5-4-3 rule

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The 5-4-3 rule (the IEEE way) is a simple guideline for older Ethernet networks that use repeaters in a shared, tree-shaped layout. It helps make sure signals travel fast enough to be detected properly.

Key points
- In one collision domain, you can have at most 5 segments connected by up to 4 repeaters. Of those five, no more than 3 may be mixing segments (examples: 10BASE5, 10BASE2, or 10BASE-FP). The remaining two segments are link segments (examples: 10BASE-T, 10BASE-FL, or 10BASE-FB) that join the repeaters.
- The rule is also called the 5-4-3-2-1 rule, noting there are two link segments and one collision domain.
- An alternative rule, sometimes called the Ethernet way, allows only 2 repeaters on a network and forbids hosts on the link between repeaters.
- The rule came from when 10BASE5, 10BASE2, and FOIRL were the only Ethernet options. It applies to shared 10 Mbit/s Ethernet using repeaters or FOIRL links, but not to switched Ethernet (where each switch port is its own collision domain). If parts of the network are switched, the rule ends at the switch port.
- Why it matters: signals must reach all parts of the network within a certain time. Each segment and repeater adds a delay, so limiting the number of segments and repeaters keeps the network’s timing within limits.
- What counts as a segment: in old coax networks, a segment was a single coax cable with devices tapped into it. In modern twisted-pair Ethernet, a segment is the connection between devices or between network gear.
- Repeaters work by locking onto the incoming signal’s preamble and then repeating the bits. If too many repeaters are used, preamble bits can be lost and a frame may be missed. The 5-4-3 rule helps prevent that.