This read-write value sets how many leading bits of the filter's network address must match for a route to be affected, creating a wildcard when set below 32, and per its description the network mask bits field must be 0 and is ignored whenever this differs from 32. An admin uses this to build broader wildcard filter rules that catch a whole family of networks sharing a common prefix. For example, setting this to 8 matches any route whose leftmost 8 bits equal the filter's configured network address, covering a wide swath of subnets under that top octet.