 Number Mapping, RFC 3761) is a system that uses DNS (RFC 1034) in order to translate certain telephone numbers, into URIs (RFC 2396), e.g., sip:user@ domain. ENUM exists primarily to facilitate the interconnec- tion of systems that rely on telephone numbers with those that use URIs to route transactions. E.164 is the ITU-T standard international numbering plan, under which all globally-reachable telephone numbers are organized. A telephone number cannot be routed in accordance with the traditional DNS resolution procedures standardized for SIP, which rely on SIP URIs. C 2009 by Taylor & Francis Group, LLC Secure Intelligent SIP Services INVITE Proceeding T_NULL T_Exception AUTH_TERM_ATT SELECT_FACILITY Present_Call T_Alerting T_Active 2xx sent 3xx -6xx response sent Completed T_Disconnect Confirmed Terminated FIGURE 21.4 Mapping from SIP to T.BCSM. ENUM provides a method for translating E.164 numbers into URIs, includ- ing potential SIP URIs. ENUM uses the domain 'e164.arpa' [5] to store the mapping. Numbers are converted to domain names using the schema defined in Figure 21.5. The E.164 number must be in full form, including the country code [5]. In the next section we describe the problem of locating terminals using E.164 numbers, and the problem of selecting a suitable gateway for calls from an IP telephony network to PSTN. Generally, these problems relate to mapping the name of a destination into an address, and finding the best route to the destination in a combined IP and PSTN network [16]. The Tele- phony Routing over IP (TRIP) protocol solves the gateway location problem by distributing routing information between entities on the IP network. 21.3.3 TRIP for Exporting Phone Routes The number entered by the user, usually given as an E.164 number, an URI or e-mail, must be mapped to at least one routing address. For calls from IP telephony terminals to other IP telephony terminals, the destination host address must be found. For calls over the network boundary to the PSTN, a gateway must be located. In the 