A Port is an application-specific
or process-specific software construct serving as a communications endpoint in
a computer's host operating system. The purpose of ports is to uniquely
identify different applications or processes running on a single computer and
thereby enable them to share a single physical connection to a packet-switched
network like the Internet. In the context of the Internet Protocol, a port is
associated with an IP address of the host, as well as the type of protocol used
for communication.
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| TCP/IP Packet |
The protocols that primarily use
ports are the Transport Layer protocols, such as the Transmission Control
Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol
Suite. A port is identified for each address and protocol by a 16-bit number,
commonly known as the port number. The port number, added to a computer's IP
address, completes the destination address for a communications session. That
is, data packets are routed across the network to a specific destination IP
address, and then, upon reaching the destination computer, are further routed
to the specific process bound to the destination port number.
Of the thousands of enumerated
ports, about 250 well-known ports are reserved by convention to identify
specific service types on a host.
History
The concept of port numbers was
established by the early developers of the ARPANET in informal cooperation of
software authors and system administrators.
The term port number was not yet
used at this time. It was preceded by the use of the term socket number in the
early development stages of the network. A socket number for a remote host was
a 40-bit quantity. The first 32 bits were similar to today's IPv4 address, but
at the time the most-significant 8 bits were the host number. The least-significant
portion of the socket number (bits 33 through 40) was an entity called another
Eight bit Number, abbreviated AEN, today's port number.
The 256 values of the AEN were divided
into the following ranges:
- 0 through 63: network-wide standard functions.
- 64 through 127: host-specific functions.
- 128 through 239: reserved for future use.
- 240 through 255: any experimental function.
Technical Details
Transport Layer protocols, such as
the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP),
specify a source and destination port number in their packet headers. A port
number is a 16-bit unsigned integer, thus ranging from 1 to 65535 (for TCP,
port number 0 is reserved and can't be used. For UDP the source port is
optional and a value of zero means no port). A process associates its input or
output channels via Internet sockets, a type of file descriptor, with a
transport protocol, a port number and an IP address. This process is known as
binding, and enables sending and receiving data via the network.
Port connection attempts are
frequently monitored and logged by computers. The technique of port knocking
uses a series of port connections (knocks) from a client computer to enable a
server connection.
Use in URLs
Port numbers can occasionally be
seen in a web or other service uniform resource locator (URL). By default, HTTP
uses port 80 and HTTPS uses port 443, but a URL like
http://www.example.com:8080/path/ specifies that the web resource be served by
the HTTP server on port 8080. The active transport layer protocol ports may be
discovered on many operating systems (Windows, Unix-like, z/OS) with the
command line netstat -a.
Well known Ports
The port numbers in the range from
0 to 1023 are the well-known ports or system ports. They are used by system
processes that provide widely used types of network services. On Unix-like
operating systems, a process must execute with superuser privileges to be able
to bind a network socket to an IP address using one of the well-known ports.
- 20 & 21: File Transfer Protocol (FTP)
- 22: Secure Shell (SSH)
- 23: Telnet remote login service
- 25: Simple Mail Transfer Protocol (SMTP)
- 53: Domain Name System (DNS) service
- 80: Hypertext Transfer Protocol (HTTP) used in the World Wide Web
- 110: Post Office Protocol (POP3)
- 119: Network News Transfer Protocol (NNTP)
- 143: Internet Message Access Protocol (IMAP)
- 161: Simple Network Management Protocol (SNMP)
- 194: Internet Relay Chat (IRC)
- 443: HTTP Secure (HTTPS)
- 465: SMTP Secure (SMTPS)
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