Complex sentence: The 5-4-3 rule--which was created when 10BASE5 and 10BASE2 were the only types of Ethernet network available--only applies to shared-access Ethernet backbones.
Split into simpler sentences: 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.