Chances are, there are several different URLs that point to the same content on your website. For example, the URLs http://yoursite.com, http://yoursite.com/default.aspx, http://www.yoursite.com, or http://www.yoursite.com/default.aspx are all likely valid URLs that results in the same content, namely the homepage for yoursite.com. While having four different URLs reference the same content may not seem like a big deal, it can directly impact your website's search engine placement and, consequently, it's traffic. To a search engine, those four different URLs represent four different pages, even though the all produce the same content.
To understand how allowing duplicate URLs in your website can affect your search engine placement, first understand that search engines base a page's placement in the search results based, in part, on how many other websites link to the page. Now, imagine that there are 1,000 web pages from other websites that link to your homepage. You might conclude, then, that a search engine would rank the importance of your homepage based on those 1,000 links. But consider what would happen if 25% of those links linked to http://yoursite.com, 25% to http://yoursite.com/default.aspx, and so on. Rather than your homepage reflecting 1,000 inbound links, instead the search engine assumes there are only 250 links to http://yoursite.com, only 250 links to http://yoursite.com/default.aspx, and so on. In effect, redundant URLs can dilute your search engine ranking.
A key tenet of search engine optimization is URL normalization, or URL canonicalization. URL normalization is the process of eliminating duplicate URLs in your website. This article explores four different ways to implement URL normalization in your ASP.NET website. To read the entire article, Techniques for Preventing Duplicate URLs in Your Website click here.