Port Scanner

Understanding Port Scanning

A TCP port is a virtual endpoint for network communication identified by a number between 0 and 65535. The Internet Assigned Numbers Authority (IANA) divides ports into three ranges: well-known ports (0-1023) assigned to common services like HTTP (80) and HTTPS (443), registered ports (1024-49151) used by applications like MySQL (3306) and PostgreSQL (5432), and dynamic or private ports (49152-65535) used for ephemeral connections. When a service starts on a server, it binds to a specific port and listens for incoming connections — this is called an open port.

Port scanning is the process of checking which ports on a target host are accepting connections. A TCP connect scan attempts to complete the full three-way handshake (SYN, SYN-ACK, ACK) with each target port. If the handshake completes, the port is considered open. If the server responds with a reset (RST) packet or no response at all, the port is considered closed or filtered. Our scanner uses this method with a configurable timeout of 3 seconds per port, providing a balance between accuracy and speed. For the 22 most common ports, results typically return within 10-15 seconds.

Port scanning is an essential skill for network engineers, system administrators, and security professionals. Common use cases include verifying that firewall rules are correctly configured after a network change, confirming that a newly deployed service is listening on the expected port, auditing your own infrastructure for unexpected exposed services, and troubleshooting connectivity issues between application tiers. You can scan individual ports by entering comma-separated values or use the common ports preset to check 22 well-known services in a single scan. Remember that many hosts have firewalls or intrusion detection systems that may block or rate-limit scan attempts, and you should only scan hosts you own or have explicit permission to test.

Scan Methods and Responsible Use

While our scanner uses TCP connect scanning (completing the full three-way handshake), production tools like Nmap also support SYN stealth scanning — sending only a SYN packet and interpreting the response without completing the handshake — which is faster and less likely to be logged. Service fingerprinting goes beyond open/closed state: by analysing response banners, timing, and protocol behaviour, you can identify that port 443 is running nginx 1.24.0 with TLS 1.3, or that port 22 is running OpenSSH 9.6. Common port-to-service mappings include 80 (HTTP), 443 (HTTPS), 22 (SSH), 25 (SMTP), 53 (DNS), 3306 (MySQL), and 5432 (PostgreSQL). Always respect legal boundaries — scan only systems you own or have written permission to test, use gradual rate limiting, and coordinate with security teams for authorised penetration testing.