Common Network Ports Reference

What it is

A quick reference for commonly used network ports and their associated services, useful for network troubleshooting, security auditing, and general understanding of network communication.

Installation

This is a reference document, not a software tool, so no installation is required.

Core Concepts

• Port Numbers: A 16-bit number (0-65535) that identifies a specific process or service on a network device. Ports are divided into three ranges:
  • Well-Known Ports (0-1023): Reserved for critical system services. Usually require root/administrator privileges to bind to.
  • Registered Ports (1024-49151): Available for use by applications and services. IANA (Internet Assigned Numbers Authority) maintains a registry.
  • Dynamic/Private Ports (49152-65535): Used for ephemeral ports by clients and for private/unregistered services.

Commands / Usage

This is a reference, not a command-line tool. The "usage" is understanding which port number corresponds to which service.

TCP (Transmission Control Protocol) Ports

UDP (User Datagram Protocol) Ports

Common Patterns

• Checking if a port is open (Linux/macOS):

  nc -zv 192.168.1.100 80
  # or
  telnet 192.168.1.100 22
  

  This attempts to connect to port 80 (HTTP) or 22 (SSH) on 192.168.1.100 to see if a service is listening.

• Checking if a port is open (Windows PowerShell):

  Test-NetConnection -ComputerName 192.168.1.100 -Port 443
  

  This tests connectivity to port 443 (HTTPS) on 192.168.1.100.

• Listing listening ports (Linux/macOS):

  sudo netstat -tulnp
  # or
  sudo ss -tulnp
  

  This shows TCP (t) and UDP (u) listening (l) ports, along with the process ID (p) and name.

• Listing listening ports (Windows PowerShell):

  Get-NetTCPConnection -State Listen
  

  This lists all active TCP connections in a listening state.

• Firewall rule examples (conceptual):

  • Allowing inbound HTTP traffic: iptables -A INPUT -p tcp --dport 80 -j ACCEPT (Linux)
  • Allowing inbound SSH traffic: ufw allow ssh (Ubuntu/Debian)
  • Denying all inbound traffic by default: iptables -P INPUT DROP (Linux)

Gotchas

• Port Forwarding vs. Port Mapping: While related, port forwarding (on routers) directs traffic from an external IP/port to an internal IP/port, while port mapping (e.g., in Docker) maps a host port to a container port.
• Ephemeral Ports: Client applications typically use dynamic (ephemeral) ports (49152-65535) for outgoing connections. These are assigned by the OS and can change.
• Service Misconfiguration: A service might be configured to run on a non-standard port. Always verify by checking the service’s configuration file or process list.
• Firewalls: Network firewalls (both host-based and network devices) can block access to ports, even if a service is running.
• UDP vs. TCP: Understanding whether a service uses TCP or UDP is crucial for troubleshooting. TCP is connection-oriented and reliable; UDP is connectionless and faster but less reliable. For example, DNS primarily uses UDP for queries but can use TCP for zone transfers.
• IANA Port Assignments: While IANA assigns ports, they can be overridden or used by different services, especially in custom or embedded systems. Always confirm with actual network analysis.