The Internet standards (Requests for Comments) for protocols specify that labels may contain only the ASCII letters "a" through "z" (in a case-insensitive manner), the digits "0" through "9", and the hyphen-minus character ('-'). The original specification of hostnames in RFC 952 disallowed labels from starting with a digit or with a hyphen character, and could not end with a hyphen. However, a subsequent specification (RFC 1123) permitted hostname labels to start with digits. No other symbols, punctuation characters, or white space are permitted. Internationalized domain names are stored in the Domain Name System as ASCII strings using Punycode transcription.