Choosing A Subnet Mask

roolee

OK, started studying for 291 today and I've hit a stumbling block already, was reading the initial TCP/IP section, all seemed to be sinking in then I got this question:

You are adding a new server to each of the following subnets. Determine which subnet mask you should assign to the new server.

1: Which subnet mask would you assign to the new server?

Subnet 1: Existing Computers
10.2.12.1
10.2.41.23
10.2.41.100
10.2.41.101

a)255.0.0.0(/
b)255.255.0.0(/16)
c)255.255.255.0(/24)

answer is b

2: Which subnet mask would you assign to the new server?

Subnet 2: Existing Computers
192.168.34.1
192.168.34.55
192.168.34.223
192.168.34.5

a)255.0.0.0(/
b)255.255.0.0(/16)
c)255.255.255.0(/24)

answer is c


I know this is probably really easy to figure out, but I'm just not getting my head around it, the chapter prior to this question told me how to work out which is the Network ID and which is the Host ID, but nothing on the above question!

Anyone fancy explaining this one?

Cheers,

Rich

dynamik

It probably meant to ask what is the most restrictive subnet mask you can assign and have all computers communicate. A and B work for the first one and B and C work for the second. All you need to do is look at the octets that change and pick the appropriate mask. For example, the last two octets in the first question change, so you'd need to use 255.255.0.0. If you used 255.255.255.0, the 10.2.12.x and 10.2.41.x would be on different networks.

With more complex questions, you'll probably have to find the range of the subnets in order to determine which one is correct. This was easy since the bits in the mask were either all ones or all zeros.