Reading CIDR Notation
CIDR (Classless Inter-Domain Routing) notation writes a network as an IP address followed by a slash and a prefix length, like 192.168.1.0/24. The prefix (0-32) is how many leading bits of the 32-bit address are fixed as the network portion; the remaining bits identify individual hosts within that network. A smaller prefix number means a bigger network: /24 has 256 addresses, /16 has 65,536, and /8 has over 16 million.
Network, Broadcast, and Usable Host Count Explained
The network address is the first address in the block (all host bits zero) and identifies the subnet itself, never assigned to a device. The broadcast address is the last address (all host bits one) and is reserved for sending to every host on that subnet at once. Usable hosts is the total address count minus these two reserved addresses, except for /31 (a 2-address point-to-point link, RFC 3021, where both addresses are usable) and /32 (a single host address, no network/broadcast distinction at all).
The Binary View: Network Bits vs. Host Bits
The binary view shows all 32 bits of the IP, mask, network, and broadcast addresses side by side, split into the four familiar octets. Bits highlighted as "network" are the fixed prefix bits shared by every address in the block; the remaining "host" bits are what varies between individual addresses, from all-zero (the network address) to all-one (the broadcast address). Seeing this bit-by-bit often makes subnetting click in a way decimal notation alone doesn't.
Private, Reserved, and Public Ranges Explained
Not every IP address is routable on the public internet. RFC 1918 reserves 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 for private networks (your home router almost certainly uses one of these). 127.0.0.0/8 is loopback (localhost), 169.254.0.0/16 is link-local (APIPA, assigned automatically when DHCP fails), 100.64.0.0/10 is Carrier-Grade NAT (RFC 6598, used inside ISP networks), and 224.0.0.0/4 is multicast. This tool automatically tags whichever range your input CIDR block falls into.
Splitting a Block Into Smaller Subnets
The subnet split table shows what your block looks like divided into progressively smaller subnets, one prefix size at a time (for example, splitting a /24 into two /25s, four /26s, and so on). This is the core idea behind VLSM (Variable Length Subnet Masking): carving one large allocation into right-sized pieces instead of wasting addresses on a one-size-fits-all subnet. This subnet calculator works as an ip subnet calculator, a network address calculator and broadcast address calculator in one, a subnet mask calculator, a cidr notation calculator, an ip range calculator, a vlsm calculator for splitting blocks, and a private ip range checker for RFC 1918 addresses.