What Is a Network Port?
A port is a logical number (0–65535) that identifies a specific service running on a server. The IP address gets you to the machine; the port number gets you to the right service. Think of it as an apartment building: the IP is the street address, the port is the unit number.
- Well-known ports (0–1023): Assigned by IANA to standard protocols — HTTP (80), HTTPS (443), SSH (22)
- Registered ports (1024–49151): Common applications — MySQL (3306), Redis (6379), PostgreSQL (5432)
- Dynamic ports (49152–65535): Assigned temporarily by the OS for client-side connections
Common Port Numbers Reference
| Port | Protocol | Service |
|---|---|---|
| 21 | TCP | FTP (file transfer) |
| 22 | TCP | SSH (secure remote login) |
| 25 | TCP | SMTP (mail sending) |
| 53 | TCP/UDP | DNS (domain resolution) |
| 80 | TCP | HTTP (web) |
| 443 | TCP | HTTPS (secure web) |
| 3306 | TCP | MySQL database |
| 5432 | TCP | PostgreSQL database |
| 6379 | TCP | Redis cache |
| 8080 | TCP | HTTP alternate (dev environments) |
Common Use Cases for Port Scanning
- Server operations: Verify a service is actually listening on its expected port
- Firewall debugging: Confirm firewall rules allow or block the correct ports
- Security auditing: Identify which ports are exposed and close unnecessary ones
- API connectivity testing: Confirm database or API ports are reachable from the internet (or are properly blocked)
How to Check Open Ports Online
- Go to tool.tl/portscan
- Enter a domain name or IP address
- Select ports to check or enter custom port numbers
- Results show each port's status: Open, Closed, or Filtered
Understanding Port Status
- Open: Port is reachable and a service is actively listening — connection succeeds
- Closed: Port is reachable but no service is listening — the service may be stopped
- Filtered: Firewall is dropping probe packets — port state is unknown (most common security configuration)
Combining Network Tools for Troubleshooting
- Ping: Check if a host is reachable at all (ICMP)
- Traceroute: See which routing hops packets take to reach a server
- DNS Lookup: Verify domain name resolution is working correctly
- WHOIS: Look up domain or IP ownership and registration
Frequently Asked Questions
Is port scanning legal?
Scanning your own servers is completely legal and standard practice. Scanning servers you don't own without authorization may violate laws (such as the Computer Fraud and Abuse Act in the US). Online port checkers perform external probes of public-facing ports, similar to what any internet connection test does — but high-volume scanning of others' infrastructure carries legal risk.