Editing NAPTR record (section)

== Example ==
A common use of NAPTR records is by the [[Session Initiation Protocol]], which routes telephony sessions over IP networks. For example, the URI for the US telephone number 1-800-555-1234 might be tel:+1-800-555-1234 and its domain name ''4.3.2.1.5.5.5.0.0.8.1.e164.arpa''.  A SIP client querying that name might receive:
{{sxhl|2=zone|<nowiki>
 $ORIGIN 4.3.2.1.5.5.5.0.0.8.1.e164.arpa.
 IN NAPTR 100 10 "U" "E2U+sip" "!^.*$!sip:customer-service@example.com!" .
 IN NAPTR 102 10 "U" "E2U+email" "!^.*$!mailto:information@example.com!" .
</nowiki>}}
The first record has an order value of 100, which is lower than 102 and therefore has precedence. Its preference of 10 is unimportant as there are no other rules with order 100.  The service name E2U+sip is an [[ENUM]] string indicating that the record can be used in telephone number-to-SIP-URI queries.<ref>{{cite web|title = ENUM: Dragging telephone numbers into the Internet Age|first = Rudolf|last = van der Berg|date = 2010-01-13|website = Ars Technica|url = https://arstechnica.com/business/news/2010/01/enum-dragging-telephone-numbers-into-the-internet-age.ars|access-date = 2017-06-14|archive-date = 2012-01-15|archive-url = https://web.archive.org/web/20120115181527/http://arstechnica.com/business/news/2010/01/enum-dragging-telephone-numbers-into-the-internet-age.ars|url-status = live}}</ref>  The client applies the regular expression {{code|!^.*$!sip:customer-service@example.com!}}, which replaces its entire URI ''tel:+1-800-555-1234'' with ''sip:customer-service@example.com''.  The flag ''U'' indicates that the replacement string is a SIP URI, and that no further rules should be applied.

To resolve the SIP URI, the client performs a second NAPTR lookup&mdash;on ''example.com'', yielding:
{{sxhl|2=zone|<nowiki>
 $ORIGIN example.com.
 IN NAPTR 100 10 "S" "SIP+D2U" "!^.*$!sip:customer-service@example.com!" _sip._udp.example.com.
 IN NAPTR 102 10 "S" "SIP+D2T" "!^.*$!sip:customer-service@example.com!" _sip._tcp.example.com.
</nowiki>}}
As in the first example, the client picks the first record because it has the lowest order value.  The regular expression rule replaces the query URI, this time with the domain name ''_sip._udp.example.com''.  The flag ''S'' indicates that the resulting domain name points to a [[SRV record]]. The client thus finishes with ''_sip._udp.example.com'', for which it can then fetch an SRV record to initiate a telephony call.