Domain name format, originally defined in RFC1034.

• It accepts 1 to 63 letters label in top-level domains as per original syntax definition.
• It accepts all-numeric top-level domains as opposed to the restriction in RFC3696.
  • If it is the sole part of domain name, (e.g. http://2130706433) client applications will most likely parse them as 32-bit integer representation of IPv4 address.