An alternative ENUM model called "Private ENUM" is widely used. Private ENUM uses DNS but not the public DNS database. Instead, it uses a private domain suffix and private DNS servers only accessible to specific clients. Private ENUM entries are directly provisioned by the local carrier rather than being registered through the carrier of record. The ENUM process begins by taking an E.164 number and converting it to a domain name using a specific algorithm. This domain name is then used to query NAPTR records from an ENUM server, which may return a SIP URL. If ENUM translation succeeds, the session is routed using the SIP URI. If it fails, the S-CSCF may forward to

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2. Source: Rec. ITU-T E.164 (11/2010)
CC - Country Code for geographic area
NDC - National Destination Code
SN - Subscriber Number
N - Number of digits in the country code
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5. An alternative model, often called "Private ENUM", is widely deployed. Private
ENUM uses the DNS Protocol, but not the public DNS Database. Instead, the
database either uses a private domain suffix/apex reserved for this purpose and
known to all participants, or is provided by local DNS servers which do not tie
into the public IANA-based tree, or more commonly both privacy tactics are used.
The Private ENUM DNS servers typically reside in a private or restricted IP
network, and are only accessible to specific clients. Such Private ENUM clients
are typically constrained to be ones owned and managed by the carrier,
Unlike Infrastructure ENUM, Private ENUM DNS database entries are not registered and
populated by the carrier-of-record for a given E.164 number. Instead, the private
database's administrator (the local carrier) directly provisions the entries for all E.164
numbers it cares about, based on various indirect information data sources, and sets the
entry URI values relative to their specific "view".
Source: draft-kaplan-enum-sip-routing-01
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6. Step 1: To resolve TEL URL to SIP URL
The output of the First Well Known Rule for the ENUM Application is the E.164 number
minus all non-digit characters except for the +. In order to convert this to a unique key
in this Database the string is converted into a domain-name according to this algorithm:
1. Remove all characters with the exception of the digits. For example, the First Well
Known Rule produced the Key "+442079460148". This step would simply remove
the leading "+", producing "442079460148".
2. Put dots (".") between each digit. Example 4.4.2.0.7.9.4.6.0.1.4.8
3. Reverse the order of the digits. Example: 8.4.1.0.6.4.9.7.0.2.4.4
4. Append the string ".e164.arpa" to the end. Example:
8.4.1.0.6.4.9.7.0.2.4.4.e164.arpa. This domain-name is used to request NAPTR
records which may contain the end result.
If the query success, the response of ENUM server shall return:
$ORIGIN 3.8.0.0.6.9.2.3.6.1.4.4.e164.arpa.
NAPTR 10 100 "u" "E2U+sip" "!^.*$!sip:info@domainB!"
This describes that the domain 3.8.0.0.6.9.2.3.6.1.4.4.e164.arpa. is preferably contacted
by SIP
Step 2: From the SIP URL, resolve the domain name into IP address
The ENUM/DNS translation mechanism as specified in IETF RFC 3761 [16] can be used by all IMS
nodes that require E.164 address to SIP URI resolution.
The S-CSCF shall support the ability to translate the E.164 address contained in a Request-URI in
the Tel: URI format (as specified in IETF RFC 3966 [15]) to a SIP routable SIP URI using the
ENUM/DNS translation mechanism.
If this translation succeeds, then the session shall be routed according to the returned SIP URI.
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7. If this translation fails, then the session may be forwarded to a BGCF for further
routing (e.g. to the PSTN).
Based on local configuration, the Breakout Gateway Control Function (BGCF) may
be provisioned as the contact point within an operator's network for transit IMS
scenarios. Otherwise the BGCF processes requests for routing from an S-CSCF for
the case were the S-CSCF has determined that the session cannot be routed
using DNS or ENUM/DNS .
The BGCF determines the next hop for routing the SIP message. This
determination may be based on information received in the protocol,
administrative information, and/or database access. For PSTN terminations, the
BGCF determines the network in which PSTN/CS Domain breakout is to occur. If
the routing determination is such that the breakout is to occur in the same
network in which the BGCF is located, then the BGCF shall select a MGCF that will
be responsible for the interworking with the PSTN/CS Domain.
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