In many ways, the problem of merging these two different leviathans comes down to merging their respective databases, which is to say their searchable collections of data records. A few years ago, Internet standards bodies approved a new type of data record—called ENUM, for electronic number—that could accommodate the legacy information of both telephony and the Internet. ENUM will be the bridge between the two global communications networks. Among other things, it provides a simple way to turn a phone number into an Internet address.

As we will see, ENUM offers an opportunity to revisit in a fundamental way the Internet's design, creating an open standards­based approach to the way we identify network devices and the people who use them. Openness is key if we are to ensure that tomorrow's VoIP world is as open and interoperable as the Internet itself has been. There's a danger that regions will instead be cordoned off by some private VoIP providers, such as Luxembourg-based Skype, that encourage their customers to use names specific to their systems.

In order to understand how ENUM can unify these two disparate worlds, though, we need to understand how traditional telephony and the Internet handle signaling today.

The conventional telephone system, jocularly called POTS, for "plain old telephone service," is the product of more than 100 years of evolution. The first commercial office, the Strowger exchange, which began operating in the early 1890s, was the first to replace a human operator with an electromechanical switch. (Strowger, a mortician, invented the system to get around a local operator, the wife of a competing mortician, whom he suspected of redirecting calls for Strowger to her husband.) During the last few decades, parallel work in international, commercial, and national standards has led to Signaling System 7, or SS7, which replaced electromechanical switches with electronic switching nodes.

Today, we expect a phone system to do a lot more than just complete phone calls. It also must provide toll-free numbers; services that locate the caller in an emergency (such as 911 in the United States and Canada and 999 in the United Kingdom); caller ID; automated payment for collect calls; do-not-call lists to ward off telemarketers; the ability to carry your number with you when you move; and so on. To support these services, the system needs database nodes to manage information about who is using the telephones and whom to bill for their use. The databases are owned by carriers or operated for them by specialized companies. Although a customer may influence these databases, say by changing carriers or blocking caller ID, the companies regard the databases as business assets, keep them under wraps, and charge for database lookups.

One of the basic concepts is a fully qualified telephone number, such as +1 650 381 6115 or +41 22 730 5111. This is defined by an International Telecommunication Union standard known as E.164. All ITU-T E.164 numbers are 15 digits or fewer and start with a country code (+1 for countries on the North American numbering plan, +41 for Switzerland, and so on), suballocated by the corresponding national bodies under nation-specific policies. The "+" denotes a complete phone number. Don't, however, be confused by the common U.S. phrase, "call 1-800 [number]"; services such as the toll-free numbers, 911, and directory assistance are processed in a country-, carrier-, and region-specific way and routed to their final destinations.

The Internet, on the other hand, employs several kinds of identifiers. Perhaps the most basic is the domain name, supported by a highly distributed network called the DNS, for Domain Name System. If, for instance, you point your browser at the domain name maps.yahoo.com, your computer will query your local DNS server for all the IP address records—numerical codes signifying physical machines in the Yahoo network—that are associated with that domain name. And if you write me at pvm@nominum.com, your mail server will ask for the corresponding codes (called MX, or mail exchange records) that are associated with mail for the domain name "nominum.com."

Let's look at the latter case in some detail. If your query is a very common one, then perhaps the local network server in your company will have cached the answers to it, obviating the need to relay it beyond the building's walls. If, on the other hand, the answer to the query isn't cached, your server will relay the query through the Internet to Nominum's name server, prompting the server to send you a response. That response, represented in text, would read as follows:

nominum.com. 3600 IN MX 500 mx1-above.nominum.net.

nominum.com. 3600 IN MX 5 mx2.nominum.com.

nominum.com. 3600 IN MX 10 mx1.nominum.com.

These three records identify three different mail servers; Nominum maintains two to provide redundancy against the possibility of the main mail server's failure. Each MX record contains the name of the mail server and a preference value that tells the order in which the servers should be used, beginning with the lowest value ("5," in this case).

How did your name server find Nominum's? Internet names are interpreted from right to left, beginning with the domain name's suffix—.com, .org, .arpa, .uk, .de, .aero, and so on. A local server looking for information about maps.yahoo.com, and starting with no cached information, would go to one of the 13 root servers spread out across the globe and find the identity and address for name servers having information about .com addresses. The server would then redirect the query to a yahoo.com DNS server, which would send back the correct numerical address for maps.yahoo.com.

The Internet uses domain names to create easily remembered Uniform Resource Locators (URLs) and Uniform Resource Identifiers (URIs) such as http://www.nominum.com and ftp://nominum.com. The first is the commonly seen hypertext transport protocol for Web pages; the second is the venerable file-transfer protocol. A URI is seen in sip://paf@swip.net; SIP (Session Initiation Protocol) can set up audio streams that are functionally identical to phone calls.

One of the more remarkable aspects of DNS is that the ownership, control, and operation of the system are distributed widely. Nominum.com is one of the approximately 40 million domains that are separately owned under .com. Other suffixes carry, in total, about an additional 100 million such domains, all registered to companies, groups, and individuals in return for annual fees.