y issues. And of course, SIP is a moving target, with rapid development in progress. It is likely to be three to five years before presence and real-time communication standards become stabilized. ENUM: Telephone Number Mapping How do you find a telephone number on the Internet? How do you find islands of connectivity across domain boundaries? The answer is the Electronic Number Mapping Standard (ENUM). ENUM, defined in RFC 2916, is an IETF protocol that will assist in the convergence of the PSTN and the IP network. It maps a telephone number from the PSTN to Internet services: You put a telephone number in and get a URL out. ENUM was developed as a solution to the question of how to find services on the Internet by using only a telephone number and how telephones, which have an input mechanism limited to 12 keys on a keypad, can be used to access Internet services. ENUM resolves a complete international telephone number to a series of URLs by using an architecture based on the Domain Name System (DNS). ENUM puts telephone numbers into the DNS, SO it allows for a wide range of applications based solely on a phone number. Probably the most exciting application is an improvement in VoIP. Other applications include addressing for fax machines, e-mail, IM, and Web sites. The possibilities are enormous. The ENUM protocol is the result of work of IETF's working group for tele- phone number mapping (www.enum.org). The charter of this group was to define a DNS-based architecture and protocols for mapping a telephone number to a uni- form resource identifier (URI) that can be used to contact a resource associated with that number. (The syntax of the URI is defined in RFC 2396.) The protocol itself is defined in the standards-track document E.164, which describes the inter- national public telecommunication telephony numbering plan. How ENUM Works ENUM makes extensive use of the Naming Authority Pointer Resource Records (which are defined in RFC 2915) in order to identify available ways or services for contacting a specifi