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I came across a site that doesn't work when I type in the address with the "www." prefix. The following is an example: if you click on this link, it won't take you to any site, but rather display an error message. By simply doing away with the "www." prefix, as in here, the site loads up just fine. I can see why a site wouldn't load when the prefix is missing, but I can't think of any reason for a site not to work with it, rather than without it.

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4 not exactly the same, but it covers your question. In short, there's no reason www should work unless a explicit subdomain is set up as such. – Journeyman Geek Jul 26 '12 at 3:03
you may be redirected to an unexpected site if you're in China, because some internet SPs hijacked DNS. – LiuYan 刘研 Jul 26 '12 at 3:49
up vote 8 down vote accepted

I found the problem. The administrator of that domain name hasn't set up an address (A or AAAA) record for www.

error@underground ~ $ host has address
error@underground ~ $ host
Host not found: 3(NXDOMAIN)

Compare this to what nearly every other site does:

error@underground ~ $ host has address has IPv6 address 2a03:2880:10:1f02:face:b00c:0:25
error@underground ~ $ host has address has IPv6 address 2a03:2880:10:1f02:face:b00c:0:25

Unless this is your domain, there's very little you can do to fix it, except perhaps contact the site and advise them of their problem and hope someone there has enough clue to fix it.

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So, is setting up an address record for www common practice in this area? I mean, why do all the other sites work with the prefix and just this one doesn't. – the_midget_17 Jul 26 '12 at 3:10
Every other site works because every other site has an address record. – Michael Hampton Jul 26 '12 at 3:11
I see... so this is merely an example of sloppy work, right? – the_midget_17 Jul 26 '12 at 3:12
It's not really a "problem"... – cutrightjm Jul 26 '12 at 3:55
@Synetech: No they can't. If there's no DNS record (whether A or CNAME or, for IPv6, AAAA) for www.…, then the browser can't send a request to it, and so can't receive any redirect in return either. – Ilmari Karonen Jul 26 '12 at 12:09

Your question is based on a somewhat false premise. There is no special reason and should go to the same site or both be valid other than that this is what people expect. As far as computers are concerned, they are two completely different names that could go to completely different places. People are free to set them up to go to the same place, different places, or make one work and the other not however they please.

There is simply a general agreement that the host name www will be used for the web site for a domain. But there is no technical reason anybody must follow this agreement.

You question is based on the false premise that and are somehow the "same site", one with a prefix and one without. In fact, they are two completely different potential site names that both happen to be under the control of whoever owns or manages

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Interesting indeed. Thanks for explaining so neatly. – the_midget_17 Jul 26 '12 at 3:21
There is also no reason why and must be under the control of the same entity, just like there being no reason why com and would need to be under the control of the same entity. There is no technical reason why one cannot delegate from in exactly the same way that is delegated from com. – Michael Kjörling Jul 26 '12 at 9:34

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