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Lesson 6 Ephemeral and reserved port numbers
Objective Describe the significance of ephemeral and reserved port numbers.

Ephemeral and Reserved Port Numbers

Ephemeral port numbers

A network server process must use a “well-known” port number so that potential clients can locate it. The client also needs a port number to create a TCP/IP connection, but this port number need not be well known. Clients are assigned port numbers by the operating system, as part of the sequence of system calls that create a network connection. These port numbers are called “ephemeral” because they are valid only for the life of the connection and have no special significance.

Reserved port numbers

Port numbers in the range 1 to 1023 are considered “reserved” or “privileged.” TCP/IP conventions require that a connection using such low port numbers have special privileges, such as root privileges on the originating machine. On UNIX machines, a process cannot open a connection on low port numbers without root permissions.

Ephemeral port

An ephemeral port is a short-lived transport protocol port for Internet Protocol (IP) communications allocated automatically from a predefined range by the IP stack software. An ephemeral port is typically used by
  1. the Transmission Control Protocol (TCP),
  2. User Datagram Protocol (UDP), or
  3. Stream Control Transmission Protocol (SCTP)
as the port assignment for the client end of a client - server communication to a well-known port on a server. On servers, ephemeral ports may also be used as the port assignment on the server end of a communication. This is done to continue communications with a client that initially connected to one of the well-known service listening ports of the server. File Transfer Protocol (FTP) and Remote Procedure Call (RPC) applications are two protocols that can behave in this manner. Note that the term "server" here includes workstations running network services that receive connections initiated from other clients (e.g. Remote Desktop Protocol). The allocations are temporary and only valid for the duration of the communication session. After completion (or timeout) of the communication session, the ports become available for reuse.
Since the ports are used on a per request basis they are also called dynamic ports.

TCP UDP Port Numbers - Quiz

Click the Quiz link below to take a short multiple-choice quiz on client/server processes, TCP, UDP, and port numbers.
TCP UDP Port Numbers - Quiz

TCP/IP Illustration