--- title: "Port Reference Guide" description: "Interactive reference for common network ports used by system administrators and security architects. Search by port number or service, filter by protocol and category, with security notes." url: https://findutils.com/network/port-reference-guide/ category: network --- # Port Reference Guide Interactive reference for common network ports used by system administrators and security architects. Search by port number or service, filter by protocol and category, with security notes. **Use this tool:** [Port Reference Guide](https://findutils.com/network/port-reference-guide/) ## Programmatic access - REST id `port-reference-guide`: POST https://api.findutils.com/api/tools/port-reference-guide/execute (reference: https://findutils.com/api/port-reference-guide/) - MCP tool `port_reference_guide` on https://mcp.findutils.com (reference: https://findutils.com/mcp/port-reference-guide/) ## Why Use Port Reference Guide? Understanding network ports is essential for system architects designing secure infrastructure, firewall administrators writing access control rules, and security teams performing penetration tests or incident response. This interactive reference covers the most commonly used TCP and UDP ports with security ratings and detailed notes. Use it to quickly identify what services run on specific ports, assess their security implications, and make informed decisions about which ports to open or block in your firewall configurations. ## Tips for Network Port Security - Always replace insecure protocols with encrypted alternatives: use SSH (22) instead of Telnet (23), SFTP (22) instead of FTP (21), and HTTPS (443) instead of HTTP (80). - Apply the principle of least privilege to firewall rules. Only open the specific ports your services require, and restrict source IP ranges whenever possible. - Use port knocking or VPN access for sensitive management ports like SSH (22) and RDP (3389) to reduce exposure to brute-force attacks. - Run regular port scans on your own infrastructure with tools like nmap to detect unintended open ports or services that should have been decommissioned. - Document every open port in your infrastructure with a justification. Unexplained open ports are a common finding in security audits and penetration tests. ## Frequently Asked Questions ### What is a network port? A network port is a virtual endpoint for communication. It's a 16-bit number (0-65535) that identifies a specific process or service on a device. When combined with an IP address, a port creates a unique socket for network communication. For example, web servers typically listen on port 443 (HTTPS). ### What is the difference between TCP and UDP? TCP (Transmission Control Protocol) provides reliable, ordered delivery with error checking and flow control. It's used for web browsing, email, and file transfers. UDP (User Datagram Protocol) is faster but unreliable — it doesn't guarantee delivery or order. It's used for DNS queries, video streaming, and gaming. ### Which ports should I block in my firewall? Block all ports that aren't needed for your services. Common ports to block include: 23 (Telnet - unencrypted), 21 (FTP - unencrypted), 135-139 (NetBIOS), 445 (SMB), and 1433/3306 (databases) from external access. Always use the principle of least privilege — only open ports that are absolutely necessary. ### What are well-known ports? Well-known ports (0-1023) are assigned by IANA for common services. Registered ports (1024-49151) are for vendor applications. Dynamic/private ports (49152-65535) are used for temporary connections. On Unix/Linux systems, binding to well-known ports requires root privileges. ### How do I check which ports are open on my server? Use `netstat -tlnp` or `ss -tlnp` on Linux to see listening ports. On Windows, use `netstat -an`. For external scanning, tools like nmap can probe a host's open ports. Always ensure you have authorization before scanning — unauthorized port scanning may violate security policies or laws. ### What is port forwarding and when do I need it? Port forwarding redirects traffic arriving on one port of a router or firewall to a different internal IP address and port. You need it when hosting services behind NAT, such as running a web server at home on port 80, a game server, or enabling remote access to devices on your local network. ### Why are some ports marked as insecure in this guide? Ports are marked insecure when the default protocol running on them transmits data in plaintext without encryption. For example, port 21 (FTP), port 23 (Telnet), and port 80 (HTTP) send credentials and data unencrypted. Use their secure alternatives: SFTP on port 22, SSH on port 22, and HTTPS on port 443. ### Can two services use the same port number? Not simultaneously on the same IP address and protocol. However, one service can use TCP port 53 (DNS over TCP) while another uses UDP port 53 (DNS over UDP) on the same host, because TCP and UDP are treated as separate namespaces by the operating system. ### What is the difference between port scanning and port knocking? Port scanning probes a host to discover which ports are open and what services are running. Port knocking is a security technique where a specific sequence of connection attempts to closed ports triggers the firewall to open a service port temporarily, adding an extra layer of access control. ### How do I find which process is using a specific port? On Linux, run `sudo lsof -i:PORT` or `ss -tlnp | grep PORT`. On macOS, use `lsof -nP -iTCP:PORT`. On Windows, run `netstat -ano | findstr PORT` to get the PID, then `tasklist /FI "PID eq PID_NUMBER"` to identify the process name. ## Related Tools - [DNS Lookup](https://findutils.com/network/dns-lookup/) - [Security Headers Analyzer](https://findutils.com/network/security-headers-analyzer/) - [SSL Certificate Checker](https://findutils.com/network/ssl-certificate-checker/) - [DNS Security Scanner](https://findutils.com/network/dns-security-scanner/) - [IP Address Lookup](https://findutils.com/network/ip-address-lookup/) - [Subnet Calculator](https://findutils.com/network/subnet-calculator/) - [URL Safety Checker](https://findutils.com/security/url-safety-checker/) - [WHOIS Lookup](https://findutils.com/network/whois-lookup/)