Port checker online tool: test if any port is open or closed

Check whether a specific port on any host is open, closed, or filtered using the port checker online tool at ToolHQ. Enter a hostname or IP address and a port number, and the tool immediately tells you whether a service is listening and accepting connections on that port.

A port checker works by attempting a real TCP connection to the host and port you specify. If the connection succeeds, the port is open. If the connection is refused or times out, the port is closed or blocked by a firewall. This simple test resolves a surprisingly wide range of network problems: firewall rules that block services, port forwarding configurations that are not working, and services that have stopped listening.

Knowing the state of a port before escalating to server logs or infrastructure tickets saves hours of debugging. Whether you are a developer, a sysadmin, or a DevOps engineer, a quick port check is one of the first steps in any network troubleshooting workflow.

Key Takeaways

• A port check sends a TCP connection attempt to the host and port you specify
• "Open" means a service is listening and accepting connections; "closed" or "filtered" means it is not
• Common use: verify firewall rules, confirm a service is running, test port forwarding
• Port numbers range from 0 to 65535; well-known services use ports 0 to 1023
• Only the host and port you enter are checked, no personal data stored

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What ports are and how the checker works

Every networked service communicates over a port: a numbered endpoint that distinguishes one service from another on the same host. When you visit a website, your browser connects to port 443 (HTTPS) or port 80 (HTTP). When you SSH into a server, your client connects to port 22. When a database application connects to MySQL, it uses port 3306.

Ports are specified by the IANA port registry and described in Wikipedia's list of TCP/UDP port numbers. There are 65,535 port numbers in total, divided into three ranges:

• 0 to 1023: Well-known ports, assigned to standard services (HTTP, HTTPS, SSH, FTP, SMTP)
• 1024 to 49151: Registered ports, used by vendor applications (databases, application servers)
• 49152 to 65535: Dynamic and private ports, used for temporary connections and custom applications

ToolHQ's port checker performs a TCP connection attempt to the host and port you specify. If the TCP handshake completes, the port is open. If the handshake is refused immediately, the port is closed. If the attempt times out, the port is filtered by a firewall that silently drops connection attempts. Only the host and port you enter are checked, no personal data stored.

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When you need a port checker

Verifying firewall rules: After deploying a new server or updating firewall rules, confirm that the correct ports are accessible from the outside before testing from your application.

Checking if a service is running: If an application is supposed to be listening on port 8080 but nothing connects, a port check immediately tells you whether the service is up or the firewall is blocking it.

Diagnosing port forwarding: Home and small business routers use port forwarding to expose services to the internet. A port check from outside your network confirms whether the forwarding rule is working.

DevOps deployment checks: After container or VM deployment, verify that services like Redis (6379), PostgreSQL (5432), or MongoDB (27017) are accessible only from authorised internal networks and not exposed to the public internet.

Mini-story: A startup deployed a new API server and immediately got reports from the mobile team that they could not connect. The developer checked the application logs, which showed the service was running and listening on port 3000. She ran a port check from ToolHQ and saw the port was filtered, not closed. That meant the firewall was blocking external traffic, not the application. She opened port 3000 in the security group and connections worked immediately. Total debugging time: four minutes.

Test your own host and port now at ToolHQ's port checker.

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How to use the port checker step by step

1. Open the tool. Visit ToolHQ's port checker.
2. Enter the host. Type the domain name (e.g. api.example.com) or IP address of the server you want to test.
3. Enter the port number. Type the port number (e.g. 443, 3306, 8080). Use the reference table below if you need to look up a common service's port.
4. Run the check. Click "Check Port". The tool attempts a TCP connection and reports the status.
5. Interpret the result. Open means the service is reachable. Closed means nothing is listening. Filtered means a firewall is silently blocking the connection.

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Common port reference table

Use this table to quickly identify which port a service uses. When a port check returns "closed" or "filtered," start by confirming the service is configured to listen on the correct port number.

Port	Protocol	Service	Common use
22	TCP	SSH	Secure shell access to servers
25	TCP	SMTP	Email sending (often blocked by ISPs)
53	TCP/UDP	DNS	Domain name resolution
80	TCP	HTTP	Unencrypted web traffic
110	TCP	POP3	Email retrieval (older protocol)
143	TCP	IMAP	Email retrieval (modern)
443	TCP	HTTPS	Encrypted web traffic
3306	TCP	MySQL	MySQL database connections
3389	TCP	RDP	Windows Remote Desktop
5432	TCP	PostgreSQL	PostgreSQL database connections
6379	TCP	Redis	Redis cache/queue connections
8080	TCP	HTTP alt	HTTP alternative; common for app servers
8443	TCP	HTTPS alt	HTTPS alternative; common for admin panels
27017	TCP	MongoDB	MongoDB database connections
9200	TCP	Elasticsearch	Elasticsearch HTTP API

Security note: Database ports (3306, 5432, 6379, 27017) should never return "open" when checked from the public internet. If they do, your database is publicly accessible, which is a serious security risk. Restrict access to your internal network or a VPN immediately.

Mini-story: A small agency discovered during a routine port check that their MySQL port (3306) was open to the internet. They had forgotten to set firewall rules on a new cloud instance. Within an hour of the check, they had configured the security group to allow port 3306 only from their application server's private IP. The exposure had been live for two days, but no breach occurred.

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Frequently asked questions

What does "filtered" mean in a port check? Filtered means a firewall is silently dropping connection attempts to that port. Unlike "closed," which sends back a connection refusal immediately, "filtered" results in a timeout because the firewall does not respond at all.

Can I check UDP ports with this tool? The tool performs TCP checks. UDP port checking works differently, since UDP is connectionless and does not perform a handshake. Most services that matter for availability testing (web servers, databases, SSH) use TCP.

Why is port 25 often blocked? Port 25 (SMTP) is commonly blocked by internet service providers to prevent misconfigured servers and compromised machines from being used to send spam. If you need to send email from a server, use port 587 (SMTP with authentication) or port 465 (SMTPS) instead.

Can I check a port on my own machine? Yes. Use your machine's public IP address to test which ports are visible from the outside. This is especially useful for verifying that local services are not accidentally exposed to the internet.

How is this different from a network scan? A port checker tests one specific port on one host. A network scanner (like Nmap) scans a range of ports or hosts systematically. The port checker is for quick, targeted verification; network scanning is for comprehensive security audits.

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The short version

A port checker sends a TCP connection attempt to a host and port and tells you whether it is open, closed, or filtered by a firewall. ToolHQ's port checker is the fastest way to verify firewall rules, confirm a service is running, or diagnose a connectivity problem, without installing Nmap or any other tool. Use the 15-port reference table above to identify service ports, and remember that database ports (MySQL 3306, PostgreSQL 5432, Redis 6379, MongoDB 27017) should never be open to the public internet.

Check any port now at ToolHQ.

Related tools: HTTP headers checker | SSL checker | DNS lookup | IP address validator