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 10 Mbit/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.