DHCP addresses and address ranges
Any container type can have associated addresses ranges; subnets must have associated address ranges. Each range within a container must be a subset of the range and must not overlap with ranges of other containers.
For example, if a class is defined within a subnet and the class has a range, the range must be a subset of the subnet range. Also, the range within that class container cannot overlap with any other ranges at its level.
Ranges can be expressed on the container line and modified by range and exclude statements to allow for disjoint address sets associated with a container. If you have the top ten addresses and the second ten addresses of a subnet available, the subnet can specify these addresses by range in the subnet clause to reduce both memory use and the chance of address collision with other clients not in the specified ranges.
After an address has been selected, any subsequent container in the list that contains address ranges is removed from the list along with its children. Network-specific options in removed containers are not valid if an address is not used from within that container.