Subnet LedgerCIDR planner and inspectorruns in your browser, nothing is sent

A /24 has 256 addresses. You can use 254. In AWS, 251.

Every subnet loses addresses before the first server gets one. Pick the rule, drag the prefix, then plan a whole network without running out of room.

1 / How many can you use?

254usable addresses in a /24

256total

2lost

0.8%overhead

Prefix length

/24

usablereservednot in block

2 / Guess first

3 / Plan a network

List every subnet by how many hosts it needs. The planner picks the smallest block that fits under the rule chosen above, sorts big to small so every block stays aligned, and shows what is left.

Parent block

4 / Inspect any CIDR

Accepts 10.1.2.3/20, 10.1.2.3 255.255.240.0 or a bare address (a /32).

Is this address inside?

Sources

All rules and ranges below were opened and read on 13 Nov 2026. The arithmetic is plain powers of two.

CIDR prefixes, masks and block sizes: RFC 4632, Classless Inter-domain Routing (primary)
/31 point-to-point links with two usable addresses: RFC 3021 (primary)
Private ranges 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16: RFC 1918 (primary)
Shared address space 100.64.0.0/10: RFC 6598 (primary)
Link-local 169.254.0.0/16: RFC 3927 (primary)
Documentation ranges 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24: RFC 5737 (primary)
This network, loopback, multicast, reserved and limited broadcast blocks: RFC 5735 (primary, a later registry exists in RFC 6890)
First four and last address reserved in every subnet, subnets from /28 to /16: AWS VPC subnet sizing documentation (primary)
Azure reserves the first four and the last address, five per subnet: Azure Virtual Network FAQ (primary)

Other clouds may reserve a different number. This tool models the three rules above and applies AWS's /28 minimum in AWS / Azure mode.