Question

Divide the 20.20.0.0/16 subnets into 10 large networks of 4096 IPs each, 16 medium-sized networks of 1024 IPs each, and 10 small sized networks of 128 IPs each. (please show all subnet Mask)

Answer 1

Given class less IP is 20.20.0.0/16

Here 16 indicates 1st 16 bits are for network id 20.20

Now the rest 16 bits are for host id.

Now in the 16 bits of host id we have to borrow some bits to create subnets.

The requirement here is 10 large networks of 4096 IPs

                                   16 medium-sized networks of 1024 IPs

                                   10 small sized networks of 128 IPs

Now take the requirement 10 large networks of 4096 IPs

for 10 medium-sized networks we need 4 bits to represent and they are from 0000 to 1001 (0 to 9)

and the 10 networks are as below

20.20.00000000.00000000

20.20.00010000.00000000

20.20.00100000.00000000

.

.

.

20.20.10010000.00000000

Hence 4 bits of host id are used so in remaining 12 bits we will have 2¹² IPs (4096).

Hence subnet Mask for this large networks is 255.255.11110000.00000000 = 255.255.240.0

Now take the requirement 16 medium-sized networks of 1024 IPs

for 16 medium-sized networks we need 4 bits to represent and they are from 0000 to 1111

but if we use these in the 1st 4 bits then 0000 to 1001 will clash with the above network

also the above network used 00,01,10 in its 1st 2 bits hence we need to use 11 as 1st

2 bits of this network to distinguish between above network and this network

Hence the 16 sub networks are as below

20.20.11000000.00000000

20.20.11000100.00000000

20.20.11001000.00000000

20.20.11001100.00000000

.

.

.

20.20.11111100.00000000

Hence 6 bits of host id are used so in remaining 10 bits we will have 2¹⁰ IPs (1024).

Hence subnet Mask for this network is 255.255.11111100.00000000 = 255.255.252.0

Now take the requirement 10 small sized networks of 128 IPs each

Here there are only 128 IPs which need only 7 bits and rest of 9 bits are for network

Hence subnet Mask for this network is 255.255.11111111.10000000 = 255.255.255.128