Determining Subnet mask for a given address pool?

[WhereBob]

Can anyone help me figure out how to calculate a specfic subnet mask for a given address pool?

Example:
What subnet mask you would assign to the address pool of 207.46.200.0 - 207.46.207.255? My book says 255.255.248.0

I must be doing something wrong because I could determine several subnet masks that would work for that pool not just one. Whats the formula to use?

I'm simply figuring a subnet mask that would put 207.46.200.0 and 207.46.207.255 on the same subnet. Thus 255.255.248.0, 255.255.240.0, 255.255.224.0, 255.255.192.0 and 255.255.128.0 would all work right.
What am I missing?

Any help is much appreicated

 

[jportz28]

just use the default class c mask, 255.255.255.0

 

[WhereBob]

Ok this is a question in a book I am studying on Routing with Windows Server2003. In the chapter review one of the questions asks

"Using a calculator, determine which subnet mask you must assign to the address pool 207.46.200.0 - 207.46.207.255"

From what I know and have read in my studies there are several subnet masks I could assign to this pool. What am I missing here? How do they come up with one definitive subnet mask of 255.255.248.0?

Any help would be much appreciated as I am going to be tested on this stuff at the end of the semester. I'm taking this course online so there is no instructor to ask.

 

[WhereBob]

No one can help me with this one? I'm going to be tested on this stuff pretty soon and I would like to understand how they derived with a specific subnet mask. I have searched the web with no concrete answers.

 

[jmacavali]

Ask the instructor to better expalin it to you. I used to remember subnetting, but I have no idea now. Did you google subnet calculator? It does say you can use one.

 

[WhereBob]

Ask the instructor to better expalin it to you. I used to remember subnetting, but I have no idea now. Did you google subnet calculator? It does say you can use one.

As I said I'm taking the course on-line but thanks

. I'm taking this course online so there is no instructor to ask.

 

[WhereBob]

Just incase someone else has the same Q
Ok I finally figured it out. Since the address is an interval of 8 (200.000 - 207.255 so the next address starts with 208.000) this would be derived by 256 - 8 = 248. So the subnet mask must be 255.255.248.0 because the host address is using 5 extra bits from the orginally assigned subnet mask of 255.255.0.0 which gives you 3 bits left for addresses. 3 bits or 2 to the power of 3 = 8
If you need a more indepth explantion you are welcome to email or PM me anytime.