TCP ports. TCP and UDP port list

In computer networks, the port is the endpoint of communication in the operating system. This term is also used for hardware devices, but in software it is a logical construction that identifies a specific process or type of service.

The port is always associated with the IP address of the host and the type of the communication protocol, and thus ends the assignment of the session address. It is identified for each address and protocol using a 16-bit number, commonly known as the port number. Specific port numbers are often used to identify specific services. Of the thousands listed, 1024 well-known port numbers are protected in accordance with the agreement to determine specific types of services on the host. Protocols that primarily 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 execute more than one program at one time, and were connected to modern networks with packet switching. In the client-server architecture model, applications, ports and network clients are connected to service initiation, provide multiplexing services after the initial communication is associated with a known port number, and it is released by switching each instance of the request service to the leased line. There is a connection to a specific number, and as a result, additional customers can be serviced without waiting.

Details

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are used to specify the destination port number and source in their segment headers. The port number is a 16-bit unsigned integer. Thus, it can be in the range from 0 to 65535.

However, TCP ports can not use the number 0. A source port for UDP is optional, and a value of zero means its absence.

The process connects its input or output channels via the Internet socket (file descriptor type) via transport protocol, port number and IP address. This process is known as binding, and it enables the transmission and reception of data through the 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 the incoming network packets (by mapping the IP address and number). Only one process can be bound to a specific IP address and a combination of ports using the same transport protocol. Common application failures, sometimes referred to as port conflicts, arise when several programs try to communicate with the same port numbers on the same IP address using the same protocol.

How they are used

Applications that implement common services often use a specially reserved and well-known list of TCP and UDP ports for receiving service requests from clients. This process is known as listening, and it involves receiving a request from a well-known port and establishing a dialog between the server and the one-to-one client, using the same local port number. Other clients can continue to connect - this is possible, since the TCP connection is identified as a chain consisting 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), as a rule, uses small port numbers - less than 1024. Many operating systems require special privileges for applications to bind to them, because they are often considered critical for the operation of IP networks. On the other hand, the final connection client, 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 packet, and they can easily be interpreted not only by the sending and receiving computers, but also by other components of the network infrastructure. In particular, firewalls are generally configured to distinguish between packets depending 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 one computer is known as scanning them. This is usually associated with either attempts at a malicious failure, or network administrators are looking for possible vulnerabilities to help prevent such attacks.

The actions aimed at how to open the TCP port are often monitored and logged using computers. This technique uses a number of spare connections 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 requires 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, the 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) that is required for client applications in e-mail on user machines to receive e-mail messages from the server. POP services listen to the numbers from TCP port 110. The above services can both be run on the same host computer. When this happens, the port number distinguishes 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, clients and the server individually use specific TCP ports assigned to the IANA. A good example is DHCP, where the client in all cases uses UDP 68, and the server is UDP 67.

Use in URLs

Port numbers are sometimes clearly visible on the Internet or other unified pointers to information resources (URLs). By default HTTP uses TCP port 80, and HTTPS - 443. However, there are other variations. For example, the URL http://www.example.com:8080/path/ indicates that the web browser connects to the 8080 instead of the HTTP server.

TCP and UDP port list

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 the registration of frequently used port numbers for known Internet services.

Port numbers are divided into three ranges: well known, registered and dynamic or private. Well-known (also known as system ones) are numbers from 0 to 1023. Requirements for new assignments in this range are more stringent than for other registrations.

Well-known examples

The 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 use case of this range is for temporary ports.

History of creation

The concept of the port number was created by the early ARPANET developers in the context of 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 most significant were the first 8 bits. The least significant part of the number (bits 33 to 40) denoted another object, called AEN. This is the prototype of the modern port number.

On March 26, 1972, it was first proposed to create a socket number catalog in RFC 322. Network administrators were called upon to describe each permanent 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 on the network. In May 1972, for the first time, official port number assignments, network services were documented, and a special administrative function was proposed to maintain this registry.

The first list of TCP ports had 256 AEN values, which were divided into the following ranges:

• 0 to 63: the standard functions of the entire network
• 64 to 127: host-specific functions
• 128 to 239: reserved for future use
• From 240 to 255: any experimental function.

The Telnet service received the first formal assignment of value 1. At the beginning of ARPANET, the AEN term was also used to refer to the socket name that was used with the original connection protocol (MSP) and the Network Management Program (NCP) component. At the same time NCP was the precursor of modern Internet protocols using TCP / IP ports.