4 Replies Latest reply: Jun 19, 2015 11:55 PM by Conwyn

# I need to find three subnets in a B class network with a C class custom netmask.

Hello,

I'm studying for CCT and during our subnetworks classes, I have a teacher who asked us a difficult question and I'm really having a hard time getting the answer to it. Please help me.

So here I go:

I have this ip: 141.25.137.0 / 26 and I'm required to divide this netspace into 3 subnets of my choosing in regards to the following request: one of the subnets requires 20 stations, another needs to have 10 and another one needs to have 12 stations.

So considering I have a B class netspace that looks like this after prefixing it : 141.25.137. (11xxxxxx) this last byte is in binary.

I applied the formula 2^3rd power - 2 = 6 subnets  (I needed only 3 so the remaining 3 are future developing). I couldn't go only 2^2nd power because I couldn't cram 3 subnets into 4 -2 zone.

So alas, this is what I came up with: 141.25.137.(11111xxx) / 29 . The wildcards here are only 2^3 = 8 and I can't have the required stations in each subnet.

Please tell me what to do.

• ###### 1. Re: I need to find three subnets in a B class network with a C class custom netmask.

So you have 141.25.137.0/26 and you need 3 subnets with 20, 12 and 10 hosts.

128      64       32       16       8         4         2        1

2^7     2^6     2^5     2^4     2^3     2^2     2^1     2^0

For 20 hosts

32-2 < 20

2^5-2 < 20

So we need 5 bits in the host part 32 - 5 = /27 = 255.255.255.224

Which finally gives you a range of 141.25.137.0 through 141.25.137.31

Next we'll work from 141.25.137.32

For 12 hosts

16-2 < 12

2^4 < 12

So we need 4 bits in the host part 32 - 4 = /28 = 255.255.255.240

Which finally gives you a range of 141.25.137.32 through 141.25.137.47

Next we'll work from 141.25.137.48

For 10 hosts

16-2 < 10

2^4 < 10

So again we need 4 bits in the host part 32 - 4 = /28 = 255.255.255.240

Which finally gives you a range of 141.25.137.48 through 141.25.137.63

I hope the process is clear enough

• ###### 2. Re: I need to find three subnets in a B class network with a C class custom netmask.

Hello Rama,

What you're saying is correct when we deal with C class networks.

Class B Subnetting with a Class C Address - YouTube  explains why you can't do how you suggested.

• ###### 3. Re: I need to find three subnets in a B class network with a C class custom netmask.

What Rama explained is valid, and it would be valid regardless of the network class. The concept of borrowing or reserving the host bits would not change in that case since we are subnetting here based on how many hosts are needed in each subnet.

Regards,

Aref

• ###### 4. Re: I need to find three subnets in a B class network with a C class custom netmask.

It is worth remember subneting is just block building in powers of two and yes you do have to learn them.

so 20 12 10 fit into 32 16 16 .

32+16+16=64 so fits into 64

So all the subnets can be contained in a block of 64.

141.25.137.0 / 26

32-26= 6

2 to the power of 6 is is 64 so looking good. So we have 141.25.137.0 to 141.25.137.63 for our block of 64.

32+16+16  so 0-31,32-47,48-63

Regards Conwyn