IP Subnet Calculator

• •Use the IP Subnet Calculator when you need accurate results quickly without the risk of manual computation errors or unit conversion mistakes.
• •Use it to verify calculations made by hand or in spreadsheets — an independent check can catch errors before they lead to costly decisions.
• •Use it to explore how changing input parameters affects the output — a quick way to develop intuition and identify the most influential variables.
• •Use it when collaborating with others to ensure everyone is working from the same numbers and applying the same assumptions.

The IP Subnet Calculator is a free, browser-based calculation tool for engineers, students, and technical professionals. Find the network and broadcast addresses, usable host range, host count and subnet and wildcard masks of an IPv4 address with a CIDR prefix or subnet mask, including /31 and /32 It implements standard formulas. Follow the units shown beside each field and use unit selectors where available. All calculations are performed in your browser with no data sent to a server. Use this calculator as a quick reference and sanity-check tool during design, analysis, and learning. Always verify results against primary engineering references and applicable standards for any safety-critical application.

An IPv4 address is a 32-bit number, written as four decimal octets from 0 to 255 separated by dots, such as 192.168.1.130. A subnet splits those 32 bits into a network part and a host part. The prefix length in CIDR notation, the /24 in 192.168.1.130/24, is the number of leading bits that belong to the network. The subnet mask writes the same thing as an address whose first 24 bits are 1: 255.255.255.0. So /24 and 255.255.255.0 mean the same thing. The wildcard mask is the mask inverted, 0.0.0.255, the form access control lists and some routing configurations use. Three bit operations give everything else. The network address is the address AND the mask, with every host bit set to 0: 192.168.1.0. The broadcast address is the network address OR the wildcard mask, with every host bit set to 1: 192.168.1.255. The subnet holds 2^(32 − prefix) addresses, 256 for a /24. For prefixes up to /30 (/31 and /32 are the exceptions below), the network and broadcast addresses are reserved, so the usable hosts run from the network address plus one to the broadcast address minus one, and there are 2^(32 − prefix) − 2 of them: 192.168.1.1 to 192.168.1.254, 254 hosts. Two prefixes are different. A /31 has only two addresses, and reserving both would leave none, so RFC 3021 lets a point-to-point link use both, with no network or broadcast address: 10.0.0.0/31 has the usable addresses 10.0.0.0 and 10.0.0.1. A /32 is a single address, used for loopback interfaces and host routes. At the other end, /0 covers the whole IPv4 address space. Common subnet sizes include /8 (16,777,216 addresses, 16,777,214 usable), /16 (65,536 addresses, 65,534 usable), /24 (256 addresses, 254 usable), the usual size for a local network, and /30 (4 addresses, 2 usable), the traditional size for a link between two routers before /31 became common. Variable-length subnet masking (VLSM) lets one network use different sizes side by side, such as a /24 for a server VLAN and a /31 for a router link. This calculator reads the address strictly: four numbers from 0 to 255 and nothing else. It refuses 256.1.2.3 or 1.2.3 rather than guessing what was meant, and it refuses leading zeros, because some software reads 010 as the octal number 8. It does the bit arithmetic on unsigned 32-bit numbers and counts addresses with integers that cannot overflow, so a /0 shows its 4,294,967,296 addresses exactly.

• •Planning an office or VLAN address plan: see how many hosts a /24, /25 or /26 leaves and where each subnet starts and ends before assigning DHCP ranges.
• •Configuring a router or firewall: interface, route and access-list statements ask for the network address and the subnet mask or, in many access lists, the wildcard mask.
• •Troubleshooting connectivity: two devices reach each other without a router only if they are in the same subnet. Enter each address with its mask and compare the network addresses.
• •Point-to-point links between routers: a /31 uses both of its addresses (RFC 3021), where a /30 spends two of its four on the network and broadcast addresses.
• •Studying for networking exams: the worked examples and the prefix table cover the counts, masks and special cases that subnetting questions turn on.

• !Forgetting the two reserved addresses: up to /30 the network and broadcast addresses are not usable hosts, so a /24 has 254 hosts, not 256. A /31 (RFC 3021) and a /32 are the exceptions.
• !Confusing the prefix length with the number of host bits. They add up to 32, so a /26 leaves 6 host bits: 64 addresses.
• !Not planning for growth: a subnet sized exactly for today's devices leaves no room. Allocate one at least twice as large as you need now.
• !Writing octets with leading zeros, such as 192.168.001.010: some software reads 010 as octal 8, so the calculator refuses it.
• !Using a mask with a gap, such as 255.0.255.0: a subnet mask's 1 bits must all come first.
• !Mixing up the subnet mask and the wildcard mask in an access list: the wildcard 0.0.0.255 is the mask 255.255.255.0 inverted.