It defaults to port 80, but you can force a different port by adding a colon and the port number after the computer name or IP address (no spaces).
If data comes back from the query, ID Serve displays it all. If data does not come back, the message, in my experience, will either be "The port is closed, so our connection attempt was refused" or "No response was received from the machine and port at that IP.
The program is free, portable and from a trustworthy source. Before connecting to a VPN, tell it to examine either your Wi-Fi or Ethernet connection to confirm the program is working.
The machine may be offline or the connection port may be stealthed".
ID Serve is limited to TCP (no UDP) and does not support HTTPS.
While working a DNS spoofability test, Gibson accidentally discovered that he crashed some routers just by sending them legit DNS requests.
This is a bit dated (Gibson has no creation dates on the pages of his site) but it takes only a few seconds to verify that your router does not fall prey to this attack. Speed Guide can also test individual ports at their Security Scan page where you can enter any port number and chose to test UDP and/or TCP.
I don't think a browser can test a UDP port, it is limited to TCP.