DNS offers a good case study with its quadrupling of options due to IPv6. A name server hosts many "resource records," consisting of a domain name and associated information. Traditionally, requesters could only send DNS queries to look up resource records associated with a given domain name over the IPv4 protocol. Furthermore, if the associated information included an IP address, it could only be an IPv4 address (a so-called "A" record).

Now, if a name server is IPv6-enabled, requesters can send queries over either IPv4 or IPv6. In addition, the associated information can include either an IPv4 address or an IPv6 address (a so-called "AAAA" or "quad-A" record -- four times as many bits). The two choices are orthogonal, so overall there are four times as many options as before. This initial complexity is just the starting point, however, because of the recursive nature of DNS, which may result in transactions with additional name servers, some of which may be IPv6-enabled, and some not, in the search for an ultimate IPv4 or IPv6 address.

Operators of the authoritative name servers for large (TLDs) have a privileged "observation point" for the transition from IPv4 to IPv6, relative to the "zone" or set of domain names for which the name server is authoritative. Verisign has been studying trends in the zones it operates name servers for -- including the DNS root, .com and .net, such as:

• The percentage of domain names in a given zone that are served by an IPv6-enabled name server (vs. IPv4-only).

• The percentage of DNS queries received via the IPv6 protocol (vs. IPv4).