# was I wrong with my answer?

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42. You are designing an IP addressing scheme for a company using classful addressing. You have been assigned a class C network address 192.168.24.0 and you have to divide it into 4 subnets with at minimum of 25 hosts each. What subnet should you use?
a. 255.255.0.0
b. 255.255.255.0
c. 255.255.255.192
d. 255.255.255.224
e. 255.255.255.240
f. 255.255.255.129

(because I thought 255.255.255.192 gave you 2x2 = 4 subnets and 2x2x2x2x2x2-2 = 62 hosts - in range with the question)

- I think I used classless or CIDR to work out the question with subnets being 2 to the power ? and hosts being 2 to the power ? minus 2.

the way i did it 255.255.255.224 would = 2x2x2 = 8 subnets and 2x2x2x2x2-2 = 30 hosts

I've looked in the sybex fifth edition and can't find classfull subnetting, does anyone have a link i can use or tips on classfull subnetting.

Explanation:
In a classfull addressing scheme answer d is correct. The 224 equals 3 borrowed bits (128+64+32) using the formula (2^n)-2 where n is the amount of borrowed bits leaves (2x2x2)-2 = 6 subnets with a maximum of 30 hosts each.

• Member Posts: 800

this is because to get a minimum of 25 hosts you need to have a mask of 255.255.255.224

if you change that mask to binary you get 11111111.11111111.11111111.11100000

that leaves you 5 bits left for host addresses.

2^5 - 2 = 30 hosts.
2^3 -2 = 6 Subnets (no subnet zero)
2^3 = 8 (subnet zero)

Therefore D is the best answer for the question.