An NAPTR (Name Authority Pointer) record specifies a rule that uses a substitution expression to rewrite a string into a domain name or URI (Uniform Resource Identifier). A URI is either a URL (Uniform Resource Locator) or URN (Uniform Resource Name) that identifies a resource on the Internet.
NAPTR records are usually used to map E.164 numbers to URIs or IP addresses. An E.164 number is a telephone number, 1-555-123-4567 for example, in a format that begins with a country code, followed by a national destination code and a subscriber number. (E.164 is an international telephone numbering system recommended by the International Telecommunication Union.) Thus, NAPTR records allow us to use telephone numbers to reach devices, such as fax machines and VoIP phones, on the Internet.
To map an E.164 to a URI, the E.164 number must first be transformed into a domain name. ENUM (E.164 Number Mapping) specifies a method for converting E.164 numbers to domain names. For example, using the method specified by ENUM, the telephone number 1-555-123-4567 becomes the domain name 7.6.5.4.3.2.1.5.5.5.1.e164.arpa. For details about ENUM, refer to RFC 3761, The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM).
After the E.164 number is converted to a domain name, a DNS client can then perform a DNS lookup for the NAPTR records of the domain name. The following example illustrates how a DNS client processes NAPTR records.
In this example, the telephone number 1-555-123-4567 is converted to the domain name 7.6.5.4.3.2.1.5.5.5.1.e164.arpa. The DNS client then sends a query to the Infoblox DNS server for the NAPTR records associated with 7.6.5.4.3.2.1.5.5.5.1.e164.arpa. The Infoblox DNS server returns the following NAPTR record:
The DNS client then examines the fields in the NAPTR record as follows:
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