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Suppose an ISP owns the block of addresses of the form 131.210.16.0/21. Suppose it wants to create four subnets from this block, with each block having the same number of IP addresses. i. What are the prefixes (of form a.b.c.d/x) for the four subnets? ii. What is the number of host address that can be supported per subnet?
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ISP owns the block 131.210.16.0
Mask is given to be 255.255.248.0

10000011.11010010.00010000.00000000
11111111.11111111.11111000.00000000

We have to make 4 subnets, so we need 2 bits for subnets.

11111111.11111111.11111(11)0.00000000

Subnets:
131.210.16.0  10000011.11010010.00010000.00000000
131.210.18.0  10000011.11010010.00010010.00000000
131.210.20.0  10000011.11010010.00010100.00000000
131.210.22.0  10000011.11010010.00010110.00000000

Subnet Mask:
255.255.254.0 or /23

The remaining bits (32 - 23 = 9) can be used for hosts.
So we can have 2^9 - 2 = 512 - 2 = 510 usable Hosts per subnet.

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