Subnetting a Subnet

[yominator]

Hi,

I've been given a network address of 129.33.112.0/20 and been asked to subdivide it into 16 networks. Can anyone help me with this as Im finding it quite difficult.

After converting it to binary, i cant figure out if i borrow the bits from the host portion and if so, what part of the binary address makes up the host portion.

Yom.

 

[DistraughtSysop]

Hi there. Is this for a school or other study assignment?

 

[yominator]

Its for a study assignment. I'd love to be able to see other completed examples so that i might be able to see how its done

 

[DistraughtSysop]

Okay, to start with - do you know what the subnet mask for that ip would be?

Would you be able to determine the subnet if it was 129.33.112.0/28?

 

[Slacker]

just wanting to see if I am correct.. 255.255.255.240

this will be coming up in classes this fall

 

[yominator]

That would equate to 11111111.11111111.11111111.11110000. The first 28 bits belong to the network and the remaining 4 belong to the host. Converting the network bits to 1s and the host bits to zero, that would equate to 255.255.255.240. Is that correct?

 

[yominator]

I suppose what I need to know in order to proceed is that having converted 129.33.112.0 to binary 10000001.00100001.01110000.00000000, i dont know whether to treat that address as Class B type or a Class C type. If I knew this, I might be able to correctly align it for subnetting.

 

[yominator]

The rule seems to state that an address beginning with 129 is in the Class B range but when I try to subnet it into 16 networks (2 to the power of 5 or 32), i dont know which part constitutes the beginning of the host part of the address. Does it begin at the 112 (01110000) part of the address or the 0 (00000000) part of the address.

 

[Oreo]

The rule seems to state that an address beginning with 129 is in the Class B range but when I try to subnet it into 16 networks (2 to the power of 5 or 32), i dont know which part constitutes the beginning of the host part of the address. Does it begin at the 112 (01110000) part of the address or the 0 (00000000) part of the address.

Begins at the 112, as that is where you can calculate how many hosts your subnet supports.

To find the number of host bits, subtract that number from 32. So if you have /28, 32-28=4, you will have 4 host bits. The formula for finding hosts is 2 to the power of the number of bits minus 2. In this example, it'll be 2^4-2, which gives you 14.

I stole that off another forum, as I couldn't think how to explain the formula.

 

[yominator]

Thanks for that. The address I've been given is 129.33.112.0/20. I need to make 16 subnets from this. So if I begin taking bits at the 112 part of the address, I would need to take 5 bits as 2 to the power of 5 is 32-2 = 30. That would allow me to make my 16 networks. Is that correct?