I now have an Arduino responding to commands coming in from a web site, but I'd like to limit the use of the web site to a single user at a time. For example, some sites have queues so that you know you are next in line.
One example of how this would be necessary is if you had a remote controlled r/c car. Obviously, you can't have 25 people driving it all at the same time. How has anyone else solved this?
Is there an easy way to do this? I've tried Google searches, but perhaps I'm not using the right terminology?
You must use a session with a session-id, or a token.
The person who can sends the token with his command gets executed until (s)he gives away the token e.g. by a logout. YOu get the token by login
You should be able to detect the incoming (requestor) IP address. You could make sure you only service one IP address at a time, although I'm not sure how you know when they have walked away from their web browser. But the same applies to tokens.
Also be aware that if the user(s) are behind a NAT router you may get multiple clients that all appear to have the same IP address.
If you go with robtillaart's suggestion you could store the token in a cookie.