Question

Given the following requirements Classful Network: 146.220.0.0 Hosts: 9 Subnets: 512 What mask would maximize the number of hosts? Use CIDR format Question 2 Given the following requirements: Classful Network: 191.245.0.0 Hosts: 50 Subnets: 200 What mask would maximize the number of hosts? Use CIDR format. Question 3 Given the following requirements: Classful Network: 183.178.0.0 Hosts: 9 Subnets: 125 What mask would maximize the number of hosts? Use CIDR format.

I'm looking for someone to help me understand how to configure the correct mask based on hosts and subnets alotted. I've tried everything but can't seem to figure it out. Any help would be greatly appreciated.

Answer 1

If the give IP address has /x CIDR and we borrow y bits for 
subnets and z bits are left for hosts, then
x+y+z = 32
No of subnets = 2^y
No of hosts = 2^z -2


146.220.0.0 is a Class B address. 
CIDR /16.
Subnets required = 512. Hence, number of bits borrowed = 9.
Hence, CIDR /25 will give maximum hosts. 
9 bits for subnets, 7 bits for hosts.
CIDR /25 will have 512 subnets and 128-2 = 126 hosts.
Subnet mask - 255.255.255.128

191.245.0.0 is a Class B address.
CIDR /16.
Subnets required = 125. Hence, number of bits borrowed = 7.
Hence, CIDR /23 will give maximum hosts.
7 bits for subnets, 9 bits for hosts.
CIDR /23 will have 128 subnets, and 512-2 = 510 hosts.
255.255.254.0


183.178.0.0 is a Class B address.
CIDR /16.
Subnets required = 135. Hence, number of bits borrowed = 7.
Hence, CIDR /23 will give maximum hosts.
7 bits for subnets, 9 bits for hosts.
CIDR /23 will have 128 subnets and 512 -2 = 510 hosts.
255.255.254.0