iMac "Knows" My ZIP Code...

Open the Address Book app and check your personal entry. That's one possibility. It's also possible that some cookie in your browser is the culprit so you might try deleting cookies. Your zip code or town can also be deduced by your internet IP address, which is provided by your ISP.

My personal bet is that your (now incorrect) zip code is being gleaned from your IP address, in which case there's nothing you can do about it.

This is probably because of cookies that are still stored on your Mac. Try purging them using Safari's "Reset Safari" option (in the "Safari" menu). Additionally, you can try emptying caches using Safari's options as well as a program like Snow Leopard Cache Cleaner, or OnyX.

IP addresses do not have zip code information directly associated with them.

nkh wrote:
How can I rid this annoyance from my iMac?

As already mentioned, your general location can be deduced from the IP routing info your Internet service provider (ISP) uses to connect you to the Internet. There is very little you can do about that -- the routing info is inherently part of the protocol (the "P" in "IP") that powers the Internet.

However, this is not specific enough to narrow your location down to one ZIP code. Sometimes ads just use a default ZIP code (like downtown in the largest city in the area) or one related to a search (like one with a high concentration of restaurants in the area if you are searching for a place to eat). If you allow a site to place cookies on your Mac, it may use them to identify you & use your previous connections to the site to try to deduce the ZIP code you would be most interested in (or at least the one that most motivates you to buy something).

IOW, this may or may not be your actual ZIP code & deleting any cookies associated with a site may or may not change anything for the better.

nkh wrote:
My address book shows only my current ZIP!

How can I check what ZIP is associated with my IP?

In your Utilities folder launch the Network Utility app. Click the Lookup tab. Select Any/All Information. Enter your IP address and click Lookup. See what you get. Sometimes a specific city is mentioned. Sometimes the name of the ISP is there. Usually there is something there that will allow you to narrow down the location of your ISP server. Then it's a simple matter to match zip code with location.

Bottom line, there is no such thing as anonymity or privacy on the internet. If someone wants to find you bad enough they can do it with the right tools.

R C-R wrote:
As already mentioned, your general location can be deduced from the IP routing info your Internet service provider (ISP) uses to connect you to the Internet. There is very little you can do about that -- the routing info is inherently part of the protocol (the "P" in "IP") that powers the Internet.

That's precisely it.

Ad servers use the location of your ISP to provide location-targeted ads.

This becomes noticeable for me when I'm logged into my work account via VPN; suddenly all my ads are for businesses in the metropolitan location where my employer's VPN servers are located.

As I mentioned, it's likely because your ISP's servers are registered to that community.

As a test, try looking up your ISP's domain name or public IP address here:

http://www.networksolutions.com/whois/index.jsp

nkh wrote:
I'm not seeking anonymity! Frankly, I don't mind too much that someone knows where I reside. What I'd like for them to do is to get it right! However, they're assuming that I live in an extremely small rural community (where I used to live long before I converted to mac)...

There are far more important problems than this one... I was just hoping that someone would know a specific file that might contain this errant info.

To answer your question directly, there is no file on your system where this information is stored. You have absolutely no control over this.