Google is planning to restrict sub-domains appearing in the search listings ot a single domain. I will update when this change becomes live, early in 2008.

• Read more detailed analysis at SEO Roundtable - see Use of subdomains for SEO

Implications for site owners

This will only impact companies who already use or are planning to use several sub-domains for different products or content types. Examples include eBay and HowStuffworks.

I have advocated the use of sub-domains by a brand since they enable a greater "share of search" - particularly for brand searches on the search results pages with each sub-domain having 1 or 2 entries. Although there may be disadvantages in terms of distributing backlinks to different sub-domains rather than the home page.

For a brand search this has the benefit of pushing down competitors and affiliates in the listings.

So the main implication is that companies using this approach may have to increase their AdWords expenditure or use of affiliates to protect their brand for brand searches.

This algorithm / filter change should increase Google's AdWords revenue. Strange coincidence that!