Question 1

What is CIDR notation?

Accepted Answer

CIDR (Classless Inter-Domain Routing) notation is a compact way to represent an IP range. For example, 192.168.1.0/24 means the first 24 bits are the network portion and the last 8 bits are for hosts. The /24 corresponds to a subnet mask of 255.255.255.0.

Question 2

Does this tool support IPv6?

Accepted Answer

Yes — full IPv6 support using BigInt math (128-bit addresses). Calculate subnets from /0 to /128, detect special ranges (loopback ::1, link-local fe80::/10, unique local fc00::/7, multicast ff00::/8), generate PTR record names, and compute host counts (which can be astronomically large — e.g. /64 has 18 quintillion addresses).

Question 3

What is VLSM subnetting?

Accepted Answer

Variable-Length Subnet Masking (VLSM) lets you split a network into subnets of different sizes, allocating larger subnets to segments that need more hosts. Enter your base network (e.g. 192.168.1.0/24) and the host counts for each segment (e.g. [50, 25, 10]), and the tool will assign the smallest fitting prefix to each, sorted largest-first.

Question 4

What extra features does this tool have compared to others?

Accepted Answer

Beyond basic subnet math, we ship: (1) Subnet history (localStorage, last 20). (2) Cisco ACL wildcard mask conversion (inverted mask). (3) Reverse DNS (PTR) record name generation for IPv4 (in-addr.arpa) and IPv6 (ip6.arpa). (4) ASN whois + RIPEstat lookup URLs. (5) CIDR ↔ IP range conversion (range-to-CIDR aggregation). (6) CIDR merge — combine multiple CIDRs into the minimal covering set. (7) Subnet containment check — does parent CIDR contain child CIDR? (8) Binary view of IPv4 addresses and masks (32-bit). (9) CSV/JSON export of subnet details. (10) Shareable URL — encode input in fragment.

Question 5

What's the difference between total hosts and usable hosts?

Accepted Answer

Total hosts = 2^(32-prefix) for IPv4, 2^(128-prefix) for IPv6. Usable hosts = total - 2 (one for the network address, one for the broadcast address). For IPv4 /31 and /32, special handling per RFC 3021. For IPv6 /127 and /128, special handling per RFC 6164. IPv6 doesn't technically require reserving network/broadcast addresses, but we follow the conservative convention.

Question 6

Is my subnet calculation sent anywhere?

Accepted Answer

No. All subnet math (IPv4 bitwise + IPv6 BigInt) is done locally in your browser. No network requests are made. The whois/RIPEstat links are just URL generators — clicking them opens a new tab to those services, but the calculation itself never leaves your device.

IP Subnet Calculator

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