Inspect subnet ranges
See the actual start and end addresses for CIDR blocks such as 10.0.0.0/24.
CIDR Calculator
브라우저 내부 처리
Compute subnet, broadcast, and host range details from IPv4 CIDR blocks.
CIDR calculation checklist
Calculate network address, broadcast address, and usable IPv4 host ranges before changing firewall or subnet settings.
Review access rules
Check whether a firewall, allowlist, or VPN range is broader or narrower than intended before deployment.
Document network values
Record network address, broadcast address, and host count to make infrastructure notes clearer.
Errors you will hit
The host count is two short
Usable hosts exclude the network address and the broadcast address. A /24 gives 254 usable hosts, not 256.
/31 and /32 behave differently
A /32 identifies a single host. A /31 is used for point-to-point router links and uses both addresses with no broadcast (RFC 3021).
A bigger prefix means a smaller network
A /26 is smaller than a /24. As the number grows the host count shrinks — a common inversion when writing firewall or security group rules.
Things to check
IPv4 focused
This tool focuses on IPv4 CIDR calculation. IPv6 uses different notation and sizing rules.
Reserved addresses vary
Cloud networks may reserve some addresses in every subnet. Confirm usable host counts with the platform documentation.
Two addresses are not usable
The first address in an IPv4 subnet is the network address and the last is the broadcast address. A /24 gives 254 usable hosts, not 256. /31 and /32 are exceptions, used for point-to-point links and single hosts.
Keep private ranges from overlapping
The private ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. If both sides of a VPN use the same block, routing collides and the link fails. When picking a new range, avoid the very common 192.168.0.0/24.
A subnet mask and a prefix are the same thing
255.255.255.0 and /24 are two notations for one value. Device UIs tend to use masks while cloud consoles and docs use prefixes. Since you move between them often, remember that each step halves: /24 is 256, /25 is 128.
More: step-by-step usage and worked examples
How to use it
- Enter the IPv4 address and prefix length in CIDR notation.
- Review the calculated network, broadcast, and host range values.
- Compare the result with the range needed by your allowlist or route rule.
- Apply cloud-provider reserved-address rules separately when relevant.
Real input and output
A common private subnet
Input
192.168.0.0/24Output
Network 192.168.0.0
Broadcast 192.168.0.255
Host range 192.168.0.1 - 192.168.0.254
Usable hosts 254A larger block
Input
10.0.8.0/22Output
Network 10.0.8.0
Broadcast 10.0.11.255
Host range 10.0.8.1 - 10.0.11.254
Usable hosts 1,022Tools that pair well
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