A network administrator is designing an IPv4 addressing scheme and requires these subnets. Which combination of subnets and masks will provide the best addressing plan for these requirements?

Question:
A network administrator is designing an IPv4 addressing scheme and requires these subnets.

1 subnet of 100 hosts
2 subnets of 80 hosts
2 subnets of 30 hosts
4 subnets of 20 hosts

Which combination of subnets and masks will provide the best addressing plan for these requirements?

• 9 subnets of 126 hosts with a 255.255.255.128 mask
• 3 subnets of 126 hosts with a 255.255.255.192 mask
  6 subnets of 30 hosts with a 255.255.255.240 mask
• 3 subnets of 126 hosts with a 255.255.255.128 mask
  6 subnets of 30 hosts with a 255.255.255.224 mask
• 1 subnet of 126 hosts with a 255.255.255.192 mask
  2 subnets of 80 hosts with a 255.255.255.224 mask
  6 subnets of 30 hosts with a 255.255.255.240 mask

Explanation: VLSM Calculator Online

IPv4 subnets that require 100 and 80 hosts are provided by creating subnets of 126 usable addresses, each of which requires 7 host bits. The resulting mask is 255.255.255.128.

Subnets that require 30 and 20 hosts are provided by creating subnets of 30 usable addresses, each of which requires 5 host bits. The resulting mask is 255.255.255.224.

Creating nine subnets, each consisting of 126 usable addresses, would waste large numbers of addresses in the six smaller subnets.

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