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Subnetting Question about CIDR notation

Posted on 2016-09-07
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Last Modified: 2016-09-09
This is a follow up question to this one:
https://www.experts-exchange.com/questions/28967640/Subnetting-Network-with-six-subnets.html?anchor=a41784308#a41784308

I have the IP address 192.168.50.0 and I create 8 subnets.

Is the broadcast address really 192.168.50.31 - even though it's in the first subnet?

I see in online calculators that the CIDR notation is 192.168.50.0/27 Why 27?

It is unfortunately not mentioned in my network book.

Thanks for your help
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Question by:Massimo Scola
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by:Qlemo
Qlemo earned 250 total points
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192.168.50.0/27 builds 8 subnets based on 192.168.50.0/24, that much is true. /27 because you use 3 more bits starting from 24, for 8 subnets (2^3 = 8).

The broadcast address is the last one in each subnet - 192.168.50.31, 192.168.50.63, 192.168.50.95 aso.
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giltjr earned 250 total points
ID: 41788919
An IP V4 address is made up of 32 bits.  The address i broken into 2 parts.  The network part, which is common to all hosts within the same IP network/subnet. Then the host part which is unique to each host within the IP network/subnet.

When you see a /##, like /24 or /27, the number after the slash tells you how many of the 32-bit address represents the IP network/subnet.  What is left over represents the host part.

So if you have a /24, 32-24 = 8, so you have 8 bits to represent each host within the network.  With 8 bits you have 256 addresses, but 1 (the lowest number) represents the network, and 1 (the highest number) is used as the broadcast address.

With a /27, the network is represented by the 1st 27 bits in the address.  For hosts you would do 32-27 = 5, so you have 5 bits to represent hosts within your network.
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Author Closing Comment

by:Massimo Scola
ID: 41791036
Thanks so much. This was so helpful making me understand the concept of CIDR. Something which should have been mentioned a bit more in detail in the book.
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