Private IP Planner
Plan private IP address space allocation across VPCs and subnets.
Inputs
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
Values used
RFC 1918 Range = 10.0.0.0/8 (16,777,216 addresses); Number of Sites / VPCs to Allocate = 20; Addresses Needed per Site/VPC = 4,096
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
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
How it works
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
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.