Given 10.80.0.0/16 is the starting address. So, we have 2^16 = 65,536 addresses with the ISP initially.
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For the 1st group, each customer needs 256 addresses. So, 8 bits are needed to define each host. The prefix length is 32 – 8 = 24.
Therefore the addresses of 1st group are:
1st customer => 10.80.0.0/24 to 10.80 .0.255/24
2nd customer => 10.80.1.0/24 to 10.80.1.255/24
3rd customer => 10.80.2.0/24 to 10.80 .1.255/24
…………….
64th customer => 10.80.63.0/24 to 10.80.63.255/24
So, for 1st group the ISP has allocated 64*256 = 16384 addresses.
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For the 2nd group, each customer needs 128 addresses. So, 7 bits are needed to define each host. The prefix length is 32 – 7 = 25.
Therefore the addresses of 2nd group are:
1st customer => 10.80.64.0/25 to 10.80.64.127/25
2nd customer => 10.80.64.128/25 to 10.80.64.255/25
3rd customer => 10.80.65.0/25 to 10.80.65.127/25
4th customer => 10.80.65.128/25 to 10.80.65.255/25
5th customer => 10.80.66.0/25 to 10.80.66.127/25
6th customer => 10.80.66.128/25 to 10.80.66.255/25
…………...
128th customer => 10.80.127.128/25 to 10.80.127.255/25
So, for 2nd group the ISP has allocated 128*128 = 16384 addresses
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For the 3rd group, each customer needs 32 addresses. So, 5 bits are needed to define each host. The prefix length is 32 – 5= 27.
Therefore the addresses of 3rd group are:
1st customer => 10.80.128.0/27 to 10.80.128.31/27
2nd customer => 10.80.128.32/27 to 10.80.128.63/27
3rd customer => 10.80.128.64/27 to 10.80.128.95/27
4th customer => 10.80.128.96/27 to 10.80.128.127/27
……………………
64th customer => 10.80.135.224/27 to 10.80.135.255/27
So, for 3rd group the ISP has allocated 64*32 = 2048 addresses.
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For the 4th group, each customer needs 32 addresses. So, 5 bits are needed to define each host. The prefix length is 32 – 5= 27.
Therefore the addresses of 4th group are:
1st customer => 10.80.136.0/27 to 10.80.136.31/27
2nd customer => 10.80.136.32/27 to 10.80.136.63/27
3rd customer => 10.80.136.64/27 to 10.80.136.95/27
4th customer => 10.80.136.96/27 to 10.80.136.127/27
……………………
64th customer => 10.80.143.224/27 to 10.80.143.255/27
So, for 4th group the ISP has allocated 64*32 = 2048 addresses.
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Hence the total no.of addresses available after allocating is 65536 – (16384+16384 +2048+2048) = 28672 addresses