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Computer Networking FAQ #18
The 5-4-3-2-1 rule of computer network design
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This one gets asked on many entry-level and intermediate-level networking exams.
Q. "What is the 5-4-3-2-1 rule of network design?"
A. The 5-4-3-2-1 rule embodies a simple recipe for network design. It may not be easy to find examples in practice, but this rule neatly ties together several important elements of design theory... (See below)
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"All computers connected by the same length of coax, or by hubs/repeaters, are said to be in the same collision domain. That is, if any two of them transmit at the same time, a collision will occur.

If the collision domain is too large, it would be possible for two machines to transmit at the same time and to finish their transmision before they heard the other transmision. A collision would have occured, but neither of them would know so they wouldn't retransmit. Obviously this is not a good situation, hence the 5-4-3-2-1 rule."
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... . To understand this rule, it's first necessary to understand the concepts of collision domains and propagation delay. Collision domains are portions of a network. When a network packet is transmitted over Ethernet, for example, it is possible for another packet from a different source to be transmitted close enough in time to the first packet to cause a collision on the wire. The total range over which a packet can travel and potentially collide with another is its collision domain.

Propagation delays are a property of the physical medium (e.g., Ethernet). Propagation delays help determine how much of a time difference between the sending of two packets on a collision domain is "close enough" to actually cause a collision. The greater the propagation delay, the increased likelihood of collisons.

The 5-4-3-2-1 rule limits the range of a collision domain by limiting the propagation delay to a "reasonable" amount of time. The rule breaks down as follows:

5 - the number of network segments
4 - the number of repeaters needed to join the segments into one collision domain
3 - the number of network segments that have active (transmitting) devices attached
2 - the number of segments that do not have active devices attached
1 - the number of collision domains

Because the last two elements of the recipe follow naturally from the others, this rule is sometimes also known as the "5-4-3" rule for short.

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