- What is CIDR notation?
- CIDR (Classless Inter-Domain Routing) is the standard way to write an IPv4 subnet. The slash and number after the IP — 192.168.1.0/24 — give the prefix length, which is the count of leading 1-bits in the subnet mask. A /24 prefix is 24 ones followed by 8 zeros, i.e. 255.255.255.0. CIDR replaced the old A/B/C class system in 1993 (RFC 1519) because rigid 8/16/24-bit boundaries wasted enormous chunks of the address space. Modern routers, firewalls, and cloud platforms all speak CIDR; the legacy classes are educational trivia.
- Why are usable IPs (totalIPs − 2)?
- In every regular subnet, two addresses are reserved: the network address (all host bits zero) identifies the subnet itself, and the broadcast address (all host bits one) sends to every host in the subnet. Neither can be assigned to a real host. So a /24 with 256 total addresses fits 254 hosts. The exceptions are /31 (RFC 3021 point-to-point — both addresses usable for the two endpoints) and /32 (single host — the one address is itself). The calculator handles both.
- What's the difference between subnet mask and wildcard mask?
- They're bitwise inverses. A subnet mask has 1-bits where the network prefix is and 0-bits where the host bits are: 255.255.255.0 for /24. A wildcard mask flips them: 0.0.0.255 for /24. Cisco IOS access control lists historically used wildcard masks; subnet masks show up everywhere else (Linux ifconfig, Windows netsh, every router GUI). The calculator shows you both. To convert by hand: subtract each subnet octet from 255.
- What is the binary representation of the mask for?
- It's the easiest way to see the prefix length without counting. 11111111.11111111.11111111.00000000 has 24 ones, so it's a /24. Walking through the binary is also how you verify a mask is valid — every real subnet mask has all its 1-bits before all its 0-bits (a contiguous mask). 11111111.00000000.11111111.00000000 is a legal 32-bit pattern but not a legal subnet mask; the calculator refuses it.
- What does the IPv4 class actually mean?
- The classful system (1981) split IPv4 into five fixed-prefix zones based on the first octet: Class A (0-127, /8 networks, 16M hosts each), Class B (128-191, /16, 65K hosts), Class C (192-223, /24, 254 hosts), Class D (224-239, multicast), and Class E (240-255, reserved/experimental). CIDR superseded the host counts in 1993, but the multicast (D) and reserved (E) ranges are still real and matter — anything in 224.0.0.0/4 is multicast traffic and won't route normally. The calculator shows the class because some legacy documentation still references it.
- How big a subnet do I need?
- Count your hosts, add room for growth, round up to the next power of two, add 2 for network + broadcast, then map back: 30 hosts → 32 IPs total → /27. 60 hosts → 64 IPs → /26. 100 hosts → 128 → /25. 1000 hosts → 1024 → /22. Don't undersize; renumbering a subnet that ran out of room is one of the more painful Tuesday-evening tasks in network engineering. Don't oversize either; allocating a /16 to a 30-host VLAN is how cloud bills surprise people.
- What's a /31 used for if it has no broadcast address?
- Point-to-point links between two routers. Before RFC 3021 (2000), every link wasted a /30 (4 IPs to use 2). RFC 3021 said: on a point-to-point link there's no broadcast meaning anyway (the receiver is the only other endpoint), so both addresses of a /31 are usable. Result: 50% IP-space savings on link networks. The calculator reports total = 2, usable = 2 for /31, matching the RFC.
- Can this calculator handle IPv6?
- No — IPv4 only. IPv6 CIDR follows the same prefix-length idea (2001:db8::/32) but the math operates on 128-bit values and a separate widget makes the UX clearer. The IPv4 calculator covers the common operator workload: VLANs, VPC subnets, point-to-point links, ACL planning. An IPv6 version is on the build list.
- Why does the calculator change my CIDR when I enter host bits?
- Because CIDR notation describes a subnet, not a host. 192.168.1.10/24 names the host 192.168.1.10 inside the 192.168.1.0/24 subnet. When the calculator shows the canonical CIDR, it zeroes the host bits to give you the subnet identifier — 192.168.1.0/24. The 'IP address (binary)' row still shows the address you entered, so you can see exactly which bits are network and which are host.
- Does this work offline / does the calculation leave my browser?
- It works fully offline once the page is loaded — the math is plain bitwise arithmetic in JavaScript. Nothing about the IP block you typed gets sent to a server, logged, or stored. Open DevTools → Network and watch: no requests fire on input change.
