If you Google for "longest domain name" you'll find a lot of conflicting information. For example, you'll probably find more than a few sites that say that a domain name cannot be more than 63 characters. That's sort of true and sort of untrue, but to understand why takes some digging (figuratively and literally, as you'll see).
It gets pretty ridiculous. For example, at one time someone registered this silliness:
http://www.thelongestdomainnameintheworldandthenso meandthensomemoreandmore.andyoucanaddsomemoretotha taswell.andabitmore.andwhynotgoonabit.aboutwatneys redbarrel.andalizardinthebleedingbidet.andnothingt oeatexcept.britishairwayssandwiches.etcetcetcand.theinfo.net/
All of that except "www" is a domain name - subdomains of subdomains, all under "theinfo.net". To make things even more confusing, the person could have added another host called "peanuts" (or wahatever) and could have had "www" and "peanuts" served by different websites and most of us would think of www.all the rest and peanuts.all the rest as separate website domains!
Let's just ignore the confusion of website domains, shall we? Trust me, it's bad enough without that. For the moment, let's look at the definition of "domain name".
That's from RFC1034. So, if we want to get pedantic, that "theinfo.net" mess was a subdomain, not a domain. According to the same RFC, each component of a subdomain is limited to 63 characters, so THAT would be the actual limit of a domain name.
Assuming we're just counting characters, of course. What if we get into typography? An "m" is wider than an "i", so a 63 character domain name with a lot of "m"'s in it would be "longer" (wider) than one without.. silly? Sure, but the whole thing is silly, isn't it?
(63 characters when you take away ".com") tried to get recognition from Guinness World Records. Guinness responded:
That's not accurate. You can't just go on adding letters to a domain name and you can't have an infinitely long subdomain. The same RFC plainly says:
I didn't explain "label lengths" before. Again, we turn to the RFC:
Given that the "theinfo.net" silliness adds up to 242 characters (subtract the "."'s) and has 11 components, that comes to only 253, so there are 2 characters missing somewhere!
Note: the original RFC for hostnames is RFC-952, which says (in part):
That was modified by RFC-1123 to allow either a letter or a digit as the first character, but underscores are not allowed anywhere in names.
However (there's always a gotcha), that 253 character name might not work everywhere. RFC1123 says that
Notice that they are blurring host names and DNS components. That's OK; it's pretty blurry anyway. For example, is "scofaq.aplawrence.com" the host "scofaq" at "aplawrence.com" or might there be a "peanuts.scofaq.aplawrence.com"?
Hard to say, isn't it? One quick check is to ask for an "mx" record. A subdomain doesn't HAVE to have an "mx", but if it does, we know it is a subdomain and not a host.
# dig +short scofaq.aplawrence.com mx <- probably host (no answer) # dig +short google.com mx <- domain (returns hosts) 20 alt1.aspmx.l.google.com. 10 aspmx.l.google.com. 30 alt2.aspmx.l.google.com. 50 alt4.aspmx.l.google.com. 40 alt3.aspmx.l.google.com. # dig +short gmail.com mx <- domain (returns hosts) 20 alt2.gmail-smtp-in.l.google.com. 5 gmail-smtp-in.l.google.com. 40 alt4.gmail-smtp-in.l.google.com. 30 alt3.gmail-smtp-in.l.google.com. 10 alt1.gmail-smtp-in.l.google.com. # dig +short mail.google.com mx <- domain? (returns host, but note no priority) googlemail.l.google.com. # dig +short googlemail.l.google.com mx <- probably host (no answer) # dig +short plus.google.com mx <- domain (returns hosts) 40 alt3.aspmx.l.google.com. 30 alt2.aspmx.l.google.com. 20 alt1.aspmx.l.google.com. 10 aspmx.l.google.com. 50 alt4.aspmx.l.google.com. # dig +short code.google.com mx code.l.google.com. <- domain (returns host)
Back to the contenders for one last look. We have The World's Longest Single Word Domain Name! which perhaps does deserve Guinness as it is a real place name (created for publicity reasons long before the Internet). Although "llanfairpwllgwyngyllgogerychwyrndrobwyll-llantysiliogogogoch" weighs in at only 60 characters, I think it deserves recognition.
Then we have this, which is billed as "The world's longest alphabetical email address":
Remember the "would" and "should" of RFC1123 above? Point taken.
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