Need help in understanding the question

The question is :
Your Company has 3 offices. The network contains servers that run Windows Server 2003 SP2.

You need to ensure that Server 1 can communicate with computers in other offices.

Which subnet mask should you assign to server 1?
A. 255.0.0.0
B. 255.255.0.0
C. 255.255.255.128
D. 255.255.255.192

The answer is C.

I need help in understanding why the answer is C?

Thanks.
Regards.

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Comments

RobertKaucher

I am assuming you understand why answers A and B do not work.

Look at answers C and D in binary:

Answer C:
255.255.255.1000 0000
10 .10 .10 .1101 1110 Server 1 (10.10.10.222)
10 .10 .10 .1000 0001 Router 1 (10.10.10.129)
10 .10 .10 .0000 0001 Router 3 (10.10.10.1)

Answer

255.255.255.1100 0000
10 .10 .10 .1101 1110 Server 1 (10.10.10.222)
10 .10 .10 .1000 0001 Router 1 (10.10.10.129)
10 .10 .10 .0000 0001 Router 3 (10.10.10.1)

Do you see why D is not possible?

hamster84

Look at answers C and D in binary:

Answer C:
255.255.255.1000 0000
10 .10 .10 .1101 1110 Server 1 (10.10.10.222)
10 .10 .10 .1000 0001 Router 1 (10.10.10.129)
10 .10 .10 .0000 0001 Router 3 (10.10.10.1)

Answer
255.255.255.1100 0000
10 .10 .10 .1101 1110 Server 1 (10.10.10.222)
10 .10 .10 .1000 0001 Router 1 (10.10.10.129)
10 .10 .10 .0000 0001 Router 3 (10.10.10.1)

So you were saying that :
Router 1 IP is 10 .10 .10 .1000 0001 does not fit to subnet mask of 255.255.255.1100 0000.

Am I right?

Thanks
Regards.

RobertKaucher

No, at least not how I understand your response. With 255.255.255.1100 0000 as the subnet mask makes the server and the servers default gateway on different network. With no default gateway it could not communicate with anything not on its LAN.

With the net mask from answer D Server 1 is on network 192 and the router it wants to use as a DFG is on network 128.

ilcram19-2

as you can see the network is being subnetted (devided in 2)
branch office where the server is not in has 10.10.10.0/25
and where server1 is located the network is 10.10.10.128/25

you can tell by the default gateway of server 1 is using 10.10.10.129 which is the first ip
address of the subnet 10.10.10.128/25 and server1 has 10.10.10.222 so it is on the range of that subnet which is 10.10.10.129-10.10.10.254/255.255.255.128.

focus more on subnetting