Split scopes are genrally created to provide the backup cabability in DHCP Server. In case if one server goes down, the clients can still gets the Ip address from the other server.
Lets take an example how Split scopes are created.We have the range from 10.0.0.1 to 10.0.0.200 and we want to configure the split scope in 50-50 manner then
1)-We will create the scope 10.0.0.1 to 10.0.0.200 on first server(say A), we will configure the Exclusion on server A from address 10.0.0.101 to 10.0.0.200 so that it serves the addresses from 10.0.0.1 to 10.0.0.100.
2)- We will create the scope 10.0.0.1 to 10.0.0.200 on second server(say B), we will configure the Exclusion on server B from address 10.0.0.1 to 10.0.0.100 so that it serves the adderss from 10.0.0.101 to 10.0.0.200.
Now some times we want that few network devices like Printers,fax m/c should always get constant IP address, this problem can be solved by two ways
1)-We can configure the same reservation on these boxes, this will make sure that these machine are getting reserved IP address only but it can be assigned by any of the server(A or B)
2)- If we want that server A only should give IP X to client C and server B should not assign any address to that client then we can create the the reservation for client A on Server A and we can implement call out DLL on Server B that when ever this server receives the packet from client X, it should drop the packet. for more information for callout dlls one can referhttp://blogs.msdn.com/anto_rocks/archive/2005/02/25/380510.aspx