Port Checker Guide: How to Check If a Port Is Open

If an IP address is a house, then TCP Ports are the 65,535 different doors and windows on that house.

When you configure a web server, database, or game server, you are essentially telling the computer to open a specific door and listen for traffic. A Port Checker is a crucial diagnostic tool used to test whether those doors are actually open, or if a strict firewall is slamming them shut.

---

🚪 Common TCP Port Cheat Sheet

You don’t need to memorize all 65,535 ports. Only a handful are critical for modern system administration:

Port Number	Protocol Name	The Real-World Use Case
22	SSH	Remote terminal access for Linux server administration.
80	HTTP	Unencrypted, raw web traffic.
443	HTTPS	Encrypted, highly secure web traffic (SSL/TLS).
3306	MySQL	Direct connection to remote database servers.
3389	RDP	Windows Remote Desktop connections.
25565	TCP/UDP	The default port for Minecraft servers.

---

🚦 The 3 States of a Port: Open, Closed, or Filtered

When a port checker pings a server, it doesn’t just say “Yes” or “No.” The nuanced response tells you exactly where the network failure is occurring:

• 🟢 OPEN: A service (like an Apache web server) is actively running on the machine, listening to the port, and accepting connections. Everything is working perfectly.
• 🔴 CLOSED: The port checker successfully reached the server, but the server explicitly replied “There is no software running on this port right now.” This usually means your software crashed.
• 🛡️ FILTERED (Stealth): The port checker sent the request, but the server completely ignored it and dropped the packet into a black hole. This guarantees that a harsh hardware or software firewall (like AWS Security Groups) is blocking the connection entirely.

---

🚀 Is your server blocking traffic? Stop guessing and start diagnosing. Use our completely free Port Checker to safely probe external IPs and verify if critical TCP ports are open, closed, or filtered by a stealth firewall!