Network Working Group P. Faltstrom
Request for Comments: 2916 Cisco Systems Inc.
Category: Standards Track September 2000
E.164 number and DNS
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document discusses the use of the Domain Name System (DNS) for
storage of E.164 numbers. More specifically, how DNS can be used for
identifying available services connected to one E.164 number.
Routing of the actual connection using the service selected using
these methods is not discussed.
Through transformation of E.164 numbers into DNS names and the use of
existing DNS services like delegation through NS records, and use of
NAPTR  records in DNS  , one can look up what services are
available for a specific domain name in a decentralized way with
distributed management of the different levels in the lookup process.
The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
in this document are to be interpreted as described in RFC2119 .
2. E.164 numbers and DNS
The domain "e164.arpa" is being populated in order to provide the
infrastructure in DNS for storage of E.164 numbers. In order to
facilitate distributed operations, this domain is divided into
subdomains. Holders of E.164 numbers which want to be listed in DNS
Faltstrom Standards Track [Page 1]RFC 2916 E.164 number and DNS September 2000
should contact the appropriate zone administrator in order to be
listed, by examining the SOA resource record associated with the
zone, just like in normal DNS operations.
Of course, as with other domains, policies for such listings will be
controlled on a subdomain basis and may differ in different parts of
To find the DNS names for a specific E.164 number, the following
procedure is to be followed:
1. See that the E.164 number is written in its full form, including
the countrycode IDDD. Example: +46-8-9761234
2. Remove all non-digit characters with the exception of the leading
'+'. Example: +4689761234
3. Remove all characters with the exception of the digits. Example:
4. Put dots (".") between each digit. Example: 188.8.131.52.184.108.40.206.3.4
5. Reverse the order of the digits. Example: 220.127.116.11.18.104.22.168.6.4
6. Append the string ".e164.arpa" to the end. Example:
2.1 Special note about the '+'
The '+' is kept in stage 2 in section 2 to flag that the number which
the regular expression is operating on is a E.164 number. Future
work will be needed to determine how other numbering plans (such as
closed ones) might be identified. It is possible, but not definite,
that they would use a similar mechanism as the one described in this
3. Fetching URIs given an E.164 number
For a record in DNS, the NAPTR record is used for identifying
available ways of contacting a specific node identified by that name.
Specifically, it can be used for knowing what services exists for a
specific domain name, including phone numbers by the use of the
e164.arpa domain as described above.
The identification is using the NAPTR resource record defined for use
in the URN resolution process, but it can be generalized in a way
that suits the needs specified in this document.
Faltstrom Standards Track [Page 2]RFC 2916 E.164 number and DNS September 2000
It is the string which is the result of step 2 in section 2 above
which is input to the NAPTR algorithm.
3.1 The NAPTR record
The key fields in the NAPTR RR are order, preference, service, flags,
regexp, and replacement. For a detailed description, see:
o The order field specifies the order in which records MUST be
processed when multiple NAPTR records are returned in response to
a single query.