Over at Anthroblogogy, DirtCrashr reports he’s getting a lot of “uknown” visitors showing up in his Sitemeter, a phenomenon about which Phlegmfatale expressed her own curiosity. No, “unknown” is not a relative of the nefarious Sumdood; here’s a little explanation of what’s going on:

When you view a website, your web browser downloads the page by sending a GET request to a server. Here’s what that request would look like if I was looking at my own blog and clicked on the link to pdb in my blogroll:

GET /blog/ HTTP/1.1
Host: www.papadeltabravo.com
Connection: keep-alive
Keep-Alive: 300
Accept-Encoding: gzip,deflate
Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Accept-Language: en-us,en;q=0.5
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.6) Gecko/20061201 Firefox/2.0.0.6
Referer: http://www.unforgivingminute.com/blog/

For our purposes, you can ignore everything but the last line of that techno-gibberish. That Referer: line tells pdb’s web server that I’m visiting his site from a link on my site; my site referred me to his.

On the other hand, if you visit a site by manually typing in the URL, there’s no Referer: line because you’re not visiting that site via a link; it’s like a letter with no return address. Those visits will show up as “unknowns” on your Sitemeter. Likewise, people who visit your site via a bookmark also show up as “unknowns.”

Finally, be aware that some people deliberately block their browsers from including the Referer: line for privacy reasons. If you use Firefox, you can do this by going to about:config and changing the value of network.http.sendRefererHeader to 0. You can also get really geeky and use Firefox add-ons like RefControl or Tamper Data to manually control the referer your browser reports.

Oh, and I’m well aware that the proper spelling is “referrer,” but the committee that wrote the official HTTP specifications didn’t bother to spellcheck, so “referer” it is.