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So we are using CIDR for subnetting In that we just define the subnets by using / SO my question is if I am using /24 it means that I have the last octet for host. But if we use anything from /17 to /23 we say that we have borrowed bits from host space. For eg: if use /17 11111111.11111111.**1**0000000.00000000 The 1st bit in the third octet is automatically considered as bit for subnet id So does that mean the subnet bit is octet specific because if we take \23 11111111.11111111.**1111111**0.00000000 the subnet id then has 7 bits But the networking devices only treat the IP address as a 32 bit number then how is it able to understand where the subnet id bits start using cidr. Because in CIDR the network id and subnet id are combined???

Or is it so that IP addresses considered as 32 bit numbers have only network id portion and host id portion and the subnets are only visible in the dotted decimal notation...

I am newbie please explain in a way I can easily understand Please Help Thanks in andvance

Tanmay B
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  • Ok @Zac67 does that mean that subnet mask helps in determining the subnetid portion – Tanmay B Apr 10 '20 at 10:21
  • The "subnetid portion" only exists in the exact moment when you are subnetting or supernetting. It is meaningless everywhere else - there's only a network portion and a host portion. – Zac67 Apr 10 '20 at 10:34
  • Ok that's what I wanted to know. – Tanmay B Apr 10 '20 at 10:35
  • Also in the above I aske whether it is octet specific while subnetting a network... What I mean to say is if we are using 1s and 0s in the same octet for mask it means the subnet id exists there – Tanmay B Apr 10 '20 at 10:36
  • The octet notation is only for humans. For the network it's a 32-bit integer, as you've pointed out. – Zac67 Apr 10 '20 at 10:40
  • Ok I edited the comment when you were replying please check out.... Also Thanks for helping out so quickly – Tanmay B Apr 10 '20 at 10:43
  • It's actually all in the question I linked above, check the *IPv4 math* section. – Zac67 Apr 10 '20 at 10:45

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