We have a website that is a subdomain of our primary domain, such as site.domain.com. The official URL of the website is http://site.domain.com, but some people keep referring to it as www.site.domain.com. I have a redirect rule in place to redirect www.site.domain.com to site.domain.com, but I ultimately need to explain why www.site.domain.com is bad practice (or is a bad idea), but I don't know how. Are there any guidelines or rules that dictate best practices with regard to domain names, specifically this case?
There is no real right or wrong answer to this question, it's purely down to personal preference.
The WWW prefix originated to specify web servers, for example you might have mail.site.com, vpn.site.com, san.site.com etc. www.site.com just became the standard for web sites, however it's not, and has never been required. As an interesting read, Heroku (Cloud Ruby Rails hosting) recommends using the WWW prefix (http://status.heroku.com/incident/156) but that's for reasons to do with cnames etc.
There is no hard or solid reason to, or to not, use the WWW prefix. Yes you could say not using it reduces your data transfers by a few bytes (HTTP Host header, cookies, absolute links) but you should always give both options. If you prefer no www. then set up a redirect to the main domain. The only consideration is canonical URLs (Google will see www.domain.com and domain.com as two separate sites, so it's generally good to redirect one to the other).
It depends on your audience as well. If you seek to reach Mr. and Mrs. everybody, then you should use with www. The reason for this is that they will most likely always enter the website WITH www into their browser. If they have a blog or something like that, and they link you without copy/pasting the link, they will also most likely use the site with www.
In terms of SEO that means that they will link Google (and any other search engine) to the actual content instead of linking to a redirect.
Google follows a redirect. But the count of in links is based on the URL. If you use www, then most people should link you to the correct URL to begin with.
You could always direct enquirers to No-WWW.ORG, the organizers of which make the rather pithy point that one addresses mail to
This is what the
A halfway house, that retains the rôle name, is to make
Of course, in an ideal world (as envisaged by Jim Nelson and many others), the WWW would have been brought kicking and screaming into the 21st century, instead of the WWW being the one shameful laggard, and such rôle names would be obsolete.