IP Subnet Calculator

What Is IP Subnetting?

Subnetting is the practice of dividing a single IP address range into smaller logical networks called subnets. Rather than assigning every device in an organization to one flat network, subnetting creates boundaries — each subnet operates as its own network segment, with its own network address and broadcast address bookending the usable host range.

Two things drive subnetting in practice: efficiency and security. A /24 subnet (192.168.1.0 through 192.168.1.255) contains 254 usable host addresses. Splitting it into four /26 subnets gives you four groups of 62 usable hosts each — the same 254 addresses distributed across isolated segments where a broadcast storm or security breach in one subnet doesn't automatically affect the others. Reference: RFC 1918 — Address Allocation for Private Internets

How to Use This Calculator

1. Enter an IPv4 address — either a network address (192.168.1.0) or a host address within a network (192.168.1.100). The calculator automatically finds the containing network.
2. Select a CIDR prefix (/1 through /32) from the dropdown — or type it directly after a slash in the IP field (192.168.1.0/24)
3. Click "Calculate Subnet" to see the network address, broadcast address, subnet mask, wildcard mask, first and last usable hosts, total addresses, and usable host count
4. The binary representation panel shows the IP address in binary — blue bits are the network portion, orange bits are the host portion, making the mask's function visual
5. For prefixes /24 and smaller (/25, /26, /27…), a subnet division table appears showing all subnets that fit within a /24 block

How Subnetting Works — The Math

Every IPv4 address is 32 bits. The CIDR prefix tells you how many of those bits identify the network versus the host portion. A /24 means 24 bits are network, 8 bits are host — 2⁸ = 256 total addresses, minus network and broadcast = 254 usable hosts.

CIDR Reference Table — Common Prefixes

The most-used CIDR prefixes in real network deployments, with verified host counts. Every "Usable Hosts" figure = 2^(32−prefix) − 2:

Private IP Ranges (RFC 1918)

Three address ranges are reserved for private networks and are never routed on the public internet. Any device behind a router using NAT (Network Address Translation) almost certainly has one of these addresses. Knowing the ranges helps immediately identify whether an IP is internal or public:

If you see a 169.254.x.x address on a Windows or macOS machine, it means the device tried DHCP and got no response — it assigned itself an APIPA address and is unreachable from the rest of the network until DHCP is fixed. Source: IANA — IPv4 Special-Purpose Address Registry

IPv4 Address Classes

Before CIDR (Classless Inter-Domain Routing) became the standard in 1993, IPv4 addresses were divided into fixed classes. Class-based addressing is rarely used today, but the terminology still appears in legacy documentation and some router configurations:

Most home routers use 192.168.x.x (Class C range) because it fits comfortably within the private allocation and the /24 default gives 254 host addresses — enough for any home or small office. Large enterprises and cloud providers predominantly use the 10.x.x.x range (/8) for its 16+ million addresses.

Dividing 192.168.1.0/24 into /26 Subnets

A common real-world scenario: you have a /24 network and need to split it into four equal subnets. Adding 2 bits to the prefix (/24 → /26) creates 2² = 4 subnets, each with 2⁶ − 2 = 62 usable hosts. The calculator's subnet division panel shows this automatically for the input — here's the manual breakdown:

Four subnets × 62 usable hosts = 248 usable addresses out of 254 available in the /24. The 6 addresses lost are the four network addresses and four broadcast addresses (one per subnet) minus the original /24's two (network and broadcast) that were already consumed. This tradeoff — slightly fewer usable addresses in exchange for network segmentation — is worth it for security and traffic management in most environments.

5 Practical Subnetting Tips

• Count subnet bits, not host bits. To split a /24 into subnets of at least 30 hosts each, work backward: 2ⁿ − 2 ≥ 30 → n = 5 host bits → prefix = 32 − 5 = /27 (30 usable hosts). For 60 hosts: n = 6 → /26 (62 usable). For 120 hosts: n = 7 → /25 (126 usable). Always use the smallest prefix that fits your host count, leaving room for growth.
• Use /30 for all point-to-point router links. A link between two router interfaces needs exactly 2 usable addresses. Using a /24 for this wastes 252 addresses and consumes a subnet unnecessarily. Every inter-router link in a properly designed network uses /30 (4 total, 2 usable) or /31 (RFC 3021, 2 total, no network/broadcast designation).
• 192.168.1.x is the world's most-used private subnet — and that's a problem for VPNs. When a remote employee's home router assigns 192.168.1.0/24 and the office network also uses 192.168.1.0/24, the VPN can't route correctly because both ends look like the same network. Enterprise VPN designs use less common ranges (10.100.x.x, 172.16–31.x.x) to avoid this overlap.
• The wildcard mask is the inverse of the subnet mask — useful for ACLs and routing protocols. If subnet mask = 255.255.255.192, wildcard = 0.0.0.63 (each octet: 255 minus the mask octet). Cisco ACLs use wildcard masks; most subnet calculators show both. The wildcard tells the router which bits to ignore when matching an address.
• VLSM (Variable Length Subnet Masking) lets you right-size each subnet. Instead of giving every segment the same /26, assign /30 to P2P links (2 hosts), /27 to small departments (30 hosts), /25 to large floors (126 hosts), all carved from the same /24. This wastes far fewer addresses than fixed-length subnetting and is standard practice in modern network design.

Reference: RFC 4632 — Classless Inter-domain Routing (CIDR) | IANA IPv4 Special-Purpose Registry