IP Subnet

Splitting an IPv4 Address into Network and Host Portions

A subnet mask splits an IPv4 address into a network portion and a host portion, determining which addresses belong to the same local network. Enter an IP address and its CIDR prefix length (the “/24” in an address like 192.168.1.0/24), and this calculator finds the network address, broadcast address, subnet mask, and the range of addresses actually usable by devices.

The Formula

• Subnet mask: the prefix length’s worth of leading 1 bits, followed by 0 bits (e.g. /24 → 11111111.11111111.11111111.00000000 = 255.255.255.0).
• Network address: the IP address with a bitwise AND against the subnet mask — this zeroes out the host bits, leaving just the network.
• Broadcast address: the network address with every host bit set to 1 — the highest address in the subnet.
• Usable hosts: every address in the subnet except the network and broadcast addresses, since those are reserved — $2^{32 - \vA{\text{Prefix Length}}} - 2$ for ordinary subnets.

Two special cases carve out different rules: a /31 (RFC 3021) is used for point-to-point links and treats both addresses as usable hosts, since there’s no need for a broadcast address between exactly two devices. A /32 is a single-host route — the one address is both the network and its own only usable host.

Worked Example

192.168.1.0/24 (a $\vA{/24}$ prefix):

1. Subnet mask: 255.255.255.0 (24 bits of 1s, then 8 bits of 0s).
2. Network address: 192.168.1.0.
3. Broadcast address: 192.168.1.255.
4. Usable host range: 192.168.1.1 through 192.168.1.254 — $2^{32 - \vA{24}} - 2 = 2^8 - 2 = 254$ usable addresses.

Key Factors to Consider

• Subnetting is how a large network is divided into smaller, more manageable segments. Breaking a network into subnets can improve security (isolating traffic between segments), performance (reducing broadcast traffic on any one segment), and organization — a common practical reason to plan subnets deliberately rather than using one flat, large network.
• Private IP address ranges (like 192.168.x.x, 10.x.x.x, and 172.16-31.x.x) are reserved for internal networks and never routed on the public internet. These ranges are set aside specifically for local networks like home routers and office LANs — this calculator works identically for private and public address ranges, since the subnetting math itself doesn’t depend on which range an address falls in.
• A smaller prefix number (like /16) covers a much larger address range than a larger prefix number (like /28). This inverse relationship — fewer bits reserved for the network portion means more bits (and therefore more addresses) available for hosts — can be counterintuitive at first, since the smaller number describes the bigger subnet.
• IPv6 uses a fundamentally different, much larger addressing scheme than IPv4. This calculator is specifically for IPv4 (32-bit) addresses and CIDR notation — IPv6 uses 128-bit addresses with its own distinct subnetting conventions, not covered by this tool.

Common Mistakes

• Forgetting to subtract the network and broadcast addresses when counting usable hosts. A /24 has 256 total addresses but only 254 usable ones — the first and last are reserved, a detail that’s easy to overlook when quickly estimating capacity.
• Assuming /31 and /32 follow the same reserved-address rule as every other prefix. A /31 is specifically designed for point-to-point links and treats both of its addresses as usable, and a /32 is a single address that’s both its own network and host — applying the standard “subtract 2” rule to either gives a wrong (or negative) answer.
• Confusing which prefix number describes a bigger network. A /16 covers far more addresses than a /28, even though 16 is the smaller number — fewer bits reserved for the network portion leaves more bits, and therefore more addresses, for hosts.
• Applying IPv4 subnetting math to an IPv6 address. IPv6 uses 128-bit addresses and its own distinct subnetting conventions — this calculator’s formulas only apply to 32-bit IPv4 addresses.

Useful to Know

• Setting up a home network and need to estimate the bandwidth it should support rather than its addressing? The Internet Speed Requirement Calculator answers that separate question.
• Working with numbers in a different base as part of networking or programming work? The Number Base Converter Calculator converts between binary, decimal, hexadecimal, and other bases.
• Need a strong, random password for a router or network device while you’re at it? The Password Generator Calculator creates one to your specified requirements.