actually, the domain name itself is a max of 63 and the length of the string (including all seperators) is 256....
I am trying to find the sheet I used to have to get the proper authority on it...
meanwhile, this is a pretty good writeup...
http://www.scit.wlv.ac.uk/~jphb/comms/dns.html
DNS names are constructed hierarchichally. The highest level of the hierarchy being the last component or label of the DNS address. Labels can be up to 63 characters long and are case insensitive. A maximum length of 255 characters is allowed. Labels must start with a letter and can only consist of letters, digits and hyphens. Unfortunately some administrators construct names that start with digits. This is wrong and can easily cause problems with software that simply inspects the first character of a host address to determine whether a DNS name or an IP address has been quoted.