Stay away from sub-domains unless you really need it. Try to maintain all folders like this is a directory structure like:
Doing this, maintains all link value and shares that link equity across the entire domain.
Subdomains are viewed as entirely new websites, or/and separate domains, (separate from their root domain), thus splitting up any value you build on, in links and link building and not allowing the value of those links to trickle all the way through the site/domain as it would in the URL structure above.
Cheers!
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Rob May
SEO Director