What does a port identify?
TCP and UDP use 16-bit port numbers from 0 to 65535 to associate communication with applications. An endpoint consists of an address, transport protocol and port. TCP port 53 and UDP port 53 are different endpoints. Port 0 is reserved and is not a normal target for a service test.
Common services
HTTP commonly uses TCP 80 and HTTPS TCP 443; HTTP/3 uses QUIC over UDP, commonly also 443. SSH uses TCP 22, while DNS uses UDP and TCP 53. SMTP uses TCP 25 for server traffic and ports such as 587 or 465 for submission. IMAPS uses 993 and POP3S 995. These are conventions: services can be configured differently.
Open, refused or timed out?
A successful TCP connection proves that a listener was reachable at that time, not that the application is healthy or secure. A refusal can originate from the destination or a firewall. A timeout can indicate filtering, loss, routing problems or an outage; it does not prove a particular cause.
What does our tool test?
The port checker establishes TCP connections from the server in Düsseldorf to the public destination. It does not test UDP or ports inside your private home network. Test required services deliberately and restrict administrative access to suit your operation. An open port alone does not prove a vulnerability.