Ports
Every service on a device listens behind a numbered port. This explains what ports are, what open, closed and filtered mean, and how to check a host's ports with Netutilus for Mac.
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An address gets you to a device. A port gets you to one service on it. Ports are numbered 1 to 65535, and each service listens on its own: the same server can run a website on 443 and SSH on 22 at once.
Ports you’ll meet
- 22 SSH, remote login. 3389 Remote Desktop. 5900 Screen Sharing (VNC).
- 53 DNS. 80 HTTP and 443 HTTPS, the web.
- 25, 587 and 993: sending and reading email.
- 445 file sharing (SMB). 631 printing (IPP).
- 5201 iperf3.
TCP and UDP
Most services use TCP: a connection is set up first, and data is checked and resent if lost. UDP just sends, which suits DNS, calls and games, where a late packet is useless anyway. A port number means a different service under each.
Open, closed, filtered
What a scan tells you about a port:
- Open: a service is listening and accepted the connection.
- Closed: the device answered that nothing listens there.
- Filtered: no answer at all, usually a firewall.
- Open|Filtered, for UDP only: silence, which can mean either.
Every open port is a door. Keep the ones you don’t use closed, and never expose remote-access ports to the internet without a VPN or a strong login.
Related
- Port Scan and Path Check.
- LAN Scanner’s Check Services, for a device on your network.
Run it on your own network
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More in learn
- BufferbloatWhy the delay shoots up while your line is busy, and how to fix it on the router.
- CertificatesHow a website proves who it is, and why a connection sometimes isn't trusted.
- DNSThe internet's address book, which turns names like example.com into addresses.
- Hops and TTLHow traffic crosses the internet one router at a time, and how traceroute finds each one.
- ICMP and TCP PingTwo ways to time a host, and why one may answer when the other doesn't.
- IP AddressesThe numbers devices are reached by: public and private, IPv4 and IPv6.