Port (computer networking)

Source: Wikipedia, the free encyclopedia.
This article is about software communication ports. For physical ports, see Computer port (hardware).
"port (network)" redirects here. For ports in electrical networks, see Port (circuit theory).

In

unsigned numbers
.

A port number is always associated with a

well-known port numbers. Higher-numbered ports are available for general use by applications and are known as ephemeral ports
.

Ports provide a multiplexing service for multiple services or multiple communication sessions at one network address. In the client–server model of 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 means no port. A

transport protocol, a network address such as an IP address, and a port number. This is known as binding. 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 arriving network packets
to 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 called port 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 reserved

well-known port numbers for receiving service requests from clients. This process is known as listening, 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 a tuple consisting of the local address, the local port, the remote address, and the remote port.[1] The well-known ports are defined by convention overseen by the Internet 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 an ephemeral 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: the well-known ports, the registered ports, and the dynamic or private ports.

The well-known ports (also known as system ports) are those numbered from 0 through 1023. The requirements for new assignments in this range are stricter than for other registrations.[2]

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 Telnet remote 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 the World 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 DHCPv6 IPv6 version of DHCP

The registered ports are those from 1024 through 49151. IANA maintains the official list of well-known and registered ranges.[3]

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

Network behavior

firewalls are commonly configured to differentiate between packets based on their source or destination port numbers. Port forwarding
is 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 as

cracking attempts 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 of port knocking
uses 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 of email. 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 the Simple Mail Transfer Protocol (SMTP). A standard SMTP service application listens on TCP port 25 for incoming requests. The second service is usually either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) which is used by email client applications 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 is

DHCP
in 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 other

uniform resource locators (URLs). 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 browser
connects instead to port 8080 of the HTTP server.

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 in use. 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.[4] 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 Eightbit Number, abbreviated AEN.[5] Today, network socket refers to a related but distinct concept, namely the internal address of an endpoint used only within the node.

On March 26, 1972, Vint Cerf and Jon Postel called 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".[6] 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".[5] 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 a czar, to maintain a registry.[7] 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

The Telnet service received the first official assignment of the value 1. In detail, the first set of assignments was:[7]

Port assignments in
RFC 349
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 a socket name,[8] and was used with the Initial Connection Protocol (ICP), a component of the Network Control Protocol (NCP).[9][10] NCP was the forerunner of the modern Internet protocols. Today the terminology service name is still closely connected with port numbers, the former being text strings used in some network functions to represent a numerical port number.

References

  1. doi:10.17487/RFC0793. RFC 793
    . Retrieved 1 January 2023.
  2. doi:10.17487/RFC6335. BCP 165. RFC 6335
    .
  3. ^ "Port Numbers". Internet Assigned Numbers Authority (IANA).
  4. doi:10.17487/RFC0036. RFC 36
    .
  5. ^
    doi:10.17487/RFC0433. RFC 433
    .
  6. doi:10.17487/RFC0322. RFC 322
    .
  7. ^
    doi:10.17487/RFC0349. RFC 349
    .
  8. doi:10.17487/RFC0197. RFC 197
    .
  9. ^ NIC 7104, ARPANET Protocol Handbook
  10. ^ Postel, Jon; Feinler, E. (1978). ARPANET Protocol Handbook. Menlo Park, CA: Network Information Center.
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