What open ports tell you
A server offers each service — websites, email, file transfer, databases — on a numbered port. An open port is reachable from the internet. That is what you want for a website (ports 80 and 443) or a mail server, and what you do not want for a database or remote-desktop service, which attackers scan for constantly.
Open, closed and filtered
- Open: a service accepted the connection.
- Closed: the server answered, but nothing is listening on that port.
- Filtered: no answer at all, usually because a firewall drops the connection. Some networks also block ports such as 25 on the way.
Common checks
- Email not arriving? Check 25 on your mail server, and 465 or 587 for sending from a mail app.
- Port forwarding for a camera or game server? Enter your home’s public IP (see What Is My IP) and the forwarded port.
- Securing a server? Make sure database ports (3306, 5432, 27017, 6379) and remote desktop (3389) are not open to everyone.
Frequently asked questions
How do I check if port 25 is open?
Enter the mail server’s address and port 25. Note that many home and cloud networks block outgoing port 25, so test from a server that is allowed to send mail.
Is it legal to check open ports?
Checking your own servers is normal administration. Only scan servers you own or have permission to test.
Why does my port show filtered when the service is running?
A firewall — on the server, at your hosting company or in your router — is dropping the connection. Allow the port there.