IP Subnet Calculator - 無料オンライン電卓

IPv4 subnet calculations are foundational for anyone managing networks - system administrators, network engineers, DevOps engineers, security analysts, and students preparing for certifications like Cisco CCNA, CCNP, Red Hat RHCE, AWS networking specialty, or CompTIA Network+. Given an IP and a CIDR prefix, this calculator instantly returns the network address, broadcast, valid host range, host count, classful designation, and binary representations - saving hours of error-prone manual binary arithmetic.

Subnetting is the practice of dividing a larger IP block into smaller blocks (subnets), typically to organise traffic, improve security through segmentation, or efficiently allocate addresses. Understanding CIDR notation (Classless Inter-Domain Routing) is essential - it replaced the older class-A/B/C system in the 1990s because it allowed flexible-size networks. A /24 has 254 usable hosts; /23 has 510; /22 has 1,022; each prefix bit you 'borrow' from the host portion doubles the network size. This calculator handles all CIDR values from /0 (entire IPv4 space) to /32 (single host), including the special /31 and /32 cases used for point-to-point links.

An IPv4 address is 32 bits, typically written as four dotted decimals (e.g. 192.168.1.10 = 11000000.10101000.00000001.00001010 in binary). The CIDR prefix indicates how many high-order bits are the network identifier. /24 means the first 24 bits are network, last 8 are host. The subnet mask in dotted decimal for /24 is 255.255.255.0 - the 1 bits indicate network, 0 bits indicate host.

Network address: bitwise AND of the IP and the mask, which zeros out the host bits. For 192.168.1.10/24, the network is 192.168.1.0. Broadcast address: bitwise OR of the network and the inverted (wildcard) mask, which sets all host bits to 1. For /24, broadcast is 192.168.1.255. Usable host range: network + 1 to broadcast - 1. Total usable = 2^(32-prefix) - 2. Two addresses are reserved per subnet (network and broadcast) except in special cases.

Special cases: /31 (RFC 3021) is a point-to-point link with 2 addresses, both usable (no network or broadcast reserved). /32 is a single host route, no host range. /0 represents the entire IPv4 space. Classful designation (deprecated but still referenced): Class A is 0.0.0.0-127.255.255.255 with default /8, Class B is 128-191 with default /16, Class C is 192-223 with default /24. Private RFC 1918 ranges: 10.0.0.0/8 (Class A private), 172.16.0.0/12 (Class B private), 192.168.0.0/16 (Class C private).