Which of the following IP addresses would be hosts on the same subnet given the subnet mask
255.255.255.224? (Select TWO).
A.
10.4.4.125
B.
10.4.4.158
C.
10.4.4.165
D.
10.4.4.189
E.
10.4.4.199
Explanation:
With the given subnet mask, a maximum number of 30 hosts between IP addresses 10.4.4.161
and 10.4.4.190 are allowed. Therefore, option C and D would be hosts on the same subnet, and
the other options would not.
References:
http://www.subnetonline.com/pages/subnet-calculators/ip-subnet-calculator.php
Best trick I’ve seen is to convert the final octet to binary, and see which numbers fit the mask.
Start with the subnet mask:
255.255.255.224 => 1111 1111 | 1111 1111 | 1111 1111 | 1110 0000
1111 1111 => 128+64+32+16+8+4+2+1 = 255
1110 0000 => 128+64+32 = 224
Final octet mask, derived from 224:
1110 0000
Final octet from each supplied answer, converted to binary:
.125 = 0111 1101
.158 = 1001 1110
.165 = 1010 0101
.189 = 1011 1101
.199 = 1100 0011
Our mask will look at the first 3 most significant (MSB) bits, which are the best starting at the left in our case. Why? Because it is 1110 0000.
.165 and .189 both share the same first three bits, which makes those two the right answer.
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