If VPC is assigned CIDR block, then I can create sub-nets within VPC, following this procedure:

        172        .       31        .      0        .        0
        255        .       255       .      0        .        0
      11111111           11111111        00000000         00000000

In CIDR notation, we write where 16 is the # of left most bits that cannot be re-used for host addressing.

Take LSBit for second octet(shown below), because second octet has the last bit with mask 1

1---------1--------1------1 ------1-----1-----1-----1 (second octet mask)

128-----64------32-----16------8-----4-----2-----1 (mask bit position)

This LSBit(in bold) in second octet gives first sub-network and also the range(as shown below)

       +1      First subnet range(  to 
       +1      Second subnet range(  to 
       +1      Third subnet range(  to 
       +1      Fourth subnet range(  to 
       +1      Fifth subnet range(  to 
and so on....

So, CIDR assigned to VPC is already allocating subnets(shown above) and their possible IP range for each subnet.

Why again we assign another CIDR block for a subnet in VPC? as shown below...

enter image description here

  • Forget octets, they are meaningless.
    – Ron Maupin
    Jan 5, 2019 at 20:52
  • @RonMaupin Yes in CIDR, octets are meaning less but, how would you know first subnet and its range, without knowing the octet having LSBit? Jan 5, 2019 at 20:54
  • Octets have nothing do do with addressing or subnetting; they are simply to make it easier for humans to read an IPv4 address. See this two-part answer about that.
    – Ron Maupin
    Jan 5, 2019 at 20:56
  • @RonMaupin Do you agree with subnet ranges given in the query, to be correct? with /16 Jan 5, 2019 at 21:00
  • Based on what you have written, then I do not think you really understand subnetting. The answer I linked will explain it all. The second part of the answer covers subnetting, but you really need to understand the first part of the answer before you get that far.
    – Ron Maupin
    Jan 5, 2019 at 21:03

1 Answer 1


This is nothing special to VPC.

From the diagram, is simply subnetted to, and so on.

Check out the excellent answers Ron has linked to for a more detailed explanation.


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