http://www.thelongestdomainnameintheworldandthensomeandthensomemoreandmore.com/ claims to be the world’s longest domain name. But is it? It turns out that this is a matter of definition. According to the domain registrars, the longest legal domain name is 63 characters starting with a letter or number.
If you included subdomains (which precede the primary domain name and are followed by a period) you can get longer, probably up to some limit supported by individual browser software.
If you include domain suffixes in your character count you also can get longer (+4 for .com and +6 for .co.uk, for example).
I don’t think either subdomains or suffixes should be included in the search fro the world’s largest domain name, meaning the best you can do is tie for first with 63 characters.
As Esther Dyson notes, nobody will type in these long names - they are opened by clicking links or selecting from a list.
Some other fun long domain names:
http://3.141592653589793238462643383279502884197169399375105820974944592.com/: the 3 is a subdomain, I like the photo of Dr. Evil
http://www.abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijk.com/: a free email service
http://www.llanfairpwllgwyngyllgogerychwyrndrobwyll-llantysiliogogogoch.com/: named after a Welsh Village, claims to be the world’s longest domain name (of course, by my definition, it is at best tied) and in Guinness records as the world’d longest domain name (turns out the title is for world’s longest domainnamed after a real place)
Any fun or interesting long domain names?