Non-Aligned Range: Multiple CIDR Blocks

Why Non-Aligned Ranges Produce Multiple CIDRs

CIDR blocks have two constraints: the size must be a power of 2, and the start address must be aligned to that size. When an IP range violates either constraint, it must be expressed as multiple CIDR blocks.

Example: 41-Address Range

Range: 192.168.1.10 - 192.168.1.50

This 41-address range produces 6 CIDR blocks:

CIDR	Addresses	Range
192.168.1.10/31	2	.10-.11
192.168.1.12/30	4	.12-.15
192.168.1.16/28	16	.16-.31
192.168.1.32/28	16	.32-.47
192.168.1.48/31	2	.48-.49
192.168.1.50/32	1	.50
Total	41

The Algorithm

The tool uses a greedy algorithm:

1. Start at the first address
2. Find the largest valid CIDR block starting at that address that doesn't exceed the end
3. Add it to the result
4. Move to the next address after that block
5. Repeat until the entire range is covered

Alignment Rules

A /N block must start at an address divisible by 2^(32-N):

Prefix	Block Size	Start Must Be Divisible By
/32	1	1 (any address)
/31	2	2
/30	4	4
/28	16	16
/24	256	256
/16	65,536	65,536

Minimizing CIDR Blocks

To get fewer CIDR blocks:

• Align your start address to a power-of-2 boundary
• Choose an end address that completes a power-of-2 block
• Round up the range if your use case allows slightly more IPs

For example, instead of 192.168.1.10-192.168.1.50 (6 blocks), use 192.168.1.0-192.168.1.63 (1 block: /26).

Real-World Impact

More CIDR blocks means more firewall rules. Some firewalls have rule limits, so minimizing blocks matters. If you control the range assignment, choose aligned boundaries from the start.