How do those ads know where I live?

The most obvious is one we often overlook.

Ever register with a site? I'll bet at a minimum it required a city, state or postal code. Well, you just told them where you are. That's now information that that site, and possibly other sites operated by the same entity or entities, can use to customize your experience. Including local news, weather, and advertising.

The spooky one is where you visit a site for the first time, and up pop those ads for your city.

As I've discussed on other articles, it's nearly impossible to physically locate a specific computer based on only its IP address. The reason is that the publicly available information about IP addresses only resolves down to the ISP that assigned it. If you need to get more detailed, you need the ISP's help.

But, without the ISP's help, you do have something. You have the location of the ISP.

In fact, that's exactly how I get located when I'm at home. Even though I'm located well outside of Seattle, my ISP is based there. Hence ads that use "geo-targeting" based on the IP address resolve to the greater Seattle area.

They can get it wrong, too. The most egregious are cases where they attempt to geo-target an AOL customer. Do a lookup on AOL's IP range, and it "looks like" every AOL customer lives in Manassas, Virginia, regardless of where they really are.

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The reality for most lies somewhere in between. As I said, AOL looks like it all originates in one location. Smaller ISPs can typically be located down to the primary metropolitan area that they serve. Some ISPs allocate blocks of IP addresses to specific geographic regions, so that while the ISP might be national, the IP addresses can be geo-targeted to a more specific region. In your case, looking up the IP address you posted from on http://whois.arin.net shows that it's part of a block of IPs that Comcast has allocated to the Baltimore area.

So the bottom line is that advertisers can often, though not always, make a reasonably intelligent guess as to your general location. It's nothing new really, and certainly nothing to be concerned about.