Why do some site require "www" at the front of the domain and some don't

[svidgen]

The resolution of the www host/subdomain is not automatic or necessarily configured by default when a domain name is registered and pointed at an IP address. A DNS record must be added "manually" to resolve hosts/subdomains. The www host/subdomain is typically added manually, or may be auto-added by some DNS management software, as an alias to the primary domain.

However, as I understand it, there is nothing inherent to the DNS system that will point any subdomain to the primary domain without a manually created entry.

Additionally, the web server often needs some configuration to respond properly to the various host/subdomain requests that could come in, and in some cases, apache is/can be configured to ignore requests for domains and subdomains/hosts that are not explicitly allowed.

That all makes sense in my head ... does it make sense in text?

[thedancer]

That seems like a good explaination to me.

What do you think about people putting 301 redirects on the non www. address to redirect to the www. version?

A lot of SEO companies seem to recomend it but I don't beleive it makes any difference.

[svidgen]

A lot of SEO companies seem to recomend it but I don't beleive it makes any difference.

I'd be interested to hear the reasoning behind that. The recommendation I've always heard is to avoid redirects, if possible. From what I understand, they create latency for visitors and stop some web crawlers dead in their tracks (not necessarily a bad thing).

Were you given any reason why one should implement that redirect?

[criterion9]

I was reading up on a similar topic a few months ago and there is a movement to abolish the www prefix altogether since it seems redundant.

http://no-www.org/

On the flip side there are working groups for the opposite as well:

http://www.yes-www.org/