Port Scanning Guide — Understanding Open Ports and Network Security

Understanding network ports is fundamental for anyone who manages servers, configures firewalls, or does security work. This guide explains what ports are, which ones matter, and how to safely check which ports are open on your systems.

What is a Network Port?

A port is a virtual endpoint for network communication. Your server has one IP address but can run many services simultaneously — each on a different port. Port numbers range from 0 to 65535.

Commonly Used Ports

Port   Protocol  Service
21     TCP       FTP (file transfer)
22     TCP       SSH (secure shell)
25     TCP       SMTP (email sending)
53     UDP/TCP   DNS (domain name resolution)
80     TCP       HTTP (web, unencrypted)
443    TCP       HTTPS (web, encrypted)
3306  TCP       MySQL database
5432  TCP       PostgreSQL database
6379  TCP       Redis
27017 TCP       MongoDB
8080  TCP       HTTP alternate / proxies

Open vs Closed vs Filtered Ports

• Open — a service is actively listening and accepting connections
• Closed — no service is listening, but the port is reachable
• Filtered — a firewall is blocking the port; no response

Security Best Practices

• Close all ports you are not actively using
• Never expose database ports (3306, 5432) to the public internet
• Change SSH from port 22 to a non-standard port to reduce automated attack attempts
• Use a firewall (AWS Security Groups, iptables, UFW) to whitelist only necessary IPs