Did you know? - The longest* looping alphabetical domain name here.
As each labels of a domain name may contain up to 63 characters, usually you can only put 2.423 A-to-Zs into it.
But there is a TLD .xyz, which is 0.115 A-to-Z. With this TLD, we've got total of 2.538 A-to-Zs, from M to Z, then A to Z twice!
*: As of , .xyz is the only TLD ending with xyz. For this reason, mnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvw.xyz is the longest, valid apex domain name with repeating alphabets.
Wanna go much longer?
You can add some 63-letter subdomains and get additional 2.423 A-to-Zs for each of them.
Or, you can create new TLD longer than .xyz (maybe 63-letter) and make your apex domain longer.
But don't forget that you cannot go further than 255 letters for hostnames, thus the longest possible hostname will consist of 63-letter sub-subdomain, dot, 63-letter subdomain, dot, 63-letter domain, dot plus 63-letter TLD and be 9.692 A-to-Zs.
So, what actually is the best?
Let's think of 255-letter, I mean, 9.692 A-to-Z alphabetical domains. There are possibly 26 domains, each starting with a to z.
Among these, you'll be interested in the one starting with B, right? If so, you have at-sign (@) in your mind, with some alphabetical email address ending with @bcde..., with @ as an extra A (0.038 A-to-Z).
Actually, this won't work well. As RFC 5321 limits the maximum length of a path (email address) to 256 octets. If you have 255-letter domain, you won't be able to add local part before @ to make a complete email address.
So, with any of these 255-letter domain names, can you get one of the longest possible email address with repeating alphabets? To answer this, we need to think a bit more.
The local part has maximum length of 64 octets, which is 1 octet longer than subdomain's. This means, you can replace a dot (0 A-to-Z) with a letter (0.038 A-to-Z), only when you have fully utilized all 64 octets.
For this purpose, you want a local part to be 64 octets and ending with Z, so it connects with @. The only local part satisfying this is opqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz.
Then, the domain name should be 191-letter, starting with B, contains a minimum number of dots. Since 191 equals to 63 + 1 + 63 + 1 + 63, there is only 1 possible domain name with minimum number of dots: bcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijkl.mnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvw.xyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefgh.
The best domain name is qrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyza.bcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijkl.mnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvw.xyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefgh (9.692 A-to-Z), with the longest email address firstname.lastname@example.org (9.769 A-to-Z).
Watch out! There's a pitfall!
Long hostnames are cool and legal. But, when you come to TLS, the coolness will become a problem. That's because, there is a limit of 64-characters for Common Name.
You might notice that this very site, with obviously lengthy hostname, is served via HTTPS. For this site, I set up a certificate with a shorter Common Name under another domain I own. And the lengthy name is listed as its Subject Alternative Name, which has no such limitation.
Got longer? Congratulations! Now THE LONGEST is yours!
Please send me a message with your URL and let me place a link to the champion on this page.