IP Address Calculator

Every device connected to a network needs an IP address — a unique numerical label that identifies it and allows data to be routed correctly. An IPv4 address consists of 32 bits written as four decimal numbers (octets) separated by dots, for example 192.168.1.100. Each octet can range from 0 to 255, giving roughly 4.3 billion possible addresses in total.

Subnetting is the practice of dividing a large IP address space into smaller, logically organized networks called subnets. Instead of treating all 4.3 billion addresses as a flat pool, network engineers use a subnet mask to indicate which portion of an IP address identifies the network and which portion identifies the individual host within that network. This hierarchical approach makes routing more efficient, contains broadcast traffic, improves security, and helps organizations allocate addresses without waste.

The subnet mask is also a 32-bit number written in dotted-decimal form. A mask of 255.255.255.0 means the first 24 bits represent the network portion and the remaining 8 bits represent host addresses. When the mask is applied to an IP address using a bitwise AND operation, the result is the network address — the identifier for the entire subnet. The complementary bits of the mask form the wildcard mask, which reveals how many host addresses exist in the subnet.

Understanding subnetting is fundamental to network design, server administration, cloud infrastructure, and security configuration. Whether you are setting up a home lab, configuring a corporate LAN, or planning VLAN segmentation in a data center, a reliable IP address calculator saves time and eliminates arithmetic errors that could disrupt connectivity.

Classless Inter-Domain Routing (CIDR) notation is a compact way to express an IP address together with its subnet mask. It appends a forward slash and a number to the IP address, such as 192.168.1.0/24. The number after the slash is the CIDR prefix length — it tells you how many of the 32 bits in the address are devoted to the network portion.

The prefix length maps directly to a subnet mask. A /24 prefix means 24 network bits, producing the mask 255.255.255.0. A /16 gives 255.255.0.0, and a /8 gives 255.0.0.0. You can convert any prefix length to its mask by setting the leftmost N bits to 1 and the remaining bits to 0, then reading the result as four octets. The calculator does this automatically using the expression (~0 << (32 - cidr)) >>> 0.

CIDR replaced the older classful system — which rigidly divided addresses into Class A, B, and C blocks — because it allows far more flexible allocation. An ISP can assign a /22 block (1,022 usable hosts) to a small business without wasting thousands of addresses. Organizations can further subnet that /22 into smaller /24, /26, or /30 blocks to match actual department sizes.

The table below lists the most common CIDR prefix lengths with their equivalent subnet masks and usable host counts:

The original IPv4 design divided the address space into classes based on the value of the first octet. Although CIDR has largely replaced classful routing in modern networks, IP classes remain relevant for understanding address types, firewall rules, and router behavior.

Class A addresses have a first octet between 1 and 126. The default mask is /8, allowing up to 16,777,214 hosts per network. Large organizations and early internet pioneers received Class A allocations.

Class B addresses range from 128.0.0.0 to 191.255.255.255. The default mask is /16, supporting up to 65,534 hosts. Universities and mid-sized companies commonly used Class B space.

Class C addresses span 192.0.0.0 to 223.255.255.255 with a default /24 mask and 254 usable hosts. This is by far the most common range for small networks today.

Class D (224.0.0.0–239.255.255.255) is reserved for multicast group addressing, while Class E (240.0.0.0–255.255.255.255) is reserved for experimental use.

Within each class, certain ranges are designated as private addresses that are not routed on the public internet. RFC 1918 defines three private ranges:

• 10.0.0.0/8 — Class A private range (10.x.x.x)
• 172.16.0.0/12 — Class B private range (172.16.x.x through 172.31.x.x)
• 192.168.0.0/16 — Class C private range (192.168.x.x)

Devices using private addresses access the internet through Network Address Translation (NAT), which maps private addresses to one or more public IPs. The IP address calculator identifies whether your input address falls within these private ranges and labels it accordingly, helping you quickly determine whether a host is publicly reachable or behind NAT.

Every subnet contains three special categories of addresses: the network address, the broadcast address, and the range of usable host addresses between them.

The network address is the first address in the subnet. It has all host bits set to zero and is used to identify the subnet itself in routing tables. You cannot assign the network address to a device; it belongs to the subnet as a whole. For example, in the subnet 192.168.1.0/24, the network address is 192.168.1.0.

The broadcast address is the last address in the subnet, with all host bits set to one. Packets sent to the broadcast address are delivered to every host in the subnet. Routers do not forward broadcast traffic between subnets, which is one reason subnetting reduces network congestion. In the 192.168.1.0/24 example, the broadcast address is 192.168.1.255.

The usable host range is everything between the network address and the broadcast address. For a /24 subnet, that gives addresses 192.168.1.1 through 192.168.1.254 — a total of 254 hosts. The general formula is totalHosts = 2^(32 - cidr) - 2, which matches the calculator's computation: broadcastNum - networkNum - 1.

The wildcard mask is the bitwise inverse of the subnet mask. It is used in access control lists (ACLs) on Cisco routers and firewalls to specify which bits of an address must match a rule and which bits are "don't care." A wildcard mask of 0.0.0.255 paired with network 192.168.1.0 means "match any address where the first three octets are 192.168.1."

The calculator also displays the binary representation of both the IP address and the subnet mask. Reading the binary form makes the bitwise AND operation that produces the network address visually obvious and is essential for passing networking certification exams like CompTIA Network+ or Cisco CCNA.

This subnet calculator is designed for quick, accurate network planning without manual binary arithmetic. Follow these steps to get complete subnet information for any IPv4 address:

1. Enter the IP address in dotted-decimal notation (e.g., 10.0.5.200). The calculator validates that each octet is between 0 and 255 and shows an error if the address is malformed.
2. Choose your input method. Toggle "Use CIDR notation" to enter the prefix length using the slider (0–32), or uncheck the box to type a subnet mask in dotted-decimal format. Both inputs yield the same result — the calculator converts between them automatically using the formula maskToCidr, which counts the set bits in the mask.
3. Use a quick-select subnet button for common prefix lengths (/8, /16, /24, /25, /26, /27, /28, /30) to instantly switch to a standard subnet size.
4. Read the results. The right panel displays the network address, broadcast address, first host, last host, wildcard mask, usable host count, IP class, binary representations, and whether the address is private or public.
5. Copy any value to your clipboard with the Copy button next to each result field — convenient when populating firewall rules, router configurations, or documentation.

For network engineers dealing with variable-length subnet masking (VLSM), you can run the calculator multiple times with different prefix lengths to plan how to subdivide a larger block into appropriately sized subnets for each department or server group.