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Official Google Webmaster Central Blog: Dynamic URLs vs. static URLs

Which can Googlebot read better, static or dynamic URLs?
We've come across many webmasters who, like our friend, believed that static or static-looking URLs were an advantage for indexing and ranking their sites. This is based on the presumption that search engines have issues with crawling and analyzing URLs that include session IDs or source trackers. However, as a matter of fact, we at Google have made some progress in both areas. While static URLs might have a slight advantage in terms of clickthrough rates because users can easily read the urls, the decision to use database-driven websites does not imply a significant disadvantage in terms of indexing and ranking. Providing search engines with dynamic URLs should be favored over hiding parameters to make them look static.

Let's now look at some of the widespread beliefs concerning dynamic URLs and correct some of the assumptions which spook webmasters. :)

Myth: "Dynamic URLs cannot be crawled."Fact: We can crawl dynamic URLs and interpret the different parameters. We might have problems crawling and ranking your dynamic URLs if you try to make your urls look static and in the process hide parameters which offer the Googlebot valuable information. One recommendation is to avoid reformatting a dynamic URL to make it look static. It's always advisable to use static content with static URLs as much as possible, but in cases where you decide to use dynamic content, you should give us the possibility to analyze your URL structure and not remove information by hiding parameters and making them look static.

Myth: "Dynamic URLs are okay if you use fewer than three parameters."Fact: There is no limit on the number of parameters, but a good rule of thumb would be to keep your URLs short (this applies to all URLs, whether static or dynamic). You may be able to remove some parameters which aren't essential for Googlebot and offer your users a nice looking dynamic URL. If you are not able to figure out which parameters to remove, we'd advise you to serve us all the parameters in your dynamic URL and our system will figure out which ones do not matter. Hiding your parameters keeps us from analyzing your URLs properly and we won't be able to recognize the parameters as such, which could cause a loss of valuable information.