TCP Port Check
Is a TCP port open on a host? (host:port)
Enter a value above to run TCP Port Check
e.g. example.com
Quick answer
TCP Port Check tells you whether a given TCP port on a host is open and accepting connections. "Open" means a service is listening there; "closed" or "filtered" means nothing answered or a firewall blocked the attempt.
TCP Port Check tells you whether a specific TCP port on a given host will accept a connection right now. You enter a hostname or IP address and a port number, and the tool attempts a TCP handshake to that endpoint. It reports whether the port is open (something is listening and accepting connections), closed, or unreachable due to filtering. This is the fastest way to confirm that a web server, mail server, database, SSH daemon, or any other service is actually reachable on the network before you dig into application logs or configuration.
How it works
The tool opens a TCP socket to the host and port you specify and starts the three-way handshake (SYN, SYN-ACK, ACK). If the remote service completes the handshake, the port is reported open. If the host actively rejects the attempt (RST), the port is closed and nothing is listening. If no reply comes back before the timeout, the connection is likely blocked by a firewall or the host is down. Unlike a simple ping, this checks the exact service port, not just whether the machine responds to ICMP.
How to read the results
Open means a process is bound to that port and accepting connections; your firewall and routing are fine for that service. Closed means the host answered but nothing is listening on that port, usually a stopped service or the wrong port number. Timeout or filtered means packets are being dropped silently, typically by a firewall or security group rule, or the host is offline. An open TCP port confirms reachability, but it does not verify that the application behind it is healthy or answering protocol requests correctly.
Why it matters
When a website, API, or mail server stops working, you need to know if the problem is the network path or the application itself. Checking the port from outside your network isolates that quickly. If port 443 is open but your site errors, the web server is running and the issue is higher up. If 443 times out, the traffic never reaches the service and you should look at firewall rules, security groups, or DNS pointing to the wrong IP.
Frequently asked questions
- What is the difference between closed and filtered?
- Closed means the host actively refused the connection with a TCP RST packet, so the machine is up but no service is listening on that port. Filtered (or timeout) means no response came back at all, so a firewall or security rule is dropping the packets silently, or the host is unreachable. Closed points to a service problem; filtered points to a network or firewall problem.
- Which common ports should I test?
- For web servers check 80 (HTTP) and 443 (HTTPS). For email check 25 and 587 (SMTP), 465 (SMTPS), 143/993 (IMAP), and 110/995 (POP3). SSH usually runs on 22, DNS over TCP on 53, MySQL on 3306, PostgreSQL on 5432, and RDP on 3389. Testing the exact port your service uses is more reliable than assuming defaults.
- Can this tool check UDP ports?
- No, this tool checks TCP ports only, because it relies on the TCP handshake to confirm a listening service. UDP is connectionless and gives no handshake, so an open UDP port and a filtered one often look identical from the outside. Services like HTTP, HTTPS, SSH, SMTP, and most databases use TCP, so this covers the vast majority of connectivity checks.