Using the 80/20 Rule for Servers and Scopes

To provide fault tolerance for the DHCP service within a given subnet, you might want to configure two DHCP servers to assign addresses for the same subnet. With two DHCP servers deployed, if one server is unavailable, the other server can take its place and continue to lease new addresses or renew existing clients.

For balancing DHCP server use in this case, a good practice is to use the 80/20 rule to divide the scope addresses between the two DHCP servers. If Server 1 is configured to make available most (approximately 80 percent) of the addresses, Server 2 can be configured to make the other addresses (approximately 20 percent) available to clients.

For example, in a typical subnet with the address 192.168.1.0, the first 10 addresses are reserved for static addresses, and the IP address range for the DHCP scope defined on the subnet is 192.168.1.11 through 192.168.1.254. To comply with the 80/20 rule, both Server 1 and Server 2 define the same range for the scope, but the exclusions configured on each server differ. On Server 1, the exclusion range is configured as the final 20 percent of the scope, or 192.168.1.205 through 192.168.1.254; this range allows the server to lease addresses to the first 80 percent of the scope's range. On Server 2, the exclusion range for the scope is configured as the first 80 percent of the scope, or 192.168.1.11 through 192.168.1.204. This range allows Server 2 to lease addresses to the final 20 percent of the scope's range.