was I wrong with my answer?
steveo1985
Member Posts: 60 ■■□□□□□□□□
in CCNA & CCENT
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
Answer(s): d. 255.255.255.224
Your Answer(s): c. 255.255.255.192
(because I thought 255.255.255.192 gave you 2x2 = 4 subnets and 2x2x2x2x2x22 = 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 2x2x2x2x22 = 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.
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
Answer(s): d. 255.255.255.224
Your Answer(s): c. 255.255.255.192
(because I thought 255.255.255.192 gave you 2x2 = 4 subnets and 2x2x2x2x2x22 = 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 2x2x2x2x22 = 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.
Comments

rakem Member Posts: 800the answer is D.
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.
if your subnet mask is 255.255.255.192 11111111.11111111.11111111.11000000
you get 62 hosts but not enough subnets cuz you only leave 2 bits for the subnets. So depending on if subnet zero is available the equation would be either 2^2 2 = 2 Subnets or 2^2 = 4 Subnets.CCIE# 38186
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steveo1985 Member Posts: 60 ■■□□□□□□□□oh ok thanks, so it depends on the subnet 0 in the question. Thanks for the help

rakem Member Posts: 800yea if subnet 0 was on then both answers would be correct.
and the real test will specify weather or not its on.CCIE# 38186
showroute.net