5-4-3 rule

The 5-4-3 rule, also referred to as the IEEE way, is a design guideline for Ethernet computer networks covering the number of repeaters and segments on shared-medium Ethernet backbones in a tree topology. It means that in a collision domain there should be at most 5 segments tied together with 4 repeaters, with up to 3 mixing segments (10BASE5, 10BASE2, or 10BASE-FP). Link segments can be 10BASE-T, 10BASE-FL or 10BASE-FB. This rule is also designated the 5-4-3-2-1 rule with there being two link segments (without senders) and one collision domain. An alternate configuration rule, known as the Ethernet way, allows 2 repeaters on the single network and does not allow any hosts on the connection between repeaters. The rules were created when 10BASE5, 10BASE2 and FOIRL were the only types of Ethernet networks available. The rules only apply to shared-medium 10Mbit/s Ethernet segments connected by repeaters or repeater hubs (collisions domains) and FOIRL links. The rules do not apply to switched Ethernet because each port on a switch constitutes a separate collision domain. With mixed repeated and switched networks, the rule's scope ends at a switched port. According to the original Ethernet protocol, a signal sent out over the collision domain must reach every part of the network within a specified length of time. The 5-4-3 rule ensures this. Each segment and repeater that a signal goes through adds a small amount of time to the process, so the rule is designed to minimize transmission times of the signals. For the purposes of this rule, a segment is in accordance with the IEEE definition: an electrical connection between networked devices. In the original 10BASE5 and 10BASE2 Ethernet varieties, a segment would therefore correspond to a single coax cable and any devices tapped into it – a mixing segment. On modern twisted-pair Ethernet, a network segment corresponds to the individual connection between end station to network equipment or the connections between different pieces of network equipment.