Sub domains or folders?

 

What is a sub domain? 

By way of an example, lets look at different ways in which the content on the Atracks website could be structured.  The content falls into two distinct subject areas, web services and web related training.  So currently, you will find sub folders,

http://www.atracks.co.uk/training/

and

http://www.atracks.co.uk/services/

 

However, the site could be structured site using sub-domains, thus

 

http://training.atracks.co.uk

and

http://services.atracks.co.uk

 

But what are the advantages, and disadvantages of each?

 

How do search engines look at sub domains

Sub domains live in a sort of limbo as far as search engines are concerned, and are treated in different ways in different circumstances.

As far as the search results are concerned, Google currently treats sub domains as separate sites.  This means that although at best a couple of separate pages from a site will appear on page 1 of the search results, any number of sub domains can appear.  The best example of this can be seen by doing a search on Google for Google.  At the time of writing, the only result other than a Google property to appear in the top 30 is a listing for wikipedia.  Everything else is a sub domain of Google with just a few other top level Google domains, such as google.ca, thrown in.  It is quite interesting just how many sub domains Google has developed, which must say something for the strategy! 

Even though search engines look at sub domains as separate sites when it comes to listing results, they usually consider the sub domain as part of the domain name, which of course gives an extra opportunity to include a keyword in the domain name.  This has several benefits

1. keywords in the domain name gives a (very slight) boost to rankings
2. a keyword in a sub domain helps a listing to stand out in the search results – assuming of course it is relevant to the search
3. the domain name is often used as link text so a keyword in the domain name helps to build extra link reputation

Be careful though, using sub domains simply as a way of getting extra keywords in a domain name is more likely to alert the spam police than provide extra visibility.  And it is certainly inadvisable to have multilevel sub domains, eg http://seo.training.atracks.co.uk.  Pages from such multilevel sub domains rarely, if ever, make it to the top of the rankings.  The same applies to hyphenated sub domains, eg seo-training.atracks.co.uk.  One hyphen might be OK, two or more are certainly likely to be counterproductive.

 

Sub domain and linking strategy

The big potential advantage of sub domains lies in the opportunities they offer large sites to develop their internal linking.  For linking purposes, Google sees sub domains as part of one big site, so any linking between the sub domains is seen as internal linking.  This means that multiple sub domains can link as much as they like between themselves and with the primary domain with impunity.  When well constructed, internal linking can have a very beneficial impact on rankings, simply because it can convey both link popularity and link reputation.

Just as a good proportion of backlinks (external links) should be coming into internal pages on any site, it is just as important to ensure that a sub domain has backlinks to it.  If it doesn’t, it will probably end up languishing in Google’s supplemental index which means it will not only fail to appear in any search results, it will almost certainly pull down the rankings of the primary domain and any other sub domains to which it links.

 

Sub domains and content

My golden rule is always –

If a strategy has benefit, or even potential benefit, to my human visitors and it is also likely to appeal to a search engine’s algorithm, then it worth doing.

 

So the amount of content on a site is a very good criterion for deciding whether or not sub domains are relevant.  If the site has lots of content, so much that dividing it into topic-specific sections is helpful then sub domains are often the best way of achieving this – and all the benefits of linking between the sub domains will kick.  If on the other hand, the site is small with limited content, then splitting it into sub domains is going to appear unnatural to your human visitors and will result in sub domains (which as stated above are seen as individual sites from a rankings point of view) with too little content to appeal to the search engines. The result in this situation likely to be a loss of rankings rather than an improvement.

 

Setting up sub domains retrospectively

The time to build sub domains is when new content is being developed.   

Moving pages to a sub domain once they are established in Google is fraught with hazard.  The only way to do it is to use a 301 permanent redirect.  Anything else will mean that any rankings the pages previously enjoyed will be lost, and because rankings are determined by off page factors as well as on page (eg incoming links) they won’t just regain those rankings once they are in place on the sub domain.  Using a 301 should redirect all link equity to the new address, but moving any pages is always a risky business.

 

In conclusion

Sub domains work well for large sites which have enough content to support them.  For small sites, at best they are cumbersome and not particularly user friendly and at worst can be detrimental to the site’s rankings. 

On a purely practical level, sub folders are easier to set up than sub domains and most small sites will not have necessary permissions to configure them and will need to contact their hosting provider.

 

Sally Kavanagh, August 2007