Domain:

The Internet standards (Request for Comments) for protocols mandate that component hostname 
labels may contain only the ASCII letters a through z (in a case-insensitive manner), the 
digits 0 through 9, and the hyphen (-). The original specification of hostnames in RFC 952, 
mandated that labels could not start with a digit or with a hyphen, and must not end with a 
hyphen. However, a subsequent specification (RFC 1123) permitted hostname labels to start with 
digits. No other symbols, punctuation characters, or blank spaces are permitted.