IP Subnet Calculator: CIDR, Mask, Host Range & IPv6
Calculate IPv4 and IPv6 subnets: network address, broadcast, usable host range, and prefix info. Binary visualizer, cheat sheet, and CIDR converter. Free subnet calculator for engineers.
What This Tool Does
IPv4: Enter an IP and CIDRโget network address, broadcast, usable host range, and total hosts. A binary visualizer shows network vs host bits. A cheat sheet lists typical prefixes (/8 through /32) with one click to apply. IPv6: Enter an address and prefixโget expanded/compressed format, network address, IP range, and total addresses. Hextet cards color-code Global Routing Prefix, Subnet ID, and Interface ID.
IPv4: Why We Subtract 2
Two addresses in every IPv4 subnet are reserved. The network address (host bits = 0) identifies the subnet. The broadcast address (host bits = 1) delivers to every host. So usable hosts = . A /24 has 256 total addresses but only 254 you can assign to devices.
IPv6: The Unimaginable Scale
IPv6 has 2128 addressesโroughly 340 undecillion. A /64 gives 264 addresses (18,446,744,073,709,551,616) per subnet. No broadcast, no NAT for scaling. Common prefixes: /48 for enterprise (65,536 subnets), /56 for residential (256 subnets), /64 for a single link. The :: compresses consecutive zeros but can appear only once per address.
Hex Breakdown: GRP, Subnet ID, Interface ID
For /64: hextets 0-2 (bits 0-47) are the Global Routing Prefix. Hextet 3 (bits 48-63) is the Subnet ID. Hextets 4-7 (bits 64-127) are the Interface ID. The tool updates colors dynamically as you change the prefix. IPv6 uses hexadecimal: 0-9 and A-F (10-15).
IP Subnet Calculator FAQ
? What is a subnet mask and how does CIDR relate to it?
A subnet mask marks which bits are network (1) vs host (0). CIDR condenses it: /24 means the first 24 bits are network. in dotted-decimal is the same as /24โboth mean 24 network bits, 8 host bits. Usable hosts = for IPv4 (network and broadcast addresses are reserved).
? Why does my subnet only have 254 usable hosts when it holds 256 addresses?
Two addresses in every subnet are reserved: the network address (all host bits 0) and the broadcast address (all host bits 1). You can't assign those to devices. So a /24 has 256 total addresses but only 254 usable for hosts. The formula always subtracts 2 for IPv4 subnets.
? What's the difference between IPv4 and IPv6 subnetting?
IPv4 uses 32 bits; IPv6 uses 128. IPv4 typically reserves 2 addresses (network + broadcast). IPv6 has no broadcastโit uses multicast. A standard IPv6 /64 gives 2โถโด addresses (18 quintillion) per subnet. No NAT neededโthe address space is enormous.
? When should I use /48, /56, or /64 for IPv6?
/48 is common for enterprise sitesโyou get 65,536 /64 subnets. /56 is typical for residential ISP assignmentsโ256 subnets. /64 is the standard for a single LAN or link. Most devices expect a /64; going smaller can break SLAAC and some features.
? How do I interpret the binary visualizer for the subnet mask?
The 1s (blue) are network bits; the 0s (green) are host bits. A /24 mask has 24 ones followed by 8 zerosโexactly 3 octets of network. The split shows where the "street address" ends and the "house number" begins. Same logic applies to IPv6 hextets.