Hi Jerald,
The cidrsubnet function takes a given existing prefix (/24 in this instance), applies an bitmask extended by the ‘number of additional bits’ (/25 = 1 additional bit, /26 2 additional bits) and then returns the ‘nth’ network.
So cidrsubnet(“10.221.22.0/24”, 1, 0) add an additional bit to the network bitmask, to create 2 new subnets (2^n=2 where n=number of additional bits=1) of 128 addresses (7 host bits = 2^7 = 128 addresses)
New subnets are therefore:
#0 - 10.221.22.0-127 as a.b.c.0hhhhhhh
#1 - 10.221.22.128-254 as a.b.c.1hhhhhhh
Consider your starting subnet 10.221.22.0/24 which can be represented as follows, with n showing the network part of the bitmap and h showing the host part:
nnnnnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh
You wish to further subnet this to get:
The first network of /25 (where a,b,c are the first octets of the network respectively, n are the additional subnet bits and h is the host range:
a.b.c.nhhhhh
and 1 network of /26 starting from 128 as start host address of that subnet.
a.b.c.nnhhhhhh
So there are two ways of doing this:
As you already have for the /25:
/25 = cidrsubnet("10.221.22.0/24", 1, 0)
And then for the /26. Ignore that you have calculated the /25 and just understand that you want the 3rd /26 subnet (as the first two /26 subnets 0-63 & 64-127 are the same range as your already reserved /25
/26 = cidrsubnet("10.221.22.0/24", 2, 2)
The second way, which is:
cidrsubnet(cidrsubnet("10.221.22.0/24", 1, 1), 1, 0)
In this second example you are taking the second network of the /25 (10.221.22.128-254):
a.b.c.1hhhhhhhh
and then feeding that network into another cidrsubnet to take the first network from the next additional but (/26):
a.b.c.11hhhhhh
Either are valid, but the second way explicitly shows that you are subnetting (to /26) a subnet (the second /25) of the original prefix range (The /24).
The second way would be the way you might illustrate it when working out a IP addressing scheme where you show you have allocated the first /25, and then further sub-netted the second /25:
| CIDR |
sub-Network Number (s bits) |
Network |
Subnet Mask |
Range |
| /24 |
|
10.221.22.0 |
11111111.11111111.11111111.00000000 |
10.221.22.0-256 |
| ___↳/25 |
0 |
10.221.22.0 |
11111111.11111111.11111111.s000000 |
10.221.22.0-127 |
| ___↳/25 |
1 |
10.221.22.0 |
11111111.11111111.11111111.s000000 |
10.221.22.128-255 |
| ______↳/26 |
0 |
10.221.22.0 |
11111111.11111111.11111111.1s00000 |
10.221.22.128-191 |
| ______↳/26 |
1 |
10.221.22.0 |
11111111.11111111.11111111.1s00000 |
10.221.22.192-255 |