TCP Port Checker

A port is open when a TCP connection can be established — the target host accepted the connection. This means a service is actively listening on that port and the firewall allows the traffic through. Open does not mean the service is necessarily healthy or authenticated, only that the TCP handshake succeeded.

Closed means the host sent a TCP RST (reset) packet, actively refusing the connection. The port is reachable but no service is listening. Filtered (shown as timeout) means no response arrived — the packet was silently dropped by a firewall or access control list. The distinction matters: closed means the host is up, filtered means a firewall is in the path.

Most ISPs and cloud providers block outbound port 25 to prevent spam. Email servers receive inbound mail on port 25 but clients should submit email using port 587 (SMTP submission with authentication). If you are setting up a mail server, check that your hosting provider allows inbound port 25 from other mail servers.

Nmap is a powerful local tool that can scan many ports simultaneously, detect OS and services, and use various techniques. This tool checks a single port from the server side using a simple TCP connect — useful for quick verification without installing software. For comprehensive security scanning of your own infrastructure, use nmap locally.

This tool only makes a standard TCP connection attempt — the same as any normal network request. You should only test ports on hosts you own or have permission to test. Unauthorized port scanning may violate terms of service or local laws.

Well-known ports are 0–1023, assigned by IANA to standard services (SSH=22, HTTP=80, HTTPS=443). Registered ports are 1024–49151, assigned to specific applications. Ephemeral ports are 49152–65535, used temporarily by clients for outbound connections. When you open a webpage, your browser uses a random ephemeral port as the source and connects to the server's port 443.