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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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The given block of addresses is 131.210.16.0/21. To create four subnets with equal number of IP addresses, we need to borrow 2 bits from the host portion to create 4 subnets, which gives us a block size of 2^2 = 4.

i. The prefix for the original block of addresses is 131.210.16.0/21. To create 4 subnets, we need to borrow 2 bits from the host portion, so the new prefix length will be 21 + 2 = 23. The block size for each subnet will be 2^2 = 4, so the new subnet masks will be:

Subnet 1: 131.210.16.0/23 Subnet 2: 131.210.18.0/23 Subnet 3: 131.210.20.0/23 Subnet 4: 131.210.22.0/23

ii. The original block of addresses has 2^(32-21) = 2^11 = 2048 IP addresses. Since we are creating 4 subnets, each subnet will have 2048/4 = 512 IP addresses. However, 2 IP addresses from each subnet are reserved for the network address and the broadcast address, so the number of host addresses that can be supported per subnet is 512 - 2 = 510.

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