Suppose an ISP owns the block of addresses of the form 130.64.128.0/21. The ISP

Question

Suppose an ISP owns the block of addresses of the form 130.64.128.0/21. The ISP wants to divide its address block into four equal-sized contiguous address blocks and give one of these address blocks out to each of up to four organizations (A, B, C, and D) that are supported by this ISP. (i) Provide four network addresses (of the form a.b.c.d to w.x.y.z) for organization A, B, C, and D. (ii) Provide four network addresses (of the form a.b.c.d/x) for organization A, B, C, and D. (show all the details/work)

Explanation / Answer

130.64.128.0/21 is equal to a network address 130.64.128.0 with subnet 255.255.248.0

How did we get subnet mask?

The number 21 after network address represents number of bits set in subnet

so, 11111111.11111111.11111000.00000000 is the subnet which is 255.255.248.0

How is the subnet mask useful?

Subnet mask helps in identifying whether the given ip is part of the network or not

Lets represent both network address and subnet in binary form

Network Address : 10000010 01000000 10000 000 00000000

Subnet : 11111111 11111111 11111 000 00000000

Any address in the range between given address falls into this network

10000010 01000000 10000 000 00000000

10000010 01000000 10000 111 11111111

which are 130.64.128.0 to 130.64.135.255

Total we can have 2048 ip addresses

To make it 4 equal parts each org gets 512 ip addresses

A gets addresses from 130.64.128.0 to 130.64.129.255 which can be represented as 130.64.128.0/23

B gets addresses from 130.64.130.0 to 130.64.131.255 which can be represented as 130.64.130.0/23

C gets addresses from 130.64.132.0 to 130.64.133.255 which can be represented as 130.64.132.0/23

D gets addresses from 130.64.134.0 to 130.64.135.255 which can be represented as 130.64.134.0/23

How did we get 23?

ISP is already having 21 bits as subnet and we have four organisations A, B, C and D which need 2 bits to represent. So, intotal we need 23 bits as subnet for each organisation

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