In a split-scope network, two or more Windows DHCP servers each control part of a DHCP address pool. Split-scopes can be used to provide limited redundancy between servers for a range of DHCP addresses.
If one of the servers fails, the other continues to assign leases for its part of the pool. The two servers do not communicate and therefore do not share any information, so an available address on a failed server cannot be allocated by the other server and is unavailable to the network.
When designing a network that includes split-scopes, it is important to size each pool so that if one server becomes unavailable the other server has enough addresses to support the hosts on the network for at least a short period of time. Within the same network segment the scopes are generally split in half. When the scopes are divided between two locations, the local server should contain a larger percentage of the addresses while the remote range contains fewer addresses.