A content addressable memory stores entries each comprising a rule and as part of the entry a mask identifying all the entities to which the rule is applicable. A search pattern of data and a bit mask identifying the actual entity (or entities) associated with the data is applied as a search word along with a comparison mask that excludes all the other entities from the comparison of the search word with the entry. The CAM can thereby store efficiently in a single entry a rule that may be applicable to some but not all of a multiplicity of entities such as possible ingress ports of a network unit.