DNS, as it has been described so far, is designed for situations in which there is a permanent (or at least semi-permanent) association of a hostname with an IP address. In today's networks (as you'll learn in the next hour), IP addresses are often assigned dynamically. In other words, a new IP address is assigned to a computer through Dynamic Host Configuration Protocol (DHCP) each time the computer starts. This means that if the computer is to be registered with DNS and accessible by its hostname, the DNS server must have some way to learn the IP address the computer is using.
The recent popularity of dynamic IP addressing has forced DNS vendors to adapt. Some IP implementations (including BIND) now offer dynamic update of DNS records. In a typical scenario (see Figure 11.8), the DHCP server assigns an IP address to the client and then updates the DNS server with the client's address. You'll learn more about DHCP in Hour 12, "Dynamic Host Configuration Protocol (DHCP)."