Ports and Applications

Overview

Ports are the way a client program specifies a particular server program (service) on a host.
Higher-level applications that use TCP/IP, such HTTP, have ports with pre-assigned numbers. These are known as "well-known ports" and have been assigned by the Internet Assigned Numbers Authority (IANA).
Other application processes are given port numbers dynamically for each connection.
When a service (server program) initially is started, it is said to bind to its designated port number. A client program wants to use that service, connects to that port on the server.
Key concepts


Well-known ports are:

FTP data: 20 File Transfer Protocol

FTP control 21

telnet: 23

SMTP (mail): 25 Simple Mail Transfer Protocol

DNS (UPD & TCP): 53 Domain Name Service

HTTP: 80 Hypertext Transfer Protocol

POP3: 110 Post Office Protocol version 3

IMAP: 143 Internet Message Access Protocol

SNMP (UPD & TCP): 160/161 Simple Network Management Protocol

HTTPS: 443 Secure HTTP (using SSL)

NSFTP data: 820 NetSeq FTP

NSFTP control: 821

SQUID proxy: 3128 UnixHTTP proxy server