My understanding is that when the flagcounter image loads on a page, it will query the visitors machine for an existing cookie and determine whether it is a new visit or not from that.

Some users don't allow cookies, so they will always show as new visits. I am guessing that military owned computers will be among these, which would be why people get "new" visits from UM most days and why most of my IO visits are from the same person.

In addition, in answer to another query I had, jeremy said that some browsers won't allow a domain to set more than 50 cookies, and the oldest will get deleted when this is exceeded - this would mean that as someone visits pages with many counters (eg some last.fm pages), their cookie will eventually get deleted and they will show as a new visitor again.

when you have a fc cookie on your pc, you cannot be a new visitor to the counter even if you are using a proxy. I think there is a if on the code:
if(there is cookie) new visitor = false;
else new visitor = true;

it has nothing to do with the IP, just your machine. So, if you are using a proxy, and want to be a new visitor, you can clean your cookies and just load the page ^^

Cookies are part of your web browser and it depends on your own browser settings/preferences how they are being handled. I have several different browsers installed on my computer and they all have different settings when it comes to cookies.

I have an fc cookie set for my own flag counter in Firefox. When I visit my webpage using Firefox, I’m not counted as a new visitor. Doesn’t matter which IP or proxy I use, fc won’t register my visit as new for a time period of 6 months after which the cookie will expire.

However, I don’t allow any cookies in Internet Explorer. My visits to any webpage with a flag counter image (even my own) using Internet Explorer will always be counted as new. Cookies in IE have nothing to do with my cookies in FF (or any other browser installed on my computer), even though I’m using both browsers at the same time and from the same computer.

I get what you're trying to say now. 

Well, I imagine it's logging your session time - how much time passes between two hits from the same IP address. That's how other similar cookies work, but their time frame varies from site to site. Some last 30 minutes, others more. Maybe you should do an experiment. You could also check your folder with cookies and see, if you find any info there. Or wait for Jeremy to answer your question.

Jeremy wrote:

We set a permanent cookie after a counter is displayed.

If you were to load your site using a proxy server, that server is accessing the site on your behalf, and the cookie on your computer is unable to be accessed.

Thank you, that's all i needed to know !!