A matrix is expressed using a shorthand that says what input ports can talk to what output ports. An examples of a specification is: 11:1; 1:3-5; 2:0,3; 4-7,9:4-7,9; 10: Semicolons separate expressions. Within each expression, the colon has a left hand side and a right hand side. Bridge ports on the left hand side can send packets to bridge ports on the right hand side. Commas separate items in a list of bridge ports. Hyphens are short-hand for specifying a range of numbers. If there is no right hand side, the bridge ports on the left cannot send packets to any bridge port (unless the matrix is combined with some some other matrix in a filter specification, or unless the filter is overridden). Bridge port numbers are specified in decimal. A matrix will read back in a form equivalent to the form written. It may not read back exactly as written. No single matrix defines whether a packet arriving on a port is forwarded to its destination. Various destination address, source address, protocol, and default filtering specifications are combined to determine whether the packet is forwarded. Unspecified rows are set to one's, which means that the corresponding input ports can communicate with every output port.