Snippet on Domain Canonicalization

29 Feb
Canonicalization – www vs. non-www, redirects, duplicate urls, 302 “hijacking,” etc.

Q: What is a canonical url? Do you have to use such a weird word, anyway?
A: Canonicalization is the process of picking the best url when there are several choices, and it usually refers to home pages. For example, many people would consider the below urls as


But technically all of these urls are different. A web server could return completely different content for all the urls above. When Google “canonicalizes” a url, we try to pick the url that seems like the best representative from that set.

Q: So how do I make sure that Google picks the url that I want?
A: One thing that helps is to pick the url that you want and use that url consistently across your entire site. For example, don’t make half of your links go to and the other half go to . Instead, pick the url you prefer and always use that format for your internal links.

Q: Is there anything else I can do?
A: Yes. Suppose you want your default url to be . You can make your webserver so that if someone requests, it does a 301 (permanent) redirect to . That helps Google know which url you prefer to be canonical. Adding a 301 redirect can be an especially good idea if your site changes often (e.g. dynamic content, a blog, etc.).

Q: If I want to get rid of but keep, should I use the url removal tool to remove
A: No, definitely don’t do this. If you remove one of the www vs. non-www hostnames, it can end up removing your whole domain for six months. Definitely don’t do this. If you did use
the url removal tool to remove your entire domain when you actually only wanted to remove the www or non-www version of your domain, do a reinclusion request and mention that you removed your entire domain by accident using the url removal tool and that you’d like it reincluded.

Q: So when you say www vs. non-www, you’re talking about a type of canonicalization. Are there other ways that urls get canonicalized?
A: Yes, there can be a lot, but most people never notice (or need to notice) them. Search engines can do things like keeping or removing trailing slashes, trying to convert urls with uppercase to lower case, or removing session IDs from bulletin board or other software (many bulletin board software packages will work fine if you omit the session ID).

Q: Let’s talk about the inurl: operator. Why does everyone think that if shows results that aren’t from, it must be hijacked?
A: Many months ago, if you saw, that would sometimes have content from mydomain. That could happen when the url was a 302 redirect to and we decided to show a result from Since then, we’ve changed our heuristics to make showing the source url for 302 redirects much more rare. We are moving to a framework for handling redirects in which we will almost always show the destination url. Yahoo handles 302 redirects by usually showing the destination url, and we are in the middle of transitioning to a similar set of heuristics. Note that Yahoo reserves the right to have exceptions on redirect handling, and Google does too.
Based on our analysis, we will show the source url for a 302 redirect less than half a percent of the time (basically, when we have strong reason to think the source url is correct).

Q: What are supplemental results?
A: Supplemental results usually only show up in the search index after the normal results. They are a way for Google to extend their search database while also preventing questionable pages from getting massive exposure.

Q: Okay, how about supplemental results. Do supplemental results cause a penalty in Google?
A: Nope.

Q: How to get out of Google Supplemental results?
A: If you were recently thrown into then the problem may be Google. You may just want to give it a wait, but also check to make sure you are not making errors like www vs non www, content management errors delivering the same content at multiple URLs (doing things like rotating product URLs), or too much duplicate content for other reasons (you may also want to check that nobody outside your domain is showing up in Google when you search for Site Saturation – and you can also look for duplicate content with


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