In computer networks, a port is the endpoint for communication in the operating system. This term is also used for hardware devices, but in software it is a logical construction that identifies a particular process or type of service.
The port is always associated with the host IP address and the type of communication protocol, and thus completes the assignment of the session address. It is identified for each address and protocol using a 16-bit number, commonly known as a port number. Specific port numbers are often used to identify specific services. Of the thousands of 1024 listed, well-known port numbers are protected by agreement to identify specific types of services on the host. Protocols that mainly use ports are used to control processes (for example, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) from the Internet protocol suite).
Value
TCP ports are not needed for direct point-to-point links, when computers at each end can work with only one program at a time. They became necessary after the machines were able to run more than one program at a time, and were connected to modern packet-switched networks. In the client-server architecture model , applications, ports, and network clients connect to service initiation, provide multiplexing services, after the initial data exchange is associated with a known port number, and it is released by switching each instance of service requests to a dedicated line. There is a connection to a specific number, and thanks to this, additional clients can be served without waiting.
Details
Data transmission protocols - Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) - are used to indicate the destination port number and source in their segment headers. The port number is an unsigned 16-bit integer. Thus, it can be in the range from 0 to 65535.
However, TCP ports cannot use the number 0. The source port for UDP is optional, and a value of zero means that it is not.
The process binds its input or output channels through an Internet socket (file descriptor type) using the transport protocol, port number and IP address. This process is known as linking, and it enables the transmission and reception of data over a network.
The network software of the operating system is responsible for transferring outgoing data from all application ports to the network, as well as forwarding incoming network packets (by matching the IP address and number). Only one process can be bound to a specific IP address and port combination using the same transport protocol. Common application crashes, sometimes called port conflicts, occur when several programs try to contact the same port numbers on the same IP address using the same protocol.
How are they used
Applications that implement common services often use a specially reserved and well-known list of TCP and UDP ports to receive service requests from clients. This process is known as listening, and it involves receiving a request from a well-known port and establishing a one-to-one dialogue between the server and client using the same local port number. Other clients may continue to connect — this is possible because the TCP connection is identified as a chain of local and remote addresses and ports. Standard TCP and UDP ports are defined by agreement under the control of the Internet Assigned Numbers Authority (IANA).
The core of network services (primarily WorldWideWeb) typically uses small port numbers - less than 1024. Many operating systems require special privileges for applications to bind to, because they are often considered critical for the functioning of IP networks. On the other hand, the final client of the connection, as a rule, uses a large number of them allocated for short-term use, so there are so-called ephemeral ports.
Structure
TCP ports are encoded in the header of the transport protocol packet, and they can easily be interpreted not only by the transmitting and receiving computers, but also by other components of the network infrastructure. In particular, firewalls are typically configured to distinguish packets based on their source or destination port numbers. Redirection is a classic example of this.
The practice of trying to connect to a range of ports sequentially on the same computer is known as scanning them. This is usually due to either a malicious failure attempt, or network administrators are looking for possible vulnerabilities to help prevent such attacks.
Actions aimed at opening the TCP port are often monitored and logged by computers. This technique uses a number of spare connections in order to ensure an uninterrupted connection to the server.
Examples of using
The most important example where TCP / UDP ports are actively used is the Internet mail system. The server is used to work with e-mail (sending and receiving), and generally needs two services. The first service is used for transportation by e-mail and from other servers. This is achieved using Simple Mail Transfer Protocol (SMTP). Typically, an SMTP service application listens on TCP port number 25 to process incoming requests. Another service is POP (fully - Post Office Protocol) or IMAP (or Internet Message Access Protocol) which is necessary for client applications in email on users' machines to receive email messages from the server. POP services listen on numbers from TCP port 110. The above services can both be run on the same host computer. When this happens, the port number is distinguished by the service requested by the remote device — the user's PC or some other mail server.
While the server listening port number is correctly defined (IANA calls them well-known ports), this client parameter is often selected from the dynamic range. In some cases, the clients and server individually use specific TCP ports assigned in IANA. A good example is DHCP, where the client in all cases uses UDP 68, and the server uses UDP 67.
URL application
Port numbers are sometimes clearly visible on the Internet or other unified indexes of information resources (URLs). By default, HTTP uses TCP port 80, and HTTPS uses 443. However, there are other variations. For example, the URL http://www.example.com:8080/path/ indicates that the web browser is connecting to the 8080 instead of the HTTP server.
List of TCP and UDP Ports
As already noted, the Internet Assigned Numbers Authority (IANA) is responsible for the global coordination of DNS-Root, IP addressing, and other Internet Protocol resources. This includes registering frequently used port numbers for well-known Internet services.
Port numbers are divided into three ranges: well-known, registered, and dynamic or private. Well-known (also known as system) are those having numbers from 0 to 1023. The requirements for new appointments in this range are more stringent than for other registrations.
Well-known examples
Examples on this list include:
- TCP 443 port: HTTP Secure (HTTPS).
- 21: File Transfer Protocol (FTP).
- 22: Secure Shell (SSH).
- 25: Simple Mail Transfer Protocol (SMTP).
- 53: Domain Name System (DNS).
- 80: Hypertext Transfer Protocol (HTTP).
- 119: Network News Transfer Protocol (NNTP).
- 123: Network Time Protocol (NTP) ..
- 143: Internet Message Access Protocol (IMAP)
- 161: Simple Network Management Protocol (SNMP) 1.
- 94: Internet Relay Chat (IRC).
Registered ports contain numbers from 1024 to 49151. IANA maintains an official list of known and registered ranges. Dynamic or private - from 49152 to 65535. One of the options for using this range is for temporary ports.
History of creation
The concept of the port number was created by the early developers of ARPANET in an informal collaboration between software authors and system administrators.
The term "port number" was not used at that time. The number range for the remote host was a 40-bit number. The first 32 bits were similar to today's IPv4 address, but the first 8 bits were the most significant. The least significant part of the number (bits 33 through 40) denoted another object called AEN. This is the prototype of the modern port number.
On March 26, 1972, the creation of a catalog of socket numbers in RFC 322 was first proposed. Network administrators were encouraged to describe each fixed number for its functions and network services. This catalog was subsequently published in RFC 433 in December 1972 and included a list of hosts, their port numbers, and the corresponding function used on each node in the network. In May 1972, official assignments of port numbers, network services were first documented, and a special administrative function was proposed for maintaining this registry.
The first list of TCP ports had 256 AEN values, which were divided into the following ranges:
- 0 to 63: standard network-wide features
- 64 to 127: host-specific features
- 128 to 239: reserved for future use
- From 240 to 255: any experimental function.
Telnet received its first official assignment of 1. At the beginning of the existence of ARPANET, the term AEN also referred to the socket name used with the original connection protocol (MSP) and the network management program (NCP) component. At the same time, NCP was the forerunner of modern Internet protocols using TCP / IP ports.