Confused at wording (subnettingquestions.com)

Question: You are designing a subnet mask for the 192.168.84.0 network. You want 19 subnets with up to 4 hosts on each subnet. What subnet mask should you use?

As I understand the question they want a maximum of 4 hosts per subnet. Given that logic:

2^6 network bits
2^2 host bits

I thought the answer would be 255.255.255.252...WRONG
The answer is 255.255.255.248

I understand where they put 2^3 host bits but why did they get this answer? Is there some Cisco or real world knowledge behind this answer?

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Comments

atorven

Your 192.168.84.0/30 will give you 4-2=2 hosts per subnet. Looks like you forgot to remove the 2 address which would be used for network and broadcast address.

sting_224

The way I understand it is like this:

192.168.84.0 (class c) with subnet mask 255.255.255.0 (11111111.11111111.11111111.00000000)

Since we want 19 subnets with up to 4 host, we'll take 5bits from the host portion our subnet mask (The part with all 0s)

This gives us 2^5 = 32 subnets, if we had taken 4 bits it'll give us 2^4= 16 subnets (not enough)

So the subnet mask we'll use is 255.255.255.248 (11111111.11111111.11111111.'11111'000)

Ismaeljrp

sting_224 wrote: »

The way I understand it is like this:

192.168.84.0 (class c) with subnet mask 255.255.255.0 (11111111.11111111.11111111.00000000)

Since we want 19 subnets with up to 4 host, we'll take 5bits from the host portion our subnet mask (The part with all 0s)

This gives us 2^5 = 32 subnets, if we had taken 4 bits it'll give us 2^4= 16 subnets (not enough)

So the subnet mask we'll use is 255.255.255.248 (11111111.11111111.11111111.'11111'000)

This is the correct answer OP. Whenever they ask for a certain number of hosts; remember that a host needs a valid, assignable address. Like atorven said, you forgot ( or didn't know ) to account for the reserved broadcast and subnet ID addresses.

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