IPv4 Subnet & CIDR Calculator
Calculate network address, broadcast address, netmask, wildcard mask, and usable host IP ranges with accurate /31 and /32 support.
How to Use IPv4 Subnet & CIDR Calculator
- Enter any valid IPv4 address (e.g., 192.168.1.1 or 10.0.0.1).
- Select or enter a CIDR prefix length between /0 and /32 (e.g., 24 for a 255.255.255.0 netmask).
- Click Calculate Subnet or Execute to calculate subnet boundaries, wildcard masks, and host counts.
- Inspect the usable host range, binary representations, and RFC 1918 scope classification.
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Frequently Asked Questions
What is CIDR notation and how does it relate to an IPv4 subnet mask?
Classless Inter-Domain Routing (CIDR) notation represents an IP address and its associated routing prefix using a slash followed by the count of leading 1-bits in the subnet mask (e.g., /24). An IPv4 address is 32 bits long. A /24 prefix means the first 24 bits are 1s and the remaining 8 bits are 0s, translating in dotted-decimal format to 255.255.255.0. This allows flexible allocation of IP addresses without the rigid constraints of legacy Class A, B, and C networks.
How is the usable host count calculated for standard IPv4 subnets?
For standard subnets with CIDR prefixes from /0 to /30, the number of usable host addresses is calculated using the formula: 2^(32 - prefix) - 2. The subtraction of 2 accounts for two reserved addresses that cannot be assigned to hosts: the Network Address (where all host bits are binary 0, identifying the subnet itself) and the Broadcast Address (where all host bits are binary 1, used to address all hosts on the subnet). For a /24 subnet, 2^(32 - 24) - 2 = 256 - 2 = 254 usable hosts.
How do /31 and /32 subnets differ from standard subnets?
Under RFC 3021, a /31 subnet has a 2-bit host space (2 total addresses) and is specifically designed for point-to-point router links. Because point-to-point links only connect two endpoints, dedicated network and broadcast addresses are unnecessary, meaning all 2 addresses are usable hosts. A /32 subnet has a 0-bit host space and represents a single host route or loopback interface with exactly 1 address and 0 host range.
What is a wildcard mask and how is it used in network configuration?
A wildcard mask is the bitwise inverse of a subnet mask, calculated by subtracting each octet of the netmask from 255 (e.g., 255.255.255.255 - 255.255.255.0 = 0.0.0.255). Wildcard masks are widely used in Cisco IOS Access Control Lists (ACLs) and routing protocols like OSPF and EIGRP to specify which IP bits must match (0 bits) and which can be ignored (1 bits).
What are RFC 1918 private IPv4 address spaces?
RFC 1918 reserves three blocks of IPv4 address space for private networks that are not publicly routable on the global internet: 10.0.0.0/8 (10.0.0.0 to 10.255.255.255, Class A), 172.16.0.0/12 (172.16.0.0 to 172.31.255.255, Class B), and 192.168.0.0/16 (192.168.0.0 to 192.168.255.255, Class C). EverySolve automatically detects and classifies whether your IP is private, public, loopback (127.0.0.0/8), or link-local (169.254.0.0/16).