# Subnetting, How to calculate Network prefix(Vlsm)

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Hi people,

Me again, I am looking for the calculation needed to determine the network prefix for a network.I have had the following question on the ccna assesment (I failed) I know what vlsm is I just cannot find the formula or method to determine the right network prefix, I 've done some google'ing but I cannot get what I'm looking for,maybe my search is wrong but I don't know, please help

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I think it was a temporary mediafire error, it shows properly now here.Thanks in advance.
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/27 is the correct answer. You want .37 and .62 in one network and .76 and .94 in the other network.

I'm sorry but that doesn't tell me anything. How do I calculate that?

Do you understand that /27 means that the mask is 255.255.255.224 and that the bits are 11111111 11111111 11111111 111000000? If the last 5 bits have a value of 31 (16+8+4+2+1), then a 27 bit mask allows for 8 networks having IP address ranges of 0-31, 32-63, 64-95, 96-127, etc. The goal is to make sure that each router is in a different netwok. You don't calculate it; you need to understand how it works.

GhislainG said:
Do you understand that /27 means that the mask is 255.255.255.224 and that the bits are 11111111 11111111 11111111 111000000? If the last 5 bits have a value of 31 (16+8+4+2+1), then a 27 bit mask allows for 8 networks having IP address ranges of 0-31, 32-63, 64-95, 96-127, etc. The goal is to make sure that each router is in a different netwok. You don't calculate it; you need to understand how it works.

Yes I understand that the snm would be 224 having a /27 as you use 27 bits and the 5 left are for hosts.I also understood the values you mentioned. Why do you say a 27 bit mask allows for 8 networks with those ranges, how did you figure it out? Sorry for being a pain but it is my first time doing subnetting.
Thanks for the help so far.

It's easy. Each subnet contains 32 addresses. 256 / 32 = 8.
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