Port (computer networking)

This article is about software communication ports. For physical ports, seeComputer port (hardware).

"port (network)" redirects here. For ports in electrical networks, seePort (circuit theory).

Incomputer networking,aportorport numberis a number assigned to uniquely identify a connection endpoint and to direct data to a specific service. At the software level, within anoperating system,a port is a logical construct that identifies a specificprocessor a type ofnetwork service.A port at the software level is identified for eachtransport protocoland address combination by the port number assigned to it. The most common transport protocols that use port numbers are theTransmission Control Protocol(TCP) and theUser Datagram Protocol(UDP); those port numbers are 16-bitunsigned numbers.

A port number is always associated with anetwork addressof ahost,such as anIP address,and the type of transport protocol used for communication. It completes the destination or origination address of a message. Specific port numbers are reserved to identify specific services so that an arriving packet can be easily forwarded to a running application. For this purpose, port numbers lower than 1024 identify the historically most commonly used services and are called thewell-known port numbers.Higher-numbered ports are available for general use by applications and are known asephemeral ports.

Ports provide amultiplexingservice for multiple services or multiple communication sessions at one network address. In theclient–server modelof application architecture, multiple simultaneous communication sessions may be initiated for the same service.

Port number

For TCP and UDP, a port number is a 16-bit unsigned integer, thus ranging from 0 to 65535. For TCP, port number 0 is reserved and cannot be used, while for UDP, the source port is optional and a value of zero meansno port.Aprocessassociates its input or output channels via aninternet socket,which is a type offile descriptor,associated with atransport protocol,anetwork addresssuch as anIP address,and a port number. This is known asbinding.A socket is used by a process to send and receive data via the network. The operating system's networking software has the task of transmitting outgoing data from all application ports onto the network, and forwarding arrivingnetwork packetsto processes by matching the packet's IP address and port number to a socket. For TCP, only one process may bind to a specific IP address and port combination. Common application failures, sometimes calledport conflicts,occur when multiple programs attempt to use the same port number on the same IP address with the same protocol.

Applications implementing common services often use specifically reservedwell-known port numbersfor receiving service requests from clients. This process is known aslistening,and involves the receipt of a request on the well-known port potentially establishing a one-to-one server-client dialog, using this listening port. Other clients may simultaneously connect to the same listening port; this works because a TCP connection is identified by atupleconsisting of the local address, the local port, the remote address, and the remote port.The well-known ports are defined by convention overseen by theInternet Assigned Numbers Authority(IANA). In many operating systems special privileges are required for applications to bind to these ports because these are often deemed critical to the operation of IP networks. Conversely, the client end of a connection typically uses a high port number allocated for short-term use, therefore called anephemeral port.

Common port numbers

Main article:List of TCP and UDP port numbers

IANA is responsible for the global coordination of the DNS root, IP addressing, and other protocol resources. This includes the registration of commonly used TCP and UDP port numbers for well-known internet services.

The port numbers are divided into three ranges: thewell-known ports,theregistered ports,and thedynamicorprivate ports.

The well-known ports (also known assystem ports) are those numbered from 0 through 1023. The requirements for new assignments in this range are stricter than for other registrations.

Notable well-known port numbers

Number	Assignment
20	File Transfer Protocol(FTP) Data Transfer
21	File Transfer Protocol(FTP) Command Control
22	Secure Shell(SSH) Secure Login
23	Telnetremote login service, unencrypted text messages
25	Simple Mail Transfer Protocol(SMTP) email delivery
53	Domain Name System(DNS) service
67, 68	Dynamic Host Configuration Protocol(DHCP)
80	Hypertext Transfer Protocol(HTTP) used in theWorld Wide Web
110	Post Office Protocol(POP3)
119	Network News Transfer Protocol(NNTP)
123	Network Time Protocol(NTP)
143	Internet Message Access Protocol(IMAP) Management of digital mail
161	Simple Network Management Protocol(SNMP)
194	Internet Relay Chat(IRC)
443	HTTP Secure(HTTPS) HTTP over TLS/SSL
546, 547	DHCPv6IPv6 version of DHCP

The registered ports are those from 1024 through 49151. IANA maintains the official list of well-known and registered ranges.

The dynamic or private ports are those from 49152 through 65535. One common use for this range is forephemeral ports.

Network behavior

Transport-layer protocols,such as theTransmission Control Protocol(TCP) and theUser Datagram Protocol(UDP), transfer data usingprotocol data units(PDUs). For TCP, the PDU is asegment,and for UDP it is adatagram.Both protocols use aheaderfield for indicating the source and destination port numbers. The port numbers are encoded in the transport protocolpacket header,and they can be readily interpreted not only by the sending and receiving hosts but also by other components of the networking infrastructure. In particular,firewallsare commonly configured to differentiate between packets based on their source or destination port numbers.Port forwardingis an example application of this.

Port scanning

The practice of attempting to connect to a range of ports in sequence on a single host is commonly known asport scanning.This is usually associated either with maliciouscrackingattempts or with network administrators looking for possible vulnerabilities to help prevent such attacks. Port connection attempts are frequently monitored and logged by hosts. The technique ofport knockinguses a series of port connections (knocks) from a client computer to enable a server connection.

Examples

An example of the use of ports is the delivery ofemail.A server used for sending and receiving email generally needs two services. The first service is used to transport email to and from other servers. This is accomplished with theSimple Mail Transfer Protocol(SMTP). A standard SMTP service application listens on TCP port 25 for incoming requests. The second service is usually either thePost Office Protocol(POP) or theInternet Message Access Protocol(IMAP) which is used byemail clientapplications on users' personal computers to fetch email messages from the server. The POP service listens on TCP port number 110. Both services may be running on the same host computer, in which case the port number distinguishes the service that was requested by a remote computer, be it a user's computer or another mail server.

While the listening port number of a server is well defined (IANA calls these the well-known ports), the client's port number is often chosen from the dynamic port range (see below). In some applications, the clients and the server each use specific port numbers assigned by the IANA. A good example of this isDHCPin which the client always uses UDP port 68 and the server always uses UDP port 67.

Use in URLs

Port numbers are sometimes seen in web or otheruniform resource locators(URLs). By default, HTTP uses port 80 and HTTPS uses port 443, but a URL likehttp://www.example.com:8080/path/specifies that theweb browserconnects instead to port 8080 of the HTTP server.

History

The concept of port numbers was established by the early developers of theARPANETin informal cooperation of software authors and system administrators. The termport numberwas not yet in use. It was preceded by the use of the termsocket numberin 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 calledAnother Eightbit Number,abbreviated AEN.Today,network socketrefers to a related but distinct concept, namely the internal address of an endpoint used only within the node.

On March 26, 1972,Vint CerfandJon Postelcalled for documenting the then-current usages and establishing a socket number catalog in RFC 322. Network administrators were asked to submit a note or place a phone call, "describing the function and socket numbers of network service programs at each HOST".This catalog was subsequently published as RFC 433 in December 1972 and included a list of hosts and their port numbers and the corresponding function used at each host in the network. This first registry function served primarily as documentation of usage and indicated that port number usage was conflicting between some hosts for "useful public services".The document promised a resolution of the conflicts based on a standard that Postel had published in May 1972 in RFC 349, in which he first proposed official assignments of port numbers to network services and suggested a dedicated administrative function, which he called aczar,to maintain a registry. The 256 values of the AEN were divided into the following ranges:

AEN ranges

Port number range	Assignment
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

TheTelnetservice received the first official assignment of the value 1. In detail, the first set of assignments was:

Port assignments inRFC349

Port number	Assignment
1	Telnet
3	File transfer
5	Remote job entry
7	Echo
9	Discard

In the early ARPANET, the AEN was also called asocket name,and was used with the Initial Connection Protocol (ICP), a component of theNetwork Control Protocol(NCP).NCP was the forerunner of the modern Internet protocols. Today the terminologyservice nameis still closely connected with port numbers, the former being text strings used in some network functions to represent a numerical port number.