What runs on which port, the well-known numbers, what they are for, and which ones should never face the internet.
Ports are how one machine offers many services at one address: the IP gets the packet to the building, the port gets it to the right apartment. The 0–1023 'well-known' range is assigned by IANA to specific services, and this directory covers those plus the de-facto standards above them that every admin and home-network tinkerer meets, databases, game servers, VPNs, IoT protocols and the admin consoles that ship with default ports. The notes column flags the security-critical entries, because port knowledge is mostly used in two moments: opening one on purpose, and auditing ones open by accident.
The golden rules the table embodies: a port open to the internet is a door anyone may knock on, SMB (445), RDP (3389), and database ports (3306, 5432, 27017, 6379, 9200) are knocked on constantly by automated scanners, and belong behind VPNs or firewalls with allowlists. Plain-text legacy protocols (FTP 21, Telnet 23, HTTP 80 for anything sensitive) leak credentials by design. And when something mysteriously works on a dev machine, check 3000, 5000, 8000 and 8080 first, the unofficial dev-port district where half of modern tooling lives.
How do I see which ports my computer is listening on? Windows: netstat -ano in an admin terminal. Mac/Linux: lsof -i -P | grep LISTEN or ss -tlnp. The listing shows the process beside each port.
What is the difference between TCP and UDP ports? They are separate numbering spaces, port 53 exists as both (DNS uses each for different jobs). Firewalls open them independently, which is why 'I opened 51820' sometimes still blocks WireGuard (UDP forgotten).
Is port forwarding safe? It is a calculated exposure: forward only the specific port, only to the device that needs it, keep that service updated and authenticated, and remove the rule when done. Game servers (25565) are the common household case.