GLOSSARY · TCP AND UDP PORT
Port
A port is a number from 0 to 65535 that tells a computer which program should receive incoming network data. An IP address finds the machine, and the port finds the service on it. Web servers conventionally use port 80 for HTTP and 443 for HTTPS, and SSH uses 22.
Why it matters
Every service you expose listens on a port, so open ports are what an outside attacker can reach. Closing ports you do not need is a basic security step. Ports are also used the other way round: your browser uses a temporary high-numbered port for each connection, and firewalls and proxies filter by port, which is why encrypted DNS on port 853 can be blocked while port 443 is not.
In practice
A port can be open, where something accepts the connection, closed, where the machine answers that nothing is listening, or filtered, where a firewall silently drops the attempt. Our port checker tests only your own address and a short list of common ports, to show what a home connection exposes. Port forwarding on your router is what opens one deliberately.
Related terms
- NAT: NAT is a technique where a router rewrites the addresses on outgoing traffic so that many devices with private addresses can share one public IP address.
- DoT: DNS over TLS encrypts DNS traffic between your device and a resolver using the same TLS security that protects websites, as set out in RFC 7858.
- STUN: STUN is a simple protocol where a device asks a public server, "what address and port do you see me coming from?" The server replies with the answer.
- HTTP proxy: An HTTP proxy is a proxy that understands web requests.