OCI VCN CIDR Calculator
Carve an OCI VCN into subnets and count usable hosts after the three addresses Oracle reserves in every subnet.
About OCI VCN CIDR Calculator
The VCN address is masked to its prefix to find the block, then divided by the subnet size to count how many subnets fit. Oracle reserves three addresses in every subnet — the network address, the subnet's default gateway and the broadcast address — so a /24 yields 253 assignable addresses rather than the 254 an on-premises design would assume. Subnet reservations shift the usable count for every subnet in the VCN at once, and a VCN CIDR cannot be shrunk after creation, so sizing on the real assignable figure avoids a re-address later. The smaller the subnet, the more the three reserved addresses hurt: a /29 gives you five hosts, not eight.
Formula
OCI VCN CIDR
usable hosts = 2^(32 − subnet prefix) − 3, and subnets available = 2^(subnet prefix − VCN prefix).
- 3
- Addresses Oracle reserves per subnet: the network address, the subnet gateway and the broadcast address
- subnet prefix
- Prefix length of each subnet you carve out of the VCN
- VCN prefix
- Prefix length of the VCN itself, between /16 and /30
Worked example
With the default values below, OCI VCN CIDR Calculator returns Usable Hosts per Subnet of 253. Change any input above and every figure here is recalculated in your browser.
Inputs used
- VCN network address
- 10.0.0.0
- VCN prefix lengthaccepts 16–30 bits
- 16 bits
- Subnet prefix lengthaccepts 16–30 bits
- 24 bits
- Subnets you plan to createaccepts 1–4,096 subnets
- 8 subnets
Results
- Usable Hosts per Subnet
- 253
- Subnets Available in the VCN
- 256
- VCN Address Range
- 10.0.0.0 – 10.0.255.255
- First Subnet CIDR
- 10.0.0.0/24
- Usable Hosts Across Planned Subnets
- 2,024
- Addresses Reserved by Oracle
- 24
- VCN Space Allocated
- 3.1%
- Reservation Overhead
- 1.2%
Step by step
- 1Values used
- 2OCI VCN CIDR
- 3Usable Hosts per Subnet= 253
- 4Subnets Available in the VCN= 256
- 5VCN Address Range= 10.0.0.0 – 10.0.255.255
- 6First Subnet CIDR= 10.0.0.0/24
- 7Usable Hosts Across Planned Subnets= 2,024
- 8Addresses Reserved by Oracle= 24
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is OCI VCN CIDR calculated?
usable hosts = 2^(32 − subnet prefix) − 3, and subnets available = 2^(subnet prefix − VCN prefix). The VCN address is masked to its prefix to find the block, then divided by the subnet size to count how many subnets fit. Oracle reserves three addresses in every subnet — the network address, the subnet's default gateway and the broadcast address — so a /24 yields 253 assignable addresses rather than the 254 an on-premises design would assume.
Why does OCI VCN CIDR matter?
Subnet reservations shift the usable count for every subnet in the VCN at once, and a VCN CIDR cannot be shrunk after creation, so sizing on the real assignable figure avoids a re-address later. The smaller the subnet, the more the three reserved addresses hurt: a /29 gives you five hosts, not eight.
What values do I need to enter?
This calculator takes 4 inputs: VCN network address, VCN prefix length, Subnet prefix length, Subnets you plan to create. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Why does a /24 subnet in OCI give 253 hosts instead of 254?
Oracle takes three addresses from every subnet rather than two: the network address, the address used as the subnet's default gateway, and the broadcast address. A /24 has 256 addresses, so 253 remain assignable to instances and load-balancer VIPs.