Ethernet and IEEE 802.3 implement a rule, known as the 5-4-3
rule, for the number of repeaters and segments on shared
access Ethernet backbones in a tree topology. The 5-4-3 rule
divides the network 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 between any two nodes on
the network, 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.

The Ethernet protocol requires that a signal sent out over
the LAN reach every part of the network within a specified
length of time. The 5-4-3 rule ensures this. Each 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.

The 5-4-3 rule -- which was created when Ethernet, 10Base5,
and 10Base2 were the only types of Ethernet network
available -- only applies to shared-access Ethernet
backbones. A switched Ethernet network should be exempt from
the 5-4-3 rule because each switch has a buffer to
temporarily store data and all nodes can access a switched
Ethernet LAN simultaneously.