The Kempt.net black list is used to catch and react to new hosts used for spamming purposes as quickly as possible. Via an automated process, hosts are listed within minutes of the first time they are caught sending or relaying spam and they are removed from the list a day later.
Kemptbl Reports Subnets
Subnet-based Blacklists are used to reject email from entire ranges of IP Addresses, i.e. providers that are hosting companies sending spam, as well as single IP Addresses that may fall in that range of IP Address.