DHCP/Subnetting Question

alharland

Can somebody please offer me and explaination of how the answer to the following question is achieved.

Q. You are responsible for implementing DHCP according to your company's official network plan. On one subnet, the network design specifications require that the DHCP server be assigned an address of 207.46.47.150 and that this server lease addresses to the local network segment through the scope 207.46.48.0. However, the network designer has neglected to provide you with an appropriate subnet mask to assign the server and scope. Which subnet mask can you assign to the server and scope that will place the DHCP server and clients on the same logical subnet and reserve the fewest number of bits possible for the host ID?

I work this out to be /23. But the answer according to the book is /19. Can somebody please offer an explaination as to how this is achieved.

For reference, this question is in the MS Press 2nd Edition 70-291 book, on page 8-36.

Many thanks in advance.

Adam.

motherwolf

A /19 subnet mask will give you a 255.255.224.0 mask. Take 256-224=32. This means your subnets will go in increments of 32.

So it will go like this. 207.46.0.x
|
207.46.32.x
|
207.46.64.x
|
207.46.96.x
|
etc, etc.

207.46.47.150 and 207.46.48.0. fall in between the .64 and . 96 subnet. Thus, the two machines are on the same subnet.

Hope this helps.

bighornsheep

Since somebody already explained why /19 is the correct answer, I'll try and describe how you can figure out the answer in the first place.

You dont know if the assigned address on the server is in the same network as the scope from just reading the question (it could be multihomed), but you have to assume that VLSM isn't used and regardless of whether the scope is in the same network as the assigned IP, the two addesses will have the same subnet mask.

You write out the address for the assigned IP in binary: 1100111.00101110.00101111.10010110
and the network address for the scope in binary: 11001111.00101110.00110000.00000000

the bold section is the subnet mask because it is the same for both address, so because you've assumed VLSM isn't used, /19 is the subnet mask to use for the scope option. You could then use that subnet mask to show that the IPs are in the same network, so whether your assumption for non-VLSM is true or not would not matter.

alharland

thanks for the replies. i think i understand the answer now.

adam.