CIDR Subnet Calculator (IPv4)

Networking · IPv4 · subnets

Calculate IPv4 network address, broadcast, subnet mask, and usable host range from an IP address and CIDR prefix. Plan VPC blocks and firewall rules with client-side math — no account, no upload.

CIDR → network details Netmask & host count Usable range VPC / firewall planning Pure client-side

What CIDR notation means

CIDR (Classless Inter-Domain Routing) writes a network as address/prefix, for example 10.0.0.0/24. The prefix length is the number of leading 1-bits in the subnet mask. /24 means 255.255.255.0 and 256 addresses (254 usable hosts on a classic LAN, after network and broadcast). /32 is a single host; /16 is a large private block often sliced into smaller subnets in cloud VPCs.

How to use this calculator

Enter any IPv4 address (it can be a host inside the subnet) and a prefix from 0–32.
Click calculate to see network address, broadcast, mask, total addresses, and usable range.
Use those values in security groups, route tables, VPN configs, or documentation.
Double-check overlapping ranges before peering VPCs or merging datacenter networks.

Prefix size reference

Prefix	Mask	Addresses	Typical use
`/32`	255.255.255.255	1	Single host route
`/31`	255.255.255.254	2	Point-to-point links (RFC 3021)
`/30`	255.255.255.252	4	Legacy P2P, small links
`/28`	255.255.255.240	16	Small app subnet
`/24`	255.255.255.0	256	Common LAN / AZ slice
`/16`	255.255.0.0	65 536	Large private allocation

Cloud tip: AWS, GCP, and Azure reserve extra addresses in each subnet beyond the classic network/broadcast picture. Always leave headroom and read your provider’s “usable IPs per CIDR” table before packing instances tightly.

Worked example

Input:  192.168.1.100 / 24

Network:   192.168.1.0
Broadcast: 192.168.1.255
Mask:      255.255.255.0
Hosts:     192.168.1.1 – 192.168.1.254  (typical usable)

The host 192.168.1.100 sits inside that network; the calculator normalizes to the block boundaries.

Common mistakes

Overlapping VPC CIDRs — breaks peering and hybrid connectivity; plan non-overlapping private space early.
Too-small subnets — load balancers, private endpoints, and managed services consume hidden IPs.
Public vs private confusion — RFC 1918 space (10/8, 172.16/12, 192.168/16) is not routable on the public Internet.
Off-by-one in allow lists — firewall rules with the wrong prefix can expose whole ranges or block production.

FAQ

Is IPv6 supported?

This calculator focuses on IPv4 CIDR. IPv6 uses much larger prefixes (often /64 for LANs) and different operational norms.

Does this probe my network?

No. It only does arithmetic in the browser from the values you type.

Network and broadcast always unusable?

On traditional Ethernet LANs yes for /24-style subnets; special cases like /31 and cloud provider rules differ. Check the environment you deploy into.

Related tools

Part of DevToolBox — free developer utilities that run in your browser. About · Privacy