Which of the following would the security engineer set as the subnet mask for the servers below to
utilize host addresses on separate broadcast domains?
Server 1: 192.168.100.6
Server 2: 192.168.100.9
Server 3: 192.169.100.20
A.
/24
B.
/27
C.
/28
D.
/29
E.
/30
Explanation:
Using this option will result in all three servers using host addresses on different broadcast
domains.
Each / refers to the mask length.
We only care about our final octet, so the mask will need to be *at least* 24 in length.
Why?
Because 255.255.255.X is at least 24 ones. Count them:
255 = 1111 1111
3 each 255’s = 8*3 = 24
The question is asking us to consider what kind of subnet mask will force all 3 of those IP addresses to be on different subnets. Different subnets mean different broadcast domains, which means less overall traffic for the switch/router. This is why subnet masking is A Good Thing.
A subnet mask of 255.255.255.248, or /29, looks like the following:
1111 1111 | 1111 1111 | 1111 1111 | 1111 1XXX
Which will force subnets the 3 IP addresses, by the final octet, to necessarily be on different subnets. I suppose that /30 would do the same trick, but this is the best I can figure out to understand the answer’s rationale.
0000 0110 – 6
0000 1001 – 9
0001 0100 – 20
A subnet mask smaller than /29 would put at least .6 and .9 on the same subnet. /29 is just the right size.
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Thank you for your better answer!
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