kurose and ross

Question

Consider a subnet with prefix 128.119.40.128/26. Give an example of one
IP address (of form xxx.xxx.xxx.xxx) that can be assigned to this network.
Suppose an ISP owns the block of addresses of the form 128.119.40.64/26.
Suppose it wants to create four subnets from this block, with each block
having the same number of IP addresses. What are the prefixes (of form
a.b.c.d/x) for the four subnets?

Answer 1

Given subnet  : 128.119.40.128/26 

we have 26 n/w bits and 6 host bits.

To assign any IP address then that IP must be present in this subnet.

128.119.40.10000000 Bold indicates host bits.

We can assign any address between 000001 to  111110.

That gives 128.119.40.129/26 to 128.119.40.190/26.

2)Now ISP owns  128.119.40.64/26 

to get 4 subnets we need 2 bits.

we borrow 2 bits from host id part.remaining bits in host id are 4.

basic idea is we will get the form of a.b.c.d/28.

With 4 bits we get 2^4 = 16  hosts per subnet including n/w and broadcast addresses.

1st subnet: 128.119.40.64/28 

2nd subnet: 128.119.40.80/28. (64 + 16)

3rd subnet:128.119.40.96/28    (80 + 16)

4th subnet: 128.119.40.112/28. (96+16).