Port (computer networking)

A port is part of a network address, causes the assignment of TCP and UDP connections, and data packets to the server and client programs by operating systems. For each combination of these two protocols are two ports, one each. On the client side and the server

Purpose

Ports serve two purposes:

• Primary ports are a feature to distinguish multiple connections between the same pair of endpoints.
• Ports can also identify network protocols and related networking services.

• A web browser can start another download from the same server during a download because the browser then opens another port and an additional connection to the same port 80 of the server builds. The server responds to the different ports of the browser with different in each case together related content. For a differentiation of the compounds thus meet different port numbers at only one of the two endpoints.
• The port 25 is reserved for the Simple Mail Transfer Protocol (SMTP). If a connection to port 25 on a server is established, this connection is accepted by the SMTP service on the server, if this service is available.

Operation

Start end servers require the operating system to specific ports to accept connections. These are usually 'well known ports ', since the port number will not be published or must be known. The numbers are rather assigned to the programs in question fixed.

Clients typically require the operating system to a random port (usually in the very high range above 30000 ) to establish connections. However, a client can also bind to a specific port ( call to bind () ); This is necessary for example for certain connectionless protocols, however, limited the use of this client on a process per computer.

The connection is initiated by the client that needs to know the server's port. By establishing the connection, the server learns the port on the client, so that he can answer this. In the case of the use of a 'well -known- ports ', the port number is changed on the server side in order to continue to provide the well-known, standardized port number for new connections can simultaneously.

History

Port numbers were originally called AEN, which stood for Another Eightbit Number.

1981, defined by the Transmission Control Protocol, a much greater range of 16 bits.

The standardization is largely due to initiatives by Jon Postel in the early 1970s.

Standardization

Since 2011 with RFC 6335 describes a Request for Comments the method by which the Internet Assigned Numbers Authority (IANA) assigns ports protocols. There are three different areas:

System ports

Ports 0-1023

These ports are called well known ports. New assignments are made only with the participation of the Internet Engineering Task Force ( IETF).

User ports

Ports 1024-49151

In this area can be made at the request new assignments without the participation of the IETF.

Dynamic Ports

Ports 49152-65535

This area is intended for the free disposal.

Importance

Deviations from the IANA assignments make it difficult to contact, without being able to prevent it.

An overview of mappings are the list of standard ports.

Service name

Ports, in addition to protocols also assigned names for the corresponding services. One such service name can be used for example for SRV resource records.

On Unix machines, this list is defined in the / etc / services file.

On operating systems, the Windows NT line you will find it under % WINDIR % \ system32 \ drivers \ etc \ services in the Windows 9x line directly in the Windows folder (% WINDIR % \ services ).

Port filter

A port forwarding causes a contacted port is forwarded via a second port to another system.

A port lock is blocking or throttling a certain Portes or of that access the service. Blocking the ports is used by some Internet service providers to block unwanted services. For providers services can for example be undesirable because they cause a lot of traffic, such as file-sharing.

Ports can also be blocked by a firewall. For a local firewall is normally proceed the other way around: only the actually required ports are enabled and all other ports are blocked. Thus, the points are reduced to the protected network or the PC.

Port Scanner

Port scanners are used to locate ports and can be a sign of an attack. Supports firewall or packet filter the port knocking, a port scan can be hindered by ports are opened only if accompanied by a pre-agreed sequence of packets sent.