What is the longest domain name?

Longest legal second level domain name is 63 charectors (starting with a letter)

Longest registered domain that I know of: http://3.141592653589793238462643383279502884197169399375105820974944592.com/

Longest possible domain would be limited by the systems will power with the ability to create (theoretically) infinate subdomains.

There seems to actually be a restriction on the number of subdomains and/or characters left of the dot - the maximum number allowed (including the TLD) seems to be 253 all told as in this claim of the longest domain:

"www.thelongestdomainnameintheworldandthensomeandthensomemoreandmore

.andyoucanaddsomemoretothataswell.andabitmore

.andwhynotgoonabit.aboutwatneysredbarrel.andalizardinthebleedingbidet

.andnothingtoeatexcept.britishairwayssandwiches.etcetcetcand.theinfo.net"

Take this short URL to see it http://alturl.com/yr8fq