What Is a Port Number?

Updated on May 12, 2025

Port numbers are a key part of how the internet works, allowing devices to communicate over a network. This guide covers what port numbers are, how they work, and why they’re important in networking. We’ll break it down step by step, starting with the basics.

Definition and Core Concepts

What Is a Port Number?

A port number is a 16-bit number (ranging from 0 to 65535) that serves as a communication endpoint for processes on a networked device. When an application communicates over a network using the Transmission Control Protocol (TCP) or User Datagram Protocol (UDP), the port number identifies the specific service or application on that device. It allows multiple devices and applications to communicate without confusion.

Think of a port number as a unique “channel” for communication on a device. When combined with an IP (Internet Protocol) address, it forms a socket, which uniquely identifies a connection.

Core Concepts of Network Communication

IP Address 

An IP address is the logical address of a device on a network. Just as a house has an address, every device on a network has an IP address. While the IP address identifies the device location, it is the port number that specifies the exact service or application on that device.

TCP (Transmission Control Protocol) 

TCP is a connection-oriented protocol. It establishes a reliable connection between sender and receiver using mechanisms like the three-way handshake. TCP ensures the delivery of packets in the correct order and without loss. 

UDP (User Datagram Protocol) 

UDP, unlike TCP, is connectionless and does not guarantee reliable delivery. It’s faster and more lightweight, making it ideal for applications like streaming or real-time communication where speed matters more than reliability.

Socket 

A socket is a combination of an IP address and a port number. It acts as the endpoint for two-way communication between applications.

Listening Port 

A listening port is a port number on which a server application waits for incoming client connections. For example, a web server listens on port 80 for HTTP requests or port 443 for HTTPS requests.

Port Number Ranges

There are three main categories of port numbers, each with distinct functions:

• Well-Known Ports (0–1023): Reserved for commonly used protocols and services. For example:
  • Port 80 for HTTP
  • Port 443 for HTTPS
  • Port 22 for SSH
• Registered Ports (1024–49151): Used by specific applications. For instance:
  • Port 1433 for Microsoft SQL Server
  • Port 3306 for MySQL
• Dynamic/Private Ports (49152–65535): Assigned to temporary or private connections, often by the operating system, for client-side communication.

How Port Numbers Work

Multiplexing

Port numbers enable multiplexing, allowing multiple applications to use the same IP address without interference. A single device can simultaneously handle multiple services like browsing the web, receiving emails, and making video calls because each application uses a unique port number.

An analogy would be an apartment building with one street address (the IP address) and individual apartment numbers (port numbers) for each tenant.

Demultiplexing

Demultiplexing is the reverse process, where the operating system uses port numbers to direct incoming network traffic to the correct application. For example:

• A packet arriving on port 80 is sent to the web server application.
• A packet arriving on port 25 is sent to the mail server.

Connection Establishment in TCP

Port numbers play a critical role in establishing connections via the TCP three-way handshake:

1. SYN (synchronize): The client sends a SYN packet to the server’s IP address and port number (e.g., 192.168.1.1:443).
2. SYN-ACK (synchronize-acknowledge): The server responds with a SYN-ACK packet.
3. ACK (acknowledge): The client sends an ACK packet to complete the handshake.

The SYN packet from the client has the SYN flag set. The server’s SYN-ACK packet has both the SYN and ACK flags set. The client’s final ACK packet has only the ACK flag set.

Data Transmission in TCP/UDP

Once the connection is established (in TCP) or initialized (in UDP), both protocols include port numbers in their packet headers. These headers identify the sending and receiving processes during data exchange, ensuring smooth communication.

Key Features and Components

16-Bit Integer

Port numbers are 16-bit integers, which means there are 2^16 possible ports (65536 in total). This range allows for a wide variety of applications and processes to coexist without conflict.

Protocol-Specific

Port numbers are specific to transport-layer protocols like TCP or UDP. For instance, port 80 used in TCP does not interfere with port 80 in UDP, as each protocol maintains its own namespace.

Well-Defined Assignments

The Internet Assigned Numbers Authority (IANA) standardizes port assignments for common services to ensure consistency across networks.

Use Cases and Applications

Port numbers are integral to many common networking scenarios, including:

Web Browsing (HTTP/HTTPS)

• Port 80: Used for unencrypted HTTP traffic.
• Port 443: Used for encrypted HTTPS traffic to ensure secure communication.

Email

• Port 25 (SMTP): Used for sending emails.
• Port 110 (POP3): Used for retrieving emails.
• Port 143 (IMAP): Used for managing and synchronizing emails across devices.

DNS (Domain Name System)

• Port 53: Used for resolving domain names into IP addresses. DNS operates over both TCP and UDP, depending on the size and type of the query.

FTP (File Transfer Protocol)

• Port 21: Used for the control connection in FTP. 
• Port 20: Used for the data connection in FTP’s active mode for transferring files.

SSH (Secure Shell)

• Port 22: Provides secure remote access to servers and devices via an encrypted channel.

Key Terms Appendix

• Port Number: A 16-bit integer that identifies a specific process or service on a network device. 
• IP Address: A numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication. 
• TCP (Transmission Control Protocol): A connection-oriented, reliable transport layer protocol. 
• UDP (User Datagram Protocol): A connectionless, unreliable transport layer protocol. 
• Socket: An endpoint of a two-way communication link between two programs running on a network, consisting of an IP address and a port number. 
• Multiplexing: Combining multiple data streams onto a single channel. 
• Demultiplexing: Separating multiple data streams from a single channel.