MS Press Book 70-297

Hooi

MS Press Book 70-297

Visual Studio55
On page 1-52 under "Lesson 3 Review", question 2 states:

"Your company has registered the network ID 131.107.0.0, which uses the

default subnet mask 255.255.0.0. You would like to create a custom subnet

mask that allows you to divide that network into the maximum number of

subnets that would allow you to have at least 20 hosts per subnet. What

custom subnet mask would you use?"

Question 2 Answer states:

"To start with, you know you will have to borrow some bits from the host ID

in the third and fourth octets. Any custom subnet mask with a value greater

than 240 in the tourth octet will give you fewer than 20 hosts per subnet.

Using 255.255.255.224 as a subnet mask will give you 4095 possible subnets

with 31 hosts possible per subnet. It is also common practice to subtract 2

addresses from the possible number of hosts to account fro host IDs that

contain all zeros or all ones. Even with this margin, you still have 30

possible hosts."

I apologize for re-typing all of this but my question is about the answer.

Where do they get the number 4095 possible subnets? Is this a typo and if

not, where is the 4095 coming from? I understand every other part of the

question and answer except for that. Someone who knows please help. Thanks!

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John

Registered User

Mon Mar 10 08:38:09 CDT 2008

Re:MS Press Book 70-297

"Fitz" <Fitz@discussions.microsoft.com>wrote in message

Quote

On page 1-52 under "Lesson 3 Review", question 2 states:

"Your company has registered the network ID 131.107.0.0, which uses the

default subnet mask 255.255.0.0. You would like to create a custom subnet

mask that allows you to divide that network into the maximum number of

subnets that would allow you to have at least 20 hosts per subnet. What

custom subnet mask would you use?"

Question 2 Answer states:

"To start with, you know you will have to borrow some bits from the host

ID

in the third and fourth octets. Any custom subnet mask with a value

greater

than 240 in the tourth octet will give you fewer than 20 hosts per subnet.

Using 255.255.255.224 as a subnet mask will give you 4095 possible subnets

with 31 hosts possible per subnet. It is also common practice to subtract

2

addresses from the possible number of hosts to account fro host IDs that

contain all zeros or all ones. Even with this margin, you still have 30

possible hosts."

I apologize for re-typing all of this but my question is about the answer.

Where do they get the number 4095 possible subnets? Is this a typo and if

not, where is the 4095 coming from? I understand every other part of the

question and answer except for that. Someone who knows please help.

Thanks!

To get 20 hosts per subnet, you need 5 bits from the last octet. To find

the number of addresses, take 2 and raise it to the power of N where N = the

number of bits. Therefore 2^5 = 32. You cannot have a host with all zeros

(network address) or all ones (broadcast address), therefore you subtract

two, leaving you with 30 possible hosts on the subnet. Since the

requirement was to have at least 20 hosts, a 5 bit host ID satisfies this

requirement. (A 4 bit host ID would only allow for 14 hosts, which does not

meet the requirement of 20).

Since you are using 5 bits for the host ID, the remaining 11 bits are used

for the subnet ID. To find the number of subnets, take 2 and raise it to

the power of N where N = the number of bits. Therefore, 2^11 = 2048.

Therefore, your subnet mask is 255.255.255.224, and you have 2048 subnets

consisting of 30 hosts/subnet if you started with a class B address such as

131.107.0.0/16.

That's the way I see it.

John R

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