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First I made two searches, one for the keyphrase "link exchange software" (without the quotes), and another for "allinanchor:link exchange software".
Comparing the results, I found that 16 of the first 20 sites for the allinanchor search were also found within the first 25 results for the keyword search. This shows that Google's algorithm places a lot of value on anchor text, as we know.
What about the four sites that ranked high for allinanchor, but did not rank high for the keyword search? I found that they did not rank at all for the keyword search... or not in the first 500 results anyway. What is "wrong" with them that causes Google to exclude them?
It turns out that two of those four are broken links. I guess Google removes broken links from their normal search results faster than from their allinanchor results.
What about the two remaining excluded sites? Both of these sites are up, they're both several months old and contain lots of fresh content, are optimized for the search keywords, and have pageranks of 4 and 5. (I know all this because they both belong to me, which is why I started this experiment.) They are both hosted by a reputable company where I also have other sites that rank well in Google. What could they have in common that makes Google discard them from its search results, regardless of their place in the allinanchor results?
All they have in common is a shared IP address. Those two sites and several others shared a single IP address. The sites I own that rank well in Google each have their own unique IP addresses.
So then I did this experiment for a few other popular keywords. Similar results: The few sites that ranked well for allinanchor but not for the straight keyword search, were all either broken links or hosted on shared IP addresses.
FYI, here's how you can generally tell if a site is hosted on a shared or a unique IP address. In a Windows command line, type (for instance):
ping www.site-name.com
The ping command will show the IP address for that site. Now enter that IP address in your browser's address bar. For example:
[255.255.0.0...]
If the site comes up in your browser, then it has a unique IP address. If the page that comes up is blank, or a hosting company's filler page, then the site probably has a shared IP address.
In my tests, all of the sites that ranked well with allinanchor, but poorly (or not all all) with a straight keyword search, were either offline or had a shared IP address. Mysteriously, I also found that a fraction of the sites that ranked _well_ for a keyword search also seemed to have a shared IP address.
So my hypothesis is that having a shared IP address is a necessary, but not sufficient, condition for Google sending a site that would otherwise rank well, into ranking limbo. Perhaps Google allows sites with a shared IP address to rank well if a certain condition is met, for instance if the site is old enough.
I'd love to know if anyone else has information that supports or contradicts this. Does this rule also seem to hold up for your keywords, and your sites?
I've moved those two sites of mine to unique IP addresses, with no change in ranking yet (after a week). Does anyone know how long before an IP address change might affect Google's ranking?
