with 0 to 1023 being labeled as the well-known ports. It is important to understand the numbers of some of the well-known ports, as administration often requires you to specify port assignments when working with applications and configuring services. Table 7 shows some of the most common port assignments.

Table 7 TCP/IP Port Assignments for Commonly Used Protocols

Protocol

Port Assignment

FTP

20

FTP

21

SSH

22

Telnet

23

SMTP

25

DNS

53

TFTP

69

HTTP

80

POP3

110

NNTP

119

NTP

123

IMAP4

143

HTTPS

443