Subnet Calculator
Free IPv4 CIDR Calculator

A subnet calculator takes an IP address and a prefix length (CIDR notation, e.g. 192.168.1.0/24) and computes the network address, broadcast address, first and last usable host addresses, subnet mask, wildcard mask, and the number of usable hosts. It's an essential tool for network engineers, sysadmins, and developers planning IP address allocation — whether for on-premises infrastructure, cloud VPCs, or firewall ACLs.

Enter the IP address and prefix in the calculator above, for example 10.0.0.0/8. The tool instantly shows the subnet mask (255.0.0.0 for /8), wildcard mask, network address, broadcast address, and host range. You can also enter the mask directly as 10.0.0.0 255.0.0.0 — both formats are accepted. The subnet mask is derived by setting the first N bits to 1 and the remaining 32−N bits to 0, then writing it in dotted-decimal.

/24 means the first 24 bits of the IP address are the network part and the remaining 8 bits are for hosts. This gives a subnet mask of 255.255.255.0, a total of 256 addresses (0–255 in the last octet), and 254 usable host addresses — network (.0) and broadcast (.255) are reserved. The /24 is the most common subnet size — it maps to the old "Class C" network and is the default LAN segment in most home and office networks.

A /27 subnet has 32 total addresses (2⁵ = 32, since 32−27 = 5 host bits). Two are reserved — the network address and the broadcast address — leaving 30 usable host addresses. The subnet mask is 255.255.255.224 and the wildcard mask is 0.0.0.31. A /24 can be split into exactly 8 /27 subnets.

The network address is the first address in a subnet (all host bits set to 0). It identifies the subnet itself and appears in routing tables — no device can be assigned this address. The broadcast address is the last address in the subnet (all host bits set to 1). Packets sent to the broadcast address are delivered to every host on that subnet simultaneously. Both are unusable as individual host addresses, which is why a /24 has only 254 usable hosts despite having 256 total addresses.

VLSM allows a network to be divided into subnets of different sizes rather than using a single uniform mask. For example, you can allocate a /30 (4 addresses, 2 usable) for a point-to-point link between routers, a /28 (16 addresses, 14 usable) for a small server cluster, and a /24 (256 addresses, 254 usable) for a large office LAN — all carved from the same larger block. Modern routing protocols like OSPF, EIGRP, and BGP carry the mask with every route advertisement, making VLSM the standard approach for efficient IP address utilisation.

Two addresses in every IPv4 subnet are reserved by the IP protocol: the first address (network address) identifies the subnet in routing tables, and the last address (broadcast address) is used to reach all hosts on the subnet at once. The only exceptions are /31 subnets (RFC 3021), where both addresses are usable because there are only two hosts and no need for a broadcast domain, and /32, which is a single host route where the address is the host itself.