IP Subnet Calculator - Network Address, Host Range & CIDR

How IP Subnetting Works (Visual Guide)

Every IPv4 address is a 32-bit number split into four 8-bit octets, written in dotted-decimal notation like 192.168.1.0. Subnetting divides that 32-bit space into two parts: network bits on the left identify the network, and host bits on the right identify individual devices. The subnet mask tells routers where the boundary falls. When a router receives a packet, it performs a bitwise AND between the destination IP and the subnet mask to extract the network address, then consults its routing table to decide where to forward the packet. The number of host bits determines how many devices the subnet can accommodate — with n host bits you get 2ⁿ total addresses, minus two reserved for the network address (all host bits zero) and the broadcast address (all host bits one). Subnetting lets administrators carve a large address block into smaller, manageable segments, improving security by isolating broadcast domains and simplifying routing through address summarisation.

CIDR Notation Explained with Examples

Classless Inter-Domain Routing replaced the rigid Class A / B / C system in 1993 with a flexible prefix-length notation. Instead of saying a network uses a 255.255.255.0 mask, you append /24 to the IP address. The number after the slash is the prefix length — the count of contiguous 1-bits in the mask, starting from the most significant bit. A /24 network has 24 network bits and 8 host bits, supporting 256 total addresses (254 usable). A /16 has 16 network bits and 16 host bits — 65,536 addresses. A /32 is a single host address with no host bits at all, commonly used in routing tables and access control lists. CIDR enabled variable-length subnet masking, letting network engineers allocate blocks of precisely the size they need rather than wasting addresses on oversized class boundaries. It also enabled supernetting — aggregating contiguous smaller blocks into a single larger prefix for cleaner routing tables.

Subnet Mask Quick Reference Table

The relationship between prefix length, subnet mask, available addresses and usable hosts follows a simple pattern of powers of two. At /32, the mask is 255.255.255.255 and there is exactly one address — a single host. At /31 (255.255.255.254), there are two addresses with no usable hosts under the traditional model, though RFC 3021 allows point-to-point links. Moving left, /30 (255.255.255.252) provides four addresses and two usable hosts, while /29 (255.255.255.248) gives eight addresses and six hosts. A /28 (255.255.255.240) yields 16 addresses and 14 hosts, often used for small server segments. The popular /24 (255.255.255.0) provides 256 addresses and 254 usable hosts, the sweet spot for most local area networks. At /16 (255.255.0.0) you get 65,536 addresses, and /8 (255.0.0.0) provides over 16 million — historically allocated as Class A blocks to large organisations. Each step in prefix length either doubles or halves the available address space, making mental arithmetic straightforward once you internalise the pattern.

How to Divide a Network into Subnets

Network segmentation starts with identifying how many subnets you need and how many hosts each requires. To create n equal subnets from a parent block, find the smallest power of two greater than or equal to n and borrow that many bits from the host portion. Splitting a /24 into four subnets borrows two bits (2² = 4), producing four /26 subnets with 64 addresses each (62 usable). For eight subnets, borrow three bits to get /27 blocks with 32 addresses (30 usable). When subnets need different sizes — a server farm with 100 hosts alongside several point-to-point links — you use Variable Length Subnet Masking (VLSM). VLSM allocates the largest subnet first, then subdivides the remaining space for progressively smaller requirements. This avoids the waste inherent in equal-size division and is standard practice in modern network design using OSPF or EIGRP, which carry prefix-length information in their routing updates.

Public vs Private IP Address Ranges

RFC 1918 designates three IPv4 ranges for private use behind NAT gateways: 10.0.0.0/8 (16,777,216 addresses), 172.16.0.0/12 (1,048,576 addresses), and 192.168.0.0/16 (65,536 addresses). These addresses are not routable on the public internet, so any number of organisations can reuse them internally without conflict. Public addresses are assigned by Regional Internet Registries (ARIN, RIPE NCC, APNIC, LACNIC, AFRINIC) and are globally unique and routable. Other reserved ranges include 127.0.0.0/8 for loopback, 169.254.0.0/16 for link-local auto-configuration (APIPA), 100.64.0.0/10 for carrier- grade NAT, and 224.0.0.0/4 for multicast. Understanding which ranges are private helps network engineers design addressing schemes that will never collide with public infrastructure, and knowing the reserved blocks prevents accidentally assigning addresses that routers will refuse to forward.

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