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To check if a port is open from outside, test it from a different machine, not from the server itself. On Linux or macOS run nc -zv -w 3 203.0.113.10 443; in Windows PowerShell run Test-NetConnection 203.0.113.10 -Port 443 and look for TcpTestSucceeded : True. If the test fails, check on the server that the service is listening (ss -tlnp) and that the firewall allows the port.

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Before you start

  • A port is a numbered door on the server that one service listens on, for example 22 for SSH, 80/443 for websites, 3389 for Remote Desktop.
  • A test from the server to itself (localhost or its own IP) skips the outside network and some firewalls, so it can succeed while outside connections fail. Use your home computer or another server.
  • These tools check TCP ports. UDP has no connection handshake, so a reliable outside test needs the actual application (for example the game client).

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How to test a port from another machine

-z only checks the connection, -v prints the result, -w 3 gives up after 3 seconds. Install nc with sudo apt install netcat-openbsd (Ubuntu/Debian) or sudo dnf install nmap-ncat (AlmaLinux/Rocky); macOS has it built in.
telnet 203.0.113.10 443 also works where the telnet client is installed: a blank screen or “Connected” means the port is open. On current systems nc and Test-NetConnection are easier.

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What the result means

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How to check the service is listening on the server

0.0.0.0 or * means all IPv4 addresses, [::] all IPv6 addresses. 127.0.0.1 means local only: the service can’t be reached from outside until you change its listen address. Use ss -ulnp for UDP. More in check open ports on Linux.

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How to open the port in the firewall

If you use the FREAKHOSTING Firewall panel, check its rules and filters for the IP too: see the Firewall panel overview.
Only open ports for services that need to be public. Database ports (3306, 5432, 6379) and admin panels should stay closed or be limited to your own IP. When you change firewall rules over SSH, make sure port 22 (or your SSH port) stays allowed.

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Frequently asked questions

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Why does the port work from the server but not from outside?

The service probably listens on 127.0.0.1 only, or a firewall blocks outside traffic. Check ss -tlnp for the listen address, then the firewall rules.

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Can I check a UDP port from outside?

Not reliably with general tools. Many UDP services don’t answer an empty test packet, so nmap -sU often reports open|filtered and can’t tell the difference. Confirm the service listens with ss -ulnp, allow the port in the firewall, and test with the real client.

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Why does Test-NetConnection say the ping failed but the TCP test succeeded?

The server’s firewall blocks ICMP (ping) but allows the port. The port test is what matters. To allow ping, see enable ICMP on Windows.

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Are online port checker websites reliable?

They test from their own network, which is a real outside test. They only check TCP, and some only test common ports. nc or Test-NetConnection from your own machine gives the same answer with more detail.

VPS and dedicated server guides

Every guide in this section.

Check open ports on Linux

See what listens on the server.

Open a port in Windows Firewall

Inbound rules on Windows Server.

Set up UFW

Firewall basics on Ubuntu.

Port forwarding with iptables

Forward a port to another host.

Locked out after a firewall change

Get back in safely.
Checked against Microsoft Learn (Test-NetConnection) on 4 November 2026; nc, nmap and ss commands tested on Ubuntu 24.04 LTS (netcat-openbsd, Nmap 7.94). Commands can differ on other distributions or releases.

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Last Updated: November 2026