In computer networks, a port is the end point of a connection in the operating system. The term is also used for hardware devices, but in software it is a logical construct that identifies a particular process or service.
A port is always associated with a host IP address and communication protocol type, and thus completes the session address assignment. It is identified for each address and protocol by 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 by convention to identify specific types of services on a host. Protocols that primarily use ports are for process control (such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) of the Internet protocol suite).
Meaning
TCP ports are not needed for direct point-to-point links when the computers at each end can only run one program at a time. They became necessary after the machines were able to run more than one program at the same time, and were connected to modern networks withpacket switching. In the client-server architecture model of the application, 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 request service instance to a leased line. A connection is made to a specific number, allowing additional customers to be served without waiting.
Details
Data transfer 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 a 16-bit unsigned integer. So it can be between 0 and 65535.
However, TCP ports cannot use number 0. A source port for UDP is optional, and a value of zero means none.
A process binds its input or output channels over an internet socket (file descriptor type) using a transport protocol, port number, and IP address. This process is known as bonding, and it enables data to be sent and received over the network.
The operating system's networking software is responsible for forwarding outgoing data from all application ports to the network, as well as forwarding incoming network packets (by matching IP address and number). Only one process can be boundto a specific IP address and port combination using the same transport protocol. Common application crashes, sometimes referred to as port conflicts, occur when multiple 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 shared services often use a specially reserved and well-known list of TCP and UDP ports to accept 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 conversation between the server and client using the same local port number. Other clients may continue to connect - this is possible because a TCP connection is identified as a chain of local and remote addresses and ports. Standard TCP and UDP ports are determined by convention under the control of the Internet Assigned Numbers Authority (IANA).
The core network services (primarily WorldWideWeb) tend to use small port numbers - less than 1024. Many operating systems require special privileges for applications to bind to because they are often considered critical to functioning of IP networks. On the other hand, the end client of the connection tends to use a large number of them dedicated 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 be easily 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 between packets based on their source or destination port numbers. A redirect is a classic example of this.
The practice of trying to connect to a range of ports in series on the same computer is known as port scanning. This is usually due to either malicious failure attempts or network administrators looking for possible vulnerabilities to help prevent such attacks.
The steps to open a TCP port are often monitored and logged by computers. This technique uses a number of spare connections to ensure an uninterrupted connection to the server.
Usage examples
The most important example of heavy use of TCP/UDP ports is the Internet mail system. The server is used to work with e-mail (sending and receiving), and in general needs two services. The first service is used for transport by email and from other servers. This is achieved using the Simple Mail Transfer Protocol (SMTP). Typically, the SMTP service application listens on TCP port number 25 to process incoming requests. The other service is POP (entirely Post Office Protocol) or IMAP (or Internet Message Access Protocol) which is requiredfor client applications in e-mail on users' machines to receive e-mail messages from the server. The POP services are listening on TCP port 110. The above services can both run on the same host machine. 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's listening port number is well defined (IANA calls them well-known ports), this client setting is often chosen out of the dynamic range. In some cases, clients and the server individually use specific TCP ports assigned by IANA. A good example is DHCP, where the client uses UDP 68 in all cases, and the server uses UDP 67.
Application in URLs
Port numbers are sometimes clearly visible on the Internet or other Uniform Resource Locators (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 8080 instead of the HTTP server.
TCP and UDP port list
As 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 commonly used port numbers for known internet services.
Port numbers are divided into threerange: well-known, registered and dynamic or private. Well-known (also known as system) are those with numbers from 0 to 1023. The requirements for new appointments in this range are more stringent than for other registrations.
Widely famous examples
Examples in this list include:
- TCP port 443: 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 are numbers from 1024 to 49151. The IANA maintains an official list of known and registered ranges. Dynamic or private - 49152 to 65535. One use case for this range is for ephemeral ports.
History of Creation
The concept of a port number was created by early ARPANET developers through an informal collaboration between software authors and system administrators.
The term "port number" was not yet in use at the time. The number string for the remote host was a 40-bit number. The first 32 bits were liketoday'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 entity, which was called AEN. This is the prototype of the modern port number.
On March 26, 1972, the creation of a catalog of socket numbers was first proposed in RFC 322. Network administrators were encouraged to describe each permanent number for its functions and network services. This directory was subsequently published in RFC 433 in December 1972 and included a list of hosts, their port numbers, and the corresponding function used by each node on the network. In May 1972, the official assignments of port numbers, network services, and a special administrative function for maintaining this registry were first documented.
The first list of TCP ports had 256 AEN values, which were divided into the following ranges:
- 0 to 63: Network-wide standard functions
- 64 to 127: host-specific features
- 128 to 239: reserved for future use
- 240 to 255: Any experimental function.
The Telnet service received its first official assignment of a value of 1. In the early days of ARPANET, AEN was also a socket name used with the original Connection Protocol (MSP) and Network Control Program (NCP) component. At the same time, NCP was the forerunner of modern Internet protocols using TCP/IP ports.
