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External probe
Check reachability from outside your local network to spot firewall, router, ISP, or CGNAT issues.
Free Online Tool
Open Port Checker
Run one external check and see TCP and UDP reachability side by side. Use Port Checker to verify open ports, troubleshoot port forwarding, and separate closed services from filtered or inconclusive paths.
Open port checker
LiveEnter an IP address or hostname with a target port.
Always confirm you have permission before scanning or testing endpoints you do not own.
Instant port testing
Built for real port checks
Port Checker combines a fast external test with practical context for port forwarding, firewalls, TCP, UDP, and filtered network paths.
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TCP and UDP results
See TCP and UDP side by side, including inconclusive UDP results that should not be treated as closed.
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Troubleshooting context
Use clear next steps for closed, filtered, and open-or-filtered results during network changes or outages.
How it works
How this open port checker tests TCP and UDP
The test runs from an external network, so it shows whether a TCP or UDP service is reachable from the public internet rather than only from your local machine.
01
Enter a target
Provide a public IP address, IPv6 address, or hostname and the port you want to check.
02
We probe both protocols
Port Checker sends TCP and UDP probes from an external network, similar to how remote clients would reach your service.
03
The response is interpreted
A TCP handshake or UDP response means open. Refused, timed-out, or silent responses are interpreted separately.
04
You get clear next steps
Use the result to verify port forwarding, firewall rules, cloud security groups, CGNAT, and service availability.
Result meanings
What open, closed, filtered, and open or filtered mean
TCP and UDP behave differently. Read each status with your firewall, router, ISP, NAT, and application configuration in mind.
TCP accepted a connection or UDP returned a response.
Confirm the exposed service is expected, patched, authenticated, and monitored.
The target explicitly rejected the check or reported that nothing is listening.
Check whether the service is running, listening on the right interface, and allowed through the firewall.
Packets appear to be blocked or dropped before a clear TCP response is received.
Review host firewalls, cloud security groups, ACLs, router rules, ISP filtering, and source-IP allowlists.
UDP did not respond, so the port may be open but silent, filtered, or dropped by NAT or a firewall.
Use a protocol-specific client test and compare the router WAN IP with your public IP to check for CGNAT.
Instant port testing
Popular ports at a glance
Jump into common port guides, then run an open port check to confirm TCP and UDP reachability where it matters.
Port
HTTPS
443 / TCPEncrypt web applications with TLS certificates on port 443.
Port
SFTP
22 / TCPTransfer files securely through SSH tunnels.
Port
SSH
22 / TCPManage servers remotely with encrypted shell access.
Port
SMB
445 / TCPServe file shares and authentication over TCP.
Port
HTTP
80 / TCPServe web traffic without encryption on port 80.
Port
Telnet
23 / TCPLegacy plaintext remote console access.
Port
DNS
53 / UDP / TCPProvide name resolution for infrastructure.
Port
SNMP
161/162 / UDPMonitor infrastructure with SNMP polling and traps.
Port
SIP
5060/5061 / UDP / TCP / TLSSet up VoIP signaling on SIP ports.
Port
VNC
5900 / TCPConnect to remote desktops with VNC on port 5900.
Port
RDP
3389 / TCPProvide Windows remote desktop access.
Port
NTP
123 / UDPDistribute precise time to fleet devices.
Port
ICMP
ICMP / ICMPTest reachability and latency with ICMP echo.
Port
FRP
7000 / TCPExpose internal apps through reverse tunnels.
Port
Minecraft
25565 / TCPHost multiplayer Minecraft worlds.
Port
FTP
20/21 / TCPServe classic file transfers on ports 20/21.
Port
IMAP
143 / TCPRetrieve email messages from servers.
Port
SQL
1433 / TCPExpose relational databases to applications.
FAQ
Frequently asked questions
Get clear answers about open port checks, TCP and UDP results, port forwarding, and filtered network paths.
What is an open port checker?
An open port checker tests whether a remote TCP or UDP service can be reached from the public internet. It helps confirm service exposure, port forwarding, firewall rules, and basic reachability.
What is port forwarding?
Port forwarding maps an external port on your router or firewall to an internal service. It enables access from the internet to devices on private networks.
Why is my port closed after port forwarding?
The service may not be listening, the firewall may still block inbound traffic, the router rule may point to the wrong device, or the connection may be behind double NAT or CGNAT. Compare the router WAN IP with your public IP and verify the service locally.
Why do TCP and UDP results differ?
TCP has a connection handshake, so open, closed, and filtered states are easier to infer. UDP has no handshake, so many valid UDP services stay silent unless they receive a protocol-specific request.
Why does UDP show open or filtered?
If a UDP probe receives no response, the port may be open but silent, filtered by a firewall, dropped by NAT, or blocked upstream. No response is inconclusive, so it should not be treated as closed.
Why do ISPs block common ports?
Consumer ISPs often block ports such as 25, 80, or 445 to reduce malware and spam. Contact your provider about business plans if you need them opened.
Do you store scan results or my targets?
We process the host and port you submit to run checks and keep limited technical logs for reliability and abuse prevention. See the Privacy Policy for details on retention and handling.
Does this replace production monitoring?
No. This tool is best for point-in-time diagnostics. For production systems, pair it with continuous monitoring, service health metrics, and alerting.