THE ACTUAL DOMAINS IMPLEMENTATONS
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The initial set of top-level names include: ARPA This represents the set of organizations involved in the Internet system through the authority of the U.S. Defense Advanced Research Projects Agency. This includes all the research and development hosts on the ARPANET and hosts on many other nets as well. But note very carefully that the top-level domain "ARPA" does not map one-to-one with the ARPANET -- domains are administrative, not topological. Transition In the transition from the ARPANET naming convention to the Internet naming convention, a host name may be used as a simple name for an endpoint domain. Thus, if "USC-ISIF" is an ARPANET host name, then "USC-ISIF.ARPA" is the name of an Internet domain. 10. Summary A hierarchical naming convention based on the domain concept has been adopted by the Internet community. It is an absolute naming convention defined along administrative rather than topological boundaries. This naming convention is adaptive for interoperations with other naming conventions. Thus, no standard convention needs to be imposed for interoperations among heterogeneous naming environments. This Internet naming convention allows distributed name service and naming authority functions at each domain. We have specified these functions required at each domain. Also discussed are implications on network-oriented applications, mail systems, and administrative aspects of this convention. RFC 819 August 1982; APPENDIX A The BNF Specification We present here a rather detailed "BNF" definition of the allowed form for a computer mail "mailbox" composed of a "local-part" and a "domain" (separated by an at sign). Clearly, the domain can be used separately in other network-oriented applications. ::= | "." ::= |

::= ::= | ::= | ::= | | "-"
:: = "#" | "[" "]" ::= | ::= "." "." "." one, two, or three digits representing a decimal integer. Note that the backslash, "\", is a quote character, which is used to indicate that the next character is to be used literally (instead of its normal interpretation). For example, "Joe\,Smith" could be used to indicate a single nine character user field with comma being the fourth character of the field. The simple names that make up a domain may contain both upper and lower case letters (as well as digits and hyphen), but these names are not case sensitive. Hosts are generally known by names. Sometimes a host is not known to the translation function and communication is blocked. To bypass this barrier two forms of addresses are also allowed for host "names". One form is a decimal integer prefixed by a pound sign, "#". Another form, called "dotted decimal", is four small decimal integers separated by dots and enclosed by brackets, e.g., "[123.255.37.2]", which indicates a 32-bit ARPA Internet Address in four 8-bit fields. (Of course, these numeric address forms are specific to the Internet, other forms may have to be provided if this problem arises in other transport systems.

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