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5-4-3 Rule

The 5-4-3 rule is a network configuration that can also be referred to as the IEEE way. It is a design guideline for ethernet computer networks covering the number of repeaters and segments on shared-access ethernet backbones in a tree topology. This is contrary to the ethernet way. Which means, that in a collision domain there should be at most 5 segments tied together with 4 repeaters, with 3 segments containing active senders or terminals.

What the 5-4-3 Rule Ensures

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. Because each segment and repeater that a signal goes through adds a small amount of time to the process, the rule is designed to minimize transmission times of the signals to ensure the proper length of signal travel time is achieved.

For the purposes of this rule, a segment is in accordance with the IEEE definition or is defined as an electrical connection between networked devices.

How Does it Work?

In the original 10BASE5 and 10BASE2 Ethernet varieties, a segment would therefore correspond to a single coax cable and any devices tapped into it. However, on a 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.

The 5-4-3 rule divides a collision domain into two types of physical segments: populated (user) segments, and unpopulated (link) segments. User segments have users' systems connected to them. Link segments are used to connect the network's repeaters together. The rule mandates that there can only be a maximum of five segments, connected through four repeaters, or concentrators, and only three of the five segments may contain user connections. This last requirement applies only to 10BASE5 and 10BASE2 Ethernet segments.[3]

This rule is also designated the 5-4-3-2-1 rule with there being two network segments and one collision domain.[4]

Application of the 5-4-3 Rule

The 5-4-3 rule was created when 10BASE5 and 10BASE2 were the only types of Ethernet network available. The rule only applies to shared-access ethernet backbones. Which means it does not apply to switched ethernet because each port on a switch constitutes a separate collision domain.

Ethernet Way

There is an alternate configuration to the 5-4-3 rule called the ethernet way. This configuration allows 2 repeaters on the single network and does not allow any hosts on the connection between repeaters

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