Private IP Planner
Plan private IP address space allocation across VPCs and subnets.
About Private IP Planner
RFC 1918 reserves three private address ranges for internal use: 10.0.0.0/8 (about 16.7 million addresses — the largest, best suited for big multi-site/multi-VPC deployments), 172.16.0.0/12 (about 1 million addresses), and 192.168.0.0/16 (65,536 addresses — common for small offices and home networks). Planning at scale means picking a per-site/per-VPC CIDR block size large enough for that site's address needs, then multiplying by the number of sites to check the total fits within the chosen range — while leaving room for growth and avoiding overlaps with other environments you may need to connect to.
Formula
Total addresses needed with CIDR alignment
total = per_site_block_size × number_of_sites, where block_size is the smallest power of 2 ≥ addresses needed per site
- p
- Per-site CIDR prefix (chosen so 2^(32-p) ≥ addresses needed)
- n
- Number of sites/VPCs
Worked example
With the default values below, Private IP Planner returns Recommended Per-Site Block Size of /20. Change any input above and every figure here is recalculated in your browser.
Inputs used
- RFC 1918 Rangeone of 10.0.0.0/8 (16,777,216 addresses), 172.16.0.0/12 (1,048,576 addresses), 192.168.0.0/16 (65,536 addresses)
- 10.0.0.0/8 (16,777,216 addresses)
- Number of Sites / VPCs to Allocateminimum 1
- 20
- Addresses Needed per Site/VPCminimum 1
- 4,096
Results
- Recommended Per-Site Block Size
- /20
- Total Addresses Allocated (with CIDR alignment)
- 81,920
- Range Utilization
- 0.488%
- Total Addresses in Selected Range
- 16,777,216
- Selected RFC 1918 Range
- 10.0.0.0/8
Step by step
- 1Values used
- 2Total addresses needed with CIDR alignment
- 3Recommended Per-Site Block Size= /20
- 4Total Addresses Allocated (with CIDR alignment)= 81,920
- 5Range Utilization= 0.488
- 6Total Addresses in Selected Range= 16,777,216
- 7Selected RFC 1918 Range= 10.0.0.0/8
Assumptions and limits
- The result is derived only from the 3 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 3 decimal places, 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
Which RFC 1918 range should I use for a large enterprise?
10.0.0.0/8 is generally preferred for large or growing deployments needing many sites/VPCs, since its ~16.7 million addresses provide the most room for hierarchical subnetting (region/site/subnet) without running out of space, unlike the much smaller 172.16/12 or 192.168/16 ranges.
Why allocate the same block size to every site instead of exact sizing?
Using a consistent, CIDR-aligned block size per site simplifies routing, summarization, and firewall rule management significantly — even if it means a few unused addresses per site, the operational simplicity of uniform blocks usually outweighs the minor address space waste.
What happens if I need to connect environments using overlapping RFC 1918 ranges?
Overlapping private address ranges between two environments you need to connect (e.g. via VPN, peering, or company merger) require either re-addressing one side or using NAT/overlay techniques to work around the conflict — both costly, which is why planning non-overlapping allocations from the start matters.
Should I leave room for growth when picking a per-site block size?
Yes — choosing the very next power-of-two size larger than your immediate need (rather than the bare minimum) gives headroom for organic growth without needing to renumber or expand into fragmented, non-contiguous address space later.