What Are Ephemeral Ports?

An ephemeral port is a temporary network communication endpoint used to establish a connection with a server application over the internet or a local network. Ports have port numbers, which are numeric values used to identify communication endpoints in an IP network. Port numbers help direct network traffic to the appropriate end destination and facilitate communication. The system ensures that the selected port is not in use by any other active connection to avoid conflicts.

The purpose of ephemeral ports is to allow multiple client applications to establish simultaneous connections to a server without interfering with each other. Once the communication session between the client and server is complete or terminated, the ephemeral port is released and becomes available for reuse by other applications when necessary.

Ephemeral ports and TCP/IP communication

Transmission control protocol/internet protocol (TCP/IP)  is an established procedure for the transfer of data and communication over the internet. These rules provide a reliable and standardized method for data transmission between devices. When establishing a TCP/IP connection, two endpoints are created, each with an IP number. An IP address is a unique string of numbers and periods assigned to each device connected to a computer network. The IP address identifies the device and network and allows for communication between devices over a network. 

The TCP connection typically has four identifiers: the server IP, server port, client IP, and client port. The server IP, server port, and client IP are generally known. When this connection is established, you must either have a specified client port or have a temporary ephemeral port number assigned. These temporary ports are typically assigned by each machine’s IP number, and machines use specific ranges for different purposes. After the connection ends, the ephemeral releases and can facilitate a different connection when needed.

Why are ephemeral ports important?

Ephemeral ports are a critical part of how the internet functions and are important for establishing temporary connections between clients and servers. Servers typically use fixed port numbers so clients will know where to send communications. Clients typically do not need a fixed port number because servers do not typically initiate communication with them. When the client sends a PORT command to establish a connection point, the server then responds using a temporary connection point with a specified ephemeral port number. 

Organizations can restrict ephemeral port selection to enhance security. Some organizations may have protocols related to which ports employee computers use for data access and transfer. This can limit the port options and make it harder for unauthorized users to access and attack your system.

Types of ports

In networking, you might use well-known ports (0 to 1023), registered ports (1024 to 49151), and dynamic or ephemeral ports (49152 to 65535), as defined by the Internet Assigned Numbers Authority (IANA). System processes or services (like port 80 for HTTP or port 25 for SMTP) use well-known ports, while client applications use ephemeral ports during communication sessions. Depending on your operating system, the port numbers may vary, so it can be helpful to know what ranges are most common for your type of device and system.

Related terms

• Firewall

• Domain name system

• ITIL

• Debugging

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