I never believed URLs could have a period at the end, as in www.google.de. (which obvious is not working).
However, www.youtu.be. is working perfectly well. How did they manage that?
3 Answers
From here
It's a little-known fact, but fully-qualified (unambiguous) DNS domain names have a dot at the end. People running DNS servers usually know this (if you miss the trailing dots out, your DNS configuration is unlikely to work) but the general public usually doesn't. A domain name that doesn't have a dot at the end is not fully-qualified and is potentially ambiguous. This was documented in the DNS specification, RFC 1034, way back in 1987:
Since a complete domain name ends with the root label, this leads to a printed form which ends in a dot. We use this property to distinguish between:
a character string which represents a complete domain name (often called "absolute"). For example, "poneria.ISI.EDU."
a character string that represents the starting labels of a domain name which is incomplete, and should be completed by local software using knowledge of the local domain (often called "relative"). For example, "poneria" used in the ISI.EDU domain.
The source of this content continues to explain this in more detail.
answered Dec 12, 2014 at 13:45
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7Not only does the general public not know this, but I commonly see email validators that explicitly disallow a trailing dot.– KevinCommented Dec 13, 2014 at 19:13
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3@Kevin as well they should, both RFC2822 and RFC5322 prohibit a trailing period. They define the domain part as dot-atom, which is one more more atoms (an atom being one or more characters from an alphabet given in the RFC) separated by periods. See RFC5322 §3.4.1 and §3.2.3– derobertCommented Dec 19, 2014 at 8:22
Hostnames without a trailing dot are potentially ambiguous. A trailing dot means that the hostname is fully qualified and may not be relative to the local search domain.
Imagine you are a student of the (fictive) Example University which has the second-level domain example.edu. Inside the university's campus network you can omit the .example.edu suffix for ease and lazyness. So if you want to surf to www.example.edu and are on-campus, you just have to surf to www and it works.
Now imagine the university has an institute for artificial intelligence ("AI") and their website is hosted on www.ai.example.edu. If you're on-campus, surfing to www.ai will suffice since you can omit the .example.edu suffix.
So far, so good. But now you want to surf to the "Offshore Information Services" company, which runs Anguilla's top-level domain .ai. Their website is at http://www.ai/ — but if you enter that URL into your web browser while being on campus, you'll land on the website of the institute for artificial intelligence instead.
So you need to tell the web browser that you really want to go to the website with the top-level domain .ai and not to the one with your university's subdomain .ai.example.edu. That's where the dot is important, because http://www.ai./ will always work independent of the local DNS search domain.
Analogous to the dots between the different domain levels, the last dot represents the root of the DNS, i.e. it states that the word before that dot is a top-level domain and no domain relative to the search domain.
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7It isn't the browser that does that, it is the DNS resolver. The DNS resolver on my computer is set to use a domain name as the "search domain" which causes this behavior. It is an operating system setting, not a browser setting. Commented Dec 12, 2014 at 18:15
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3Unfortunately for the students of Example University, adding the trailing . often confuses various vhosting configurations, and it'll stop working for any absolute links (which a lot of sites also use, too).– fluffyCommented Dec 12, 2014 at 21:20
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1@fluffy: Yes, of course, if the web server uses virtual host configurations, those shortened hostnames must be listed in the vhost configuration, too. Commented Dec 12, 2014 at 21:24
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1@kinokijuf: I'd consider such behaviour a bug. The browser hasn't to decided if I want to use the searchdomain feature of my DNS resolver or not. Commented Dec 12, 2014 at 21:25
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1@XTaran Yep, but whether the www.ai.example.edu site has its vhosts configured correctly or not, the DNS resolution will still screw up if
www.aihas an absolute link tohttp://www.ai/something, and if thewww.aiserver's httpd doesn't understandwww.ai.as a configured host for some reason, you still can't browse it ashttp://www.ai./either.– fluffyCommented Dec 12, 2014 at 21:51
I think the answer is that most Internet tools just drop the . or treat it as extra path information.
Note that this is a very tricky illusion. youtu.be is not youtube.be. Not sure, but this might be abuse.
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youtu.beis a domain by youtube. It's used for shorter links for example when sharing or embedding videos.– kapexCommented Dec 12, 2014 at 18:33 -
note to @CareyB: that's probably okay, since 'youtu.be' is youtube's no-longer-that-recently-introduced URL shortener for youtube videos; just pop on the ID of a video (e.g. for a video [hmm, by someone that apparently does NOT understand CC-BY-SA] about FQDN naming mentioning that the root domain is an empty string, and thus leaves a dot at the end, located at youtube.com/watch?v=qHAb3Foa1Nw you can use youtu.be/qHAb3Foa1Nw Commented Dec 12, 2014 at 18:41
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14-1; incorrect, and the youtu.be/youtube.be distinction has nothing to do with this issue at all.– fluffyCommented Dec 12, 2014 at 21:52
www.google.de.works fine for me..is the overlord TLD, then you have gTLDs :)http://example.com/path/file.vs.http://example.com/path/file) can make a difference. For example, if the path translates to a file and the underlying OS and filesystem distinguishesfile.fromfile. I guess I need not mention thathttp://example.com/path/file?foo=bar.andhttp://example.com/path/file?foo=barare also very different. urlsHostheader from an incoming request, there is no guarantee that it will treat it as if the.was not there. I just verified that with a site I am running on Amazon S3.