IP Subnet Calculator

IP subnetting is the practice of dividing a single IP network into smaller logical sub-networks (subnets). It improves security by isolating traffic between segments, reduces broadcast domains (improving performance), enables efficient use of IP address space, and simplifies routing. For example, a /24 network gives you 256 addresses for one segment, whereas splitting it into two /25 networks lets you separate two departments while keeping their traffic isolated.

CIDR (Classless Inter-Domain Routing) notation combines an IPv4 address and a prefix length into one string, like 192.168.1.0/24. The number after the slash is the prefix length — it tells you how many leading bits of the 32-bit address identify the network. /24 means 24 bits are the network portion (256 total addresses, 254 usable). /16 means 16 bits are the network (65,536 addresses). /30 means 30 bits are the network (4 total addresses, 2 usable) — ideal for point-to-point links.

A subnet mask is a 32-bit number where all network bits are 1 and all host bits are 0. For /24: the binary mask is 11111111.11111111.11111111.00000000 = 255.255.255.0. To find the network address, you bitwise-AND the IP address with the subnet mask. To find the broadcast address, you bitwise-OR the network address with the wildcard mask (bitwise inverse of the subnet mask). For /24: wildcard = 0.0.0.255, broadcast = network + 255. The binary representation makes the boundary between network and host bits visually clear.

IPv4 classful addressing divides the address space by the value of the first octet: Class A (1–126) — first bit 0, /8 default mask, 16.7M hosts per network, used for large organizations. Class B (128–191) — first two bits 10, /16 default mask, 65,534 hosts per network. Class C (192–223) — first three bits 110, /24 default mask, 254 hosts per network. Class D (224–239) — reserved for multicast groups. Class E (240–255) — reserved for experimental use. Note: 127.x.x.x is reserved for loopback. Modern routing uses CIDR and largely ignores classful boundaries, but IP class still describes the address's historical range.

RFC 1918 defines three private address ranges that are not routed on the public internet: 10.0.0.0/8 (10.0.0.0 – 10.255.255.255, ~16.7 million addresses) — used for large enterprise networks and cloud VPCs. 172.16.0.0/12 (172.16.0.0 – 172.31.255.255, ~1 million addresses) — common in Docker and Kubernetes environments. 192.168.0.0/16 (192.168.0.0 – 192.168.255.255, 65,536 addresses) — standard for home routers and small office networks. Additionally, 127.0.0.0/8 is loopback, and 169.254.0.0/16 is link-local (APIPA, assigned when DHCP fails). Use private ranges for internal networks; devices need NAT to reach the public internet.