An Internet router classifies incoming packets into flows utilizing information contained in packet headers and a table of (classification) rules. This table is called the rule table (equivalently, router table). The packet-header information that is used to perform the classification is some subset of the source and destination addresses, the source and destination ports, the protocol, protocol flags, type of service, and so on. The specific header information used for packet classification is governed by the rules in the rule table. Each rule-table rule is a pair of the form (F, A), where F is a filter and A is an action. The action component of a rule specifies what is to be done when a packet that satisfies the rule filter is received. Sample actions are drop the packet, forward the packet along a certain output link, and reserve a specified amount of bandwidth. A rule filter F is a tuple that is comprised of one or more fields. In the simplest case of destination-based packet forwarding, F has a single field, which is a destination (address) prefix and A is the next hop for packets whose destination address has the specified prefix. For example, the rule (01 *, a) states that the next hop for packets whose destination address (in binary) begins with 01 is a. IP (Internet Protocol) multicasting uses rules in which F is comprised of the two fields source prefix and destination prefix; QoS routers may use five-field rule filters (source-address prefix, destination-address prefix, source-port range, destination-port range, and protocol); and firewall filters may have one or more fields.