Check if common ports are open on a host
Note: Browser-based port checking has limitations. Results may not be 100% accurate due to CORS and firewall policies. For precise results, use nmap or similar tools.
Port Checker tests whether a specific TCP port is reachable on a public host, either an IP address (like 8.8.8.8) or a domain name (like example.com). You enter the target host and a port number from 1 to 65535, and the tool attempts a connection to report the port as open (accepting connections), closed, or filtered (blocked or dropped by a firewall).
It is used by developers verifying that a web server, database, SSH daemon, or game server is exposed correctly, by sysadmins confirming firewall and NAT port-forwarding rules, and by anyone troubleshooting why a service cannot be reached. Because the check runs from an external server, it shows what the internet actually sees rather than what is reachable only from your local network.
The tool performs a TCP connection attempt, the same three-way handshake (SYN, SYN-ACK, ACK) that any client uses to open a connection. Our server sends a SYN packet to the target host on the chosen port. If it receives a SYN-ACK, the port is reported open; if it receives a TCP RST (reset), the port is closed because nothing is listening; and if no response arrives before the timeout, the port is reported filtered, which usually means a firewall is dropping the packet.
This distinction matters for diagnosis. A closed port means the host is reachable but the service is not running or not bound to that port. A filtered or timed-out result means traffic is being blocked before it reaches the service, pointing to a firewall rule, a security group, or missing NAT port forwarding. Note that the check tests TCP only; UDP services such as DNS or SNMP cannot be confirmed this way because UDP is connectionless and produces no handshake.
Because the probe originates from our external server, it reflects reachability from the public internet. A port that appears open on your LAN can still show as filtered here if your router or cloud firewall is not forwarding it, which is exactly the failure this tool is designed to catch.
Closed means the host actively refused the connection with a TCP reset, so it is online but nothing is listening on that port. Filtered (or timed out) means no response came back at all, which typically indicates a firewall silently dropping the traffic.
No. This checker uses TCP connection attempts, which rely on the TCP handshake. UDP is connectionless and does not respond to a connect probe, so services like DNS (53), SNMP (161), or many game servers cannot be reliably confirmed this way.
The port being open only means a TCP handshake succeeded. The service behind it may still reject your request at the application layer, require authentication, filter by source IP, or expect a different protocol than the client is sending.
You can only check hosts reachable from the public internet. Private addresses (such as 192.168.x.x or 10.x.x.x) and localhost are not reachable from our server, so you would need to test against your public IP with the appropriate port forwarding in place.
Yes, it is completely free with no sign-up. We only use the host and port you submit to perform the connection test; we do not require personal information, and results are shown to you rather than published.
A single-port check against a host you own or administer is standard troubleshooting. Scanning ranges of ports on systems you do not control may violate acceptable-use policies or local law, so only check hosts you are authorized to test.
Common examples are 80 (HTTP), 443 (HTTPS), 22 (SSH), 25/587 (SMTP), 3306 (MySQL), 5432 (PostgreSQL), and 3389 (RDP). If you are unsure which port a service uses, the Port Reference tool lists standard assignments.