I would recommend using mod_rewrite or another similar method to give each professional's listing a URL similar to yourdomain.com/zip/listing-name.html, that way you could pull a list of available zip codes from the database and create an index page. This will also facilitate the creation of a sitemap. This information could then be divided by state as your site grows, and the zip code part of the url may be eventually replaced by /state/nearest-city/ which will give your site a structure users will be able to easily understand and navigate. I have an entertainment site that is built on a similar concept-it is designed to be searched but I use mod rewrite to categorize the thousands of events and generate index pages for the different categories and as a result I find many users browse the site rather than searching.

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