Re: Underscore? What is a hostname?
So I assume (without looking it up) that "wally.my_domain.co.uk" might also be valid?
Not according to RFC952 "A "name" (Net, Host, Gateway, or Domain name) is a text string up to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus sign (-), and period (.). Note that periods are only allowed when they serve to delimit components of "domain style names". ... No blank or space characters are permitted as part of a name. No distinction is made between upper and lower case. The first character must be an alpha character." modified, as previously noted, by RFC1123 to permit leading digits (allegedly at the urging of a certain Minnesota Mining and Manufacturing Co., better known these days as "3M"). DNS entries that represent things other than hosts can contain underscores, as mentioned in posts above.