1. A method of generating a representation of an access control list, the representation being utilizable in a processor, the method comprising the steps of: determining a plurality of rules of the access control list, each of at least a subset of the rules having a plurality of fields and a corresponding action; and processing the rules to generate a multi-level tree representation of the access control list, each of one or more of the levels of the tree representation being associated with a corresponding one of the fields and comprising one or more nodes each associated with a respective matching table comprising one or more entries each specifying a respective value for the corresponding one of the fields; wherein at least one level of the tree representation other than a root level of the tree representation comprises a plurality of nodes, with at least two of the nodes at that level each having a separate matching table associated therewith, each of the separate matching tables associated with that level corresponding to a respective value, for another field, specified by a corresponding one of a plurality of entries of another matching table associated with another level of the tree representation; wherein processing the rules to generate the multi-level tree representation of the access control list comprises processing a given value in a given one of the plurality of fields of a given one of the plurality of rules to determine whether any entry of the matching table associated with a given node at the level of the tree representation associated with the given field specifies a matching value and, if no entry of the matching table associated with the given node specifies a matching value, adding to the matching table of the given node a new entry specifying the given value.