CBL_Index
The CBL Index is a ratio between the number of IP addresses in a given IP subnet (Subnetwork) to the number of CBL (Composite Blocking List) listings in the subnet. It may be used to measure how "clean" (of compromised computers) a given subnet is.[1]
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The higher the number is, the "cleaner" the subnet.
The CBL index may be represented in Decibels (dB) or as CIDR suffix (*/xx).
Note: other spam researchers prefer to use a percentage of IPs that are listed in a subnet. Using percentages is better suited for "unclean" subnets because "clean" nets have significantly less than 1% of addresses listed.