TCP/IP Ports

TCP/IP Ports

Network ports are logical connections, provided by the TCP or UDP protocols at the Transport layer, for use by protocols in the upper layers of the OSI model. The TCP/IP protocol stack uses port numbers to determine what protocol incoming traffic should be directed to. Some characteristics of ports are listed below:

• Ports allow a single host with a single IP address to run network services. Each port number identifies a distinct service.
• Each host can have over 65,000 ports per IP address.
• Port use is regulated by the Internet Corporation for Assigning Names and Numbers (ICANN).

 ICANN specifies three categories for ports.

Categories	Characteristics
Well Known	Assigned for specific protocols and services Port numbers range from 0 to 1023
Registered	ICANN can assign a specific port for a newly created network service Port numbers range from 1024 to 49151
Dynamic (Private or High)	Assigned when a network service establishes contact and released when the session ends Allows applications to 'listen' to the assigned port for other incoming requests (traffic for a protocol can be received through a port other than the port which the protocol is assigned, as long as the destination application or service is 'listening' for that type of traffic on that port) Port numbers range from 49,152 to 65,535

The following table lists the well known ports that correspond to common Internet services.

Protocol(s)	Port(s)	Service
TCP	20, 21	File Transfer Protocol (FTP)
TCP UDP	22	Secure Shell (SSH)
TCP UDP	23	Telnet
TCP UDP	25	Simple Mail Transfer Protocol (SMTP)
TCP UDP	53	Domain Name Server (DNS)
UDP	67, 68	Dynamic Host Configuration Protocol (DHCP)
UDP	69	Trivial File Transfer Protocol (TFTP)
TCP	80	HyperText Transfer Protocol (HTTP)
TCP	110	Post Office Protocol (POP3)
TCP	119	Network News Transport Protocol (NNTP)
UDP	123	NTP
TCP UDP	143	Internet Message Access Protocol (IMAP4)
TCP UDP	161, 162	Simple Network Management Protocol (SNMP)
TCP UDP	389	Lightweight Directory Access Protocol
TCP	443	HTTP with Secure Sockets Layer (SSL)

Note: When creating access lists, allow only the port numbers that correspond to the services running on the servers.