IPv4 CIDR calculator - network, broadcast, host range and mask
What is the IP Subnet Calculator?
The ip subnet works like this: it performs IPv4 subnet math on an address and CIDR prefix: masking the address to its network, deriving the broadcast, first/last usable host, total and usable address counts, wildcard mask, dotted and binary mask forms, plus RFC 1918 private-range classification - and checks whether any second IP falls inside the subnet. Everything happens on your device - the input never leaves your tab unless the tool clearly states it performs a server-side lookup.
How to use the IP Subnet Calculator
Enter any IPv4 address - your own, a server’s, or a made-up one to explore.
Pick the CIDR prefix; the dropdown shows every mask in dotted-decimal with the classic ones labelled.
Read the full table - every row is copyable with one click for runbooks and docs.
Type a second address in the probe box to test membership: inside, outside, and which sibling subnet it lands in.
Features
Full RFC-correct results for every prefix /0 through /32, including the /31 point-to-point and /32 single-host special cases
Binary mask display - see exactly which bits are network vs host
Private/public classification against all three RFC 1918 ranges (10/8, 172.16/12, 192.168/16)
IP membership probe that names the subnet a foreign address actually belongs to
Common mistakes to avoid
Counting the network and broadcast addresses as usable hosts - on any prefix shorter than /31, they are reserved
Reading 192.168.x.x as "Class C private" by octet alone - classfulness died with CIDR; privacy is range-based, and 172.16.0.0/12 spans 172.16-172.31
Planning a /24 for 254 devices and forgetting gateways, DHCP margins and growth - leave headroom or plan for /23
Mixing up subnet mask (/prefix length) with usable-host count - they scale in opposite directions
Planning IP ranges for offices, labs and VPCs without spreadsheet errors
Checking whether two addresses can talk without a router
Verifying firewall and DHCP configurations against the real math
Studying for network certifications (Network+, CCNA-style questions)
Privacy
Pure arithmetic - the addresses you type never leave the page. There are no lookups, no pings and no DNS queries.
Frequently asked questions
How do I find the network address manually?
Convert the IP and the mask to binary, and keep only the bits where the mask is 1. For 192.168.1.130/26: the last octet 130 is 10000010 in binary, the /26 mask keeps the top two bits, giving 10000000 = 128 - so the network is 192.168.1.128.
Why does /24 give only 254 usable addresses, not 256?
Two addresses are always reserved: the network address itself (all host bits 0) identifies the subnet, and the broadcast address (all host bits 1) reaches every host in it. Only the rest can be assigned to devices.
What are /31 and /32 used for?
RFC 3021 permits /31 (two addresses, both usable) for point-to-point router links where a broadcast is meaningless. /32 is a single host - used for loopbacks and host routes. This tool handles both without the reserved-address subtraction.
Is my IP private or public?
Private means one of three ranges: 10.0.0.0-10.255.255.255, 172.16.0.0-172.31.255.255, 192.168.0.0-192.168.255.255. The tool flags this automatically - private addresses exist behind NAT and are not routable across the internet.
Related tools
IP Address Checker - Check your public IP, location, ISP, VPN and proxy detection