Ethernet 5-4-3 rule Explained

This tutorial explains the 5-4-3 rule in networking. Learn what the 5-4-3 rule is and in which Ethernet network architecture it is used.

What is the 5-4-3 rule in computer networking?

The 5-4-3 rule specifics that in a single collision domain there can be only five network segments in total, joined by four repeaters, but only three of those network segments can be populated with nodes.

In a network, signals get a very limited time to propagate. Because of this, if more than four hubs exist between end devices, a collision on one end of the network might not be detected by devices on the other side of the network in time for them to react properly. The 5-4-3 rule ensures that all nodes on the network can detect a collision and take appropriate steps to remove it.

In which architecture the 5-4-3 rule is used?

The 5-4-3 rule applies to an architecture that designs the network in a single collision domain. A single collision domain means a group of devices that share collision. Coaxial cables, Hubs, and Repeaters share a collision. If you use these things to build a network, you create a single collision domain network. Three Ethernet specifications use these things. These specifications are 10Base2, 10Base5, and 10BaseT.

Let's discuss how these specifications use the 5-4-3 rule.

10Base2

10Base2 is known as ThinNet. This specification uses RG-58 coaxial cables. The maximum length of a ThinNet segment is 185 meters. You can create a maximum of 5 segments. The entire cabling scheme, including all five segments, can't be longer than 925 meters. You can populate 3 segments and can put a maximum of 30 devices on each populated segment.

10Base5

10Base5 is known as ThickNet. This specification uses RG-8 cables. The maximum network segment length is 500 meters. The entire set of five segments cannot exceed 2,500 meters. You can have a maximum of 100 devices on a 10Base5 network segment.

Key points
  • ThinNet and ThickNet network segments can be joined using repeaters to form larger networks.
  • If you want to join network segments, you have to follow the 5-4-3 rule.
  • If you violate the 5-4-3 rule when implementing a network, the network will not work properly.
  • The 5-4-3 rule states that the maximum number of segments you can join is five.
  • To join these segments, you can use four repeaters.
  • You can attach computers in three segments, leaving two segments. The remaining two segments are used only for extending distances rather than hosting computers.
  • These two unpopulated segments are called inter-repeater links.
  • You can join up to three segments of 500 or 185 meters.
  • You can use two repeaters to extend the network with another two cable segments of 500 or 185 meters each, as long as these were link segments connected directly to the next repeater in line, with no intervening computers.
  • A 10Base2 network can span up to 925 meters and a 10Base5 network can span up to 2,500 meters.
  • You could put a maximum of 30 devices in a 10Base2 segment and 100 devices in a 10Base5 segment.

The following image shows the implementation of the 5-4-3 rule in 10Base2 and 10Base5 architectures.

5-4-3 rule

10BaseT

The 5-4-3 rule applies to this specification only if it uses hubs to connect devices. If you want to use hubs to create a network, you should follow the 5-4-3 rule. The rule for expanding a 10BaseT network with hubs is that no more than four hubs can be placed between two communicating workstations.

Conclusion

The 5-4-3 rule applies to a network that uses the devices that share collision. Switches do not share a collision. A collision on a switch can take place only between the switch and a single workstation. Since modern networks use switches, the 5-4-3 rule does not apply to modern networks. 10Base2, 10Base5, and10BaseT are essentially obsolete technologies. To take full advantage of current technology, networks using these technologies should upgrade as soon as circumstances permit.

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