Q1. You have a network address of 18.104.22.168 with four subnets. You want to allow for maximum number of Hosts. What is the subnet mask you need to apply?
Correct Answer: A
I. The number of subnets required are four. We need to add subnets of all ones and all zeros to this. This is because all zeros and all ones subnets belong to "this subnet" and "all subnets" broadcasts and can not be used. Therefore, the total number of subnets to be reserved is 4+2 = 6.
II. We want to implement maximum possible Hosts. Therefore, we need to minimize the number of subnets. This minimum number is 6 here. If we reserve 2 bits, it results in only 2^2=4 subnets which is less than 6. Therefore, we have to reserve 3 bits for implementing subnets, resulting in 2^3=8 subnets. This is now optimized for maximum number of Hosts (as we have optimized for minimum number of subnets).
III. Write the 3 bits available for subnetting in fouth octet in the form 11100000 (Five 0s being Host bits). The decimal equivalent is 2^7+2^6+2^5
= 128 + 64 +32 = 224.
II. Now the subnet mask required is 255.255.255.224.
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