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Subnetting Question

By david-kolevski ·
I was recently asked a question in a quiz and as easy as it looks, i get a
bit confused.

An organization is granted a block; one address is 2.2.2.64/20. The
organization needs 10 subnets. What is the subnet prefix length?

a) /20
b) /24
c) /25
d) None of the above

Answers would be greatly appreiated

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I would go with

by NetMan1958 In reply to Subnetting Question

b)24

Their assigned block has a prefix of 20 and they need to break that into 10 subnets. Obviously we can't use 20 as that is their base prefix. So we move to 24 and see if that will give us the needed number of subnets. 24 minus 20 gives us 4 bits for the subnets. 2 to the 4th power is 16. If we are using "ip subnet zero" that yields 16 subnets. Without "ip subnet zero" we lose 2 and it yields 14 subnets so either way we have at least 10.

To rule out option d) let's run it with 1 less than 24, that is 23. 23 minus 20 = 3. 2 to the 3rd power = 8. So even if we are using "subnet zero, that still doesn't give us 10 subnets.

Hope this helps.

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hi <happ new year>

by bencheart18 In reply to Subnetting Question

ans:/20, Why?

alternative formula for how to get subnet;

/20
/\
21 21 = 2 subnets
/\ /\
22 22 22 22 = 4 subnets
/\ /\ /\ /\
2323 2323 2323 2323 = 8 subnets
/\ /\ /\/\ /\/\ /\/\
2424.. .... .... .... = 16 subnets

which is # of hosts is 4094.

Why? /32-/20= 12 bits

formula:

2^n-2 = hosts
n=12

therefore;

2^8-2= 4094 hosts
2x2x2x2x2x2x2x2x2x2x2x2= 4096


hopefully it's understandable for you...

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