The port checker tests whether a specific TCP port is open on a remote host. Enter a hostname or IP address and a port number to get an instant result — open, closed, or filtered — with response time and a short explanation of what it means.
It is useful for system administrators and network engineers who need to quickly verify firewall rules, confirm service availability, or troubleshoot connectivity without installing additional tools.
Port checker
Check whether a TCP port is open on a remote host.
How to use
- Enter a hostname or IP address in the first field, such as
example.comor203.0.113.10. - Enter the port number you want to test, such as
443,22, or3389. - Click Check port and wait for the result.
- Review the status badge — OPEN, CLOSED, or FILTERED — along with the response time and a short explanation.
- Use Copy result to copy the summary to the clipboard if you need to share or document it.
Understanding the result
OPEN means the TCP connection was accepted. The host is reachable on that port and a service is actively listening. This is the expected result for publicly accessible services such as a web server on port 443 or an SSH server on port 22.
CLOSED means the connection was refused immediately. The host is reachable but no service is listening on that port, or the service is explicitly rejecting connections. This typically happens when a service is stopped or configured to reject connections from outside.
FILTERED means no response was received within 5 seconds. This usually indicates a firewall is silently dropping packets to that port rather than refusing them. A filtered result does not mean the host is down — it means something is blocking the connection before it reaches the service.
FAQ
Port checker help
CLOSED means the target host actively refused the connection — the port sent back a TCP RST packet. FILTERED means no response came back within the timeout period. A firewall is likely dropping packets silently. CLOSED confirms the host is reachable; FILTERED does not.
When a port is closed, the host sends back a refusal immediately — typically in under 1 millisecond. The tool rounds to the nearest millisecond, so very fast refusals display as 0 ms. This is correct behaviour and not a display error.
No. This tool runs checks from the zaur.it server, which has no access to your local network. Private address ranges such as 192.168.x.x, 10.x.x.x, and 172.16–31.x.x are blocked. To test ports on internal hosts, use PowerShell’s Test-NetConnection or a local port scanning tool.
No. This tool checks TCP ports only. UDP is a connectionless protocol and does not confirm whether a port is open in the same way TCP does. Reliable UDP port checking requires raw socket access, which is not available on shared hosting.
OPEN means the TCP connection was accepted — a process is listening on that port. It does not guarantee the service is functioning correctly. A web server could accept connections on port 443 but return errors, or an SSH daemon could be listening but rejecting authentication.
Common TCP ports include: 22 (SSH), 25 (SMTP), 53 (DNS), 80 (HTTP), 443 (HTTPS), 3389 (RDP), 1433 (SQL Server), 3306 (MySQL), 5985 (WinRM HTTP), 5986 (WinRM HTTPS). Use the result alongside your firewall rules to confirm expected reachability.
Practical examples
Example 1: You updated a firewall rule to allow inbound RDP on port 3389. Enter the server’s public IP and port 3389 to confirm the rule is active and the port is reachable before attempting a Remote Desktop connection.
Example 2: A web application is not loading over HTTPS. Check port 443 on the server hostname to determine whether the issue is at the network level — a FILTERED or CLOSED result confirms the connection is being blocked before reaching the application.
Example 3: You deployed a new service and want to confirm it is listening on the expected port before updating DNS or notifying users. Check the port directly against the server IP to verify the service started correctly.
Useful links
- DNS lookup — resolve hostnames to IP addresses before running a port check.
- My IP and network info — find your public IP address and connection details.
- Network diagnostics tool — check HTTP and HTTPS reachability with latency measurements.
Other tools
- DNS lookup — check A, AAAA, MX, TXT, CNAME, NS, and other record types.
- My IP and network info — inspect your public IP, ISP, country, and connection details.
- Network diagnostics tool — run HTTP reachability checks and measure response time and jitter.
- IP subnet calculator — calculate subnet ranges, masks, available hosts, and allocation details.