IP Subnet / CIDR Calculator
Enter an IP + CIDR and instantly get network address, broadcast, usable range, and a subnet-splitting table.
Binary Breakdown
Split Into Subnets
About this tool
What this tool does
The IP Subnet / CIDR Calculator takes an IPv4 address and a prefix length (or subnet mask) and works out everything you'd otherwise do by hand: network address, broadcast address, wildcard mask, the usable host range, and how many hosts actually fit. A live binary breakdown shows exactly which bits are network bits and which are host bits, so the math behind the numbers isn't a black box.
It also does the part most subnet calculators skip: splitting a network into smaller subnets. Tell it how many subnets you need, or how many hosts each one needs to support, and it generates the full table of resulting networks — the same VLSM-style planning you'd do before handing address ranges out to VLANs, sites, or cloud subnets.
How to use the subnet calculator
- Enter an IPv4 address (any address inside the network works, e.g.
192.168.1.50). - Pick the CIDR prefix length from the dropdown — the matching dotted subnet mask is shown right next to each option.
- Read off the network address, broadcast address, usable host range, and total/usable host counts.
- Check the binary breakdown to see which bits are fixed by the network (blue) and which are free for hosts (gray).
- Open Split Into Subnets, choose whether you're planning by subnet count or by hosts-per-subnet, and get the full resulting subnet table.
Common use cases
- Network design and VLSM planning — figure out how to carve a /24 into several smaller subnets for different VLANs or departments without wasting addresses.
- Cloud VPC and subnet planning — work out non-overlapping CIDR ranges before creating subnets in AWS, Azure, or GCP.
- Firewall and ACL rules — confirm the exact network and broadcast address a CIDR block covers before writing a rule.
- Studying for networking certifications — CCNA/Network+ subnetting practice with the binary math shown, not just the final answer.
Frequently asked questions
What's the difference between a subnet mask and CIDR notation?
They're two ways of writing the same thing. A subnet mask like 255.255.255.0 and a CIDR prefix like /24 both mean "the first 24 bits are the network portion" — this calculator keeps them in sync so you can think in whichever one you're used to.
Why are the network and broadcast addresses not usable for hosts?
The network address (all host bits set to 0) identifies the subnet itself, and the broadcast address (all host bits set to 1) is reserved for sending to every host on that subnet at once — neither can be assigned to a single device, which is why usable hosts is total addresses minus 2.
What happens with a /31 or /32 network?
A /31 has no network or broadcast address reserved (RFC 3021) and is commonly used for point-to-point links between two routers. A /32 is a single host route with exactly one address.
How do I know if an IP address is private or public?
Private (non-routable) ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 — anything outside those (and outside loopback/link-local) is treated as a public, internet-routable address, which this tool flags automatically.
How many /27 subnets fit inside a /24?
A /24 has 8 more host bits than a /27 gives up (27-24=3 bits), so it splits into 2³ = 8 separate /27 subnets — exactly the kind of breakdown the Split Into Subnets table generates automatically for any prefix pair.