CIDR to IP Range Calculator

What Is a CIDR to IP Range Calculator, and How Does CIDR Notation Work?

Classless Inter-Domain Routing (CIDR) is the addressing scheme that replaced the old class-based system of ip addressing across the internet protocol address space. A CIDR block is written as an IP address followed by a slash and a number — the routing prefix — that tells you how many of the address's leading bits are fixed as the network prefix and how many remain free to identify individual hosts. Everything before the slash is the network portion; everything after is the host portion, sometimes called the host identifier or rest field. Each device on a network interface pulls its address from within that boundary, and routers use that same prefix to decide where ip packets should be forwarded next across a tcp/ip network. Once you understand that split, every ip subnet you carve out this way follows the exact same rules, no matter how large or small the block is.

You'll see this same tool called by different names depending on the site — an ip subnet calculator, a subnet mask calculator, a network calculator, a cidr converter, or simply a subnetting tool. Whatever the label, it performs the same underlying slash-to-range conversion.

The Subnet Mask Formula Behind Every CIDR Calculation

Under the hood, this calculator is just doing bitwise arithmetic against the octets of your address. If A is the IP address, M is the subnet mask, and W is the wildcard mask, the network address and broadcast address come from two bitwise operations:

Every subnet mask is really a bit mask — a run of 1s followed by a run of 0s, laid out in binary format and then expressed in the familiar dot-decimal notation (255.255.255.0) so you don't have to read raw mask bits by hand. A /24, for instance, sets the first three octets to 255 and leaves the last one open, which is why 192.0.2.0/24 and 255.255.255.0 describe exactly the same network mask.

Wildcard Masks, Gateways, and Access Control Lists

The wildcard mask matters most once you start writing routing protocols configs like OSPF or access control lists — both use it to match a whole block of addresses in one rule instead of listing every host individually. It's also worth remembering that a subnet's gateway (the router that forwards traffic leaving the subnet) sits inside the usable host range, not at the network or broadcast address.

IPv4 vs IPv6 Addressing in an IPv4 Subnet Calculator

An IPv4 address is 32 bits, written as four decimal octets; an IPv6 address is 128 bits, written in hexadecimal format as eight groups separated by colons. This calculator handles both: for an ipv4 subnet calculator lookup you get a network mask, network address, and broadcast address exactly like the table above; for an IPv6 address you get a prefix length instead of a subnet mask, since IPv6 has no broadcast concept and drops the class system entirely.

Every Field This Subnet Calculator Returns

• IP Class — where the address falls among the standard address classes A through E; this doesn't apply to IPv6, since classless addressing is the default.
• IP Type — whether the address is public (routable on the internet) or private (reserved for local networks).
• Binary ID — the address written entirely in 0s and 1s.
• Integer ID — the same address as a single decimal integer.
• Hex ID — the address in hexadecimal format, the native representation for these addresses.
• Full IP Address — the unabbreviated form, useful since addresses are usually shortened by omitting leading zeros.
• Total IP Addresses — how many addresses the block contains, from a single host up to an entire /8.
• Usable Host IP Range — the first and last assignable address, excluding the network and broadcast address.
• Number of Usable Hosts — the usable host IP range minus those two reserved addresses.
• In-addr.arpa — the reverse DNS name built from the address in reverse order, used to look up a hostname from an IP.
• IPv4-Mapped Address — an IPv4 address represented inside the IPv6 space, prefixed with 80 zero bits and 16 set bits.
• 6to4 Prefix — a transition address beginning with 2002, used to route IPv4 traffic across an IPv6 network during migration.