Open port checker
Tests whether one or more TCP ports are reachable on a public host and reports each as open, closed or filtered, with the connection latency.
How the port checker works
The tool opens a TCP connection from our server to each port you enter and watches what comes back. A completed handshake means the port is open and a service is listening behind it. This is the same technique a port scanner uses, limited to the ports you specify so it stays fast and polite.
The check runs from the public internet, so it sees exactly what an outside visitor would see, not what is reachable from inside your own network. A port that is open on localhost can still be firewalled from the outside, and this is precisely the difference the tool helps you confirm.
Each port is probed in parallel with a short timeout, so a full check of several ports finishes in a couple of seconds. Latency is the time the TCP handshake took, which is a rough proxy for the network distance to the host.
How to read the results
Each port falls into one of three states, and telling them apart is the whole point:
- Open - the connection was accepted, so a service is listening and reachable. For a web server you expect 80 and 443 open.
- Closed - the host answered but refused the connection (a TCP reset). The host is alive and reachable, but nothing is listening on that port.
- Filtered - no answer arrived before the timeout. A firewall is silently dropping the packets, so you cannot tell whether a service is behind it.
Common problems
Every port shows as filtered. A firewall or security group is dropping all inbound traffic, or the host is down. Check that the host name resolves to the address you expect and that inbound rules allow the ports from outside.
The port is open locally but filtered here. The service listens only on 127.0.0.1 or on an internal interface, or a cloud security group blocks external access. Bind the service to the public interface and open the port in the firewall.
Closed when you expected open. The host is reachable but the service is not running or is listening on a different port. Confirm the service is started and check the actual listening port with a local command such as ss -tlnp.
Frequently asked questions
What is the difference between closed and filtered?
Closed means the host actively refused the connection with a TCP reset, so it is reachable but no service listens on that port. Filtered means no response arrived at all, which usually points to a firewall silently dropping the packets.
Why can a port be open on my machine but filtered here?
This tool connects from the public internet. A service bound only to localhost or an internal interface, or a firewall or cloud security group, can make a locally open port unreachable from outside. That outside view is exactly what visitors experience.
Does this scan UDP ports?
No. The checker tests TCP ports only, because TCP has a clear handshake that lets us distinguish open from closed reliably. UDP is connectionless and gives ambiguous results without sending service-specific probes.
How many ports can I test at once?
Up to 10 ports per check. Enter them separated by commas, for example 80,443,22. This keeps each check fast and avoids looking like an aggressive scan to the target host.
Watch your ports around the clock
ePulz.io can monitor a TCP port from one minute and alert you by email or Telegram the moment it stops accepting connections.
Try for free →About this tool
An open port checker tells you, from the outside, which services on a host accept connections. It is the fastest way to confirm a firewall rule, verify a freshly deployed service is reachable, or diagnose why clients cannot connect. The three states - open, closed and filtered - each point to a different cause, so reading them correctly saves a lot of guesswork.