IP Subnet Calculator — Free CIDR & Subnet Mask Tool

The 30-Second Answer

What Is a Subnet Calculator?

A subnet calculator (also called a CIDR calculator or IP subnetting calculator) computes everything about an IPv4 subnet from just an address and a prefix length. Type 192.168.1.10 with /24 and it tells you the network is 192.168.1.0, the broadcast is 192.168.1.255, devices can use 192.168.1.1 through 192.168.1.254, and there are 254 usable hosts.

IT professionals reach for it when configuring routers, firewalls, DHCP scopes, and VPC subnets — anywhere a wrong mask means an outage. The calculator above does the binary math for you, but the sections below teach you how the math works, so you can subnet in your head too.

CIDR Cheat Sheet: Subnet Masks & Usable Hosts (/8–/30)

The single most useful subnetting reference: every common prefix with its mask, wildcard, and host counts. A /24 subnet has 256 total IP addresses and 254 usable hosts — the first (network) and last (broadcast) addresses can’t be assigned to devices.

How to Use This Subnet Calculator

1. Type any IPv4 address, e.g. 192.168.1.10. Invalid addresses show a clear error.
2. Pick the CIDR prefix (/24 is the common home-network default).
3. Network, broadcast, host range, masks and IP class appear instantly as you type.
4. Change the prefix to compare subnet sizes — switch /24 to /25 and watch the network split into two 126-host halves.

Worked Example: 192.168.1.10/24

Here is exactly what the calculator returns for a typical home-network address:

Why 254 and not 256? The first address (.0) identifies the network itself and the last (.255) is the broadcast address that reaches every device on the subnet. Neither can be assigned to a device, so you always subtract 2.

How Subnet Math Works (Binary Walkthrough)

Every subnet calculation is a bitwise AND: line up the IP address and the subnet mask in binary, and AND each bit. Let’s find the network address of 192.168.1.50 with mask 255.255.255.240 (/28):

Look at the last octet: 50 = 00110010 AND 240 = 11110000 gives 00110000 = 48. The network is 192.168.1.48, the broadcast is 192.168.1.63, and devices can use .49 through .62 — 14 usable hosts, exactly what the cheat sheet says for /28.

The three formulas behind every result on this page:

Total addresses = 2^(32 − CIDR)
Usable hosts = 2^(32 − CIDR) − 2 (except /31 and /32 — see below)
Subnets from borrowing n bits = 2^n

The Magic-Number Shortcut (Mental Subnetting)

The fastest way to subnet without binary — a favorite on CCNA exams. The magic number is 256 minus the interesting octet of the mask (the octet that isn’t 0 or 255):

How to use it: count by the magic number from 0 to find the subnet boundaries. Example — which subnet is 192.168.5.73/27 in? Magic number = 256 − 224 = 32. Boundaries: 0, 32, 64, 96, 128… — 73 falls in the 64 block, so the network is 192.168.5.64, broadcast 192.168.5.95, usable .65–.94. No binary needed.

VLSM: Splitting One Network Into Unequal Subnets

VLSM (Variable Length Subnet Masking) means using different prefix lengths inside one address block so each department gets only what it needs. To split 192.168.10.0/24 into 4 equal subnets, borrow 2 bits (→ /26, mask 255.255.255.192, 62 usable hosts each):

For unequal subnets, follow the golden rule: allocate the largest requirement first, then the next largest, and so on. Carving the big blocks first prevents a small subnet from landing in the middle of space a larger one needs.

Subnet Mask vs. Wildcard Mask

A wildcard mask is the bitwise inverse of the subnet mask — every 1 becomes 0 and every 0 becomes 1:

Wildcard masks are used in Cisco access-control lists (ACLs) and OSPF network statements, where the logic is flipped from subnet masks: a 0 bit means “must match” and a 1 bit means “ignore”. So access-list 10 permit 192.168.1.0 0.0.0.255 matches the entire 192.168.1.0/24 network. Mixing the two up is one of the most common Cisco config errors.

Special Cases: /31 and /32 (RFC 3021)

Two prefixes break the “minus 2” rule. RFC 3021 defines /31 for point-to-point links between two routers: with only 2 addresses in the subnet, both are usable — there is no network or broadcast reservation. A link numbered 192.168.1.0/31 gives usable addresses 192.168.1.0 and 192.168.1.1, saving address space compared to a /30.

A /32 is a single host — one address, mask 255.255.255.255. You’ll see it on loopback interfaces and in host routes. The calculator above covers /8–/30 (the range used for real subnets); the cheat sheet includes the /31 and /32 rows for reference.

Supernetting (Route Summarization) Explained

Supernetting is subnetting in reverse: combining several small networks into one larger advertisement. Four contiguous /24 networks can be summarized as a single /22:

ISPs and backbone routers aggregate routes this way to keep global routing tables small — instead of four entries, one entry does the job. The requirement is that the networks be contiguous and align on the summary boundary.

Private IP Address Ranges (RFC 1918)

RFC 1918 reserves three blocks for internal networks. These addresses are never routed on the public internet — they’re reused inside homes, offices, and cloud VPCs behind NAT:

For reference, the classic IP classes are: Class A 1.0.0.0–126.255.255.255 (default /8), Class B 128.0.0.0–191.255.255.255 (default /16), Class C 192.0.0.0–223.255.255.255 (default /24), Class D 224.0.0.0–239.255.255.255 (multicast), and Class E 240.0.0.0–255.255.255.255 (experimental).

Common Subnetting Mistakes to Avoid

1. Forgetting the “minus 2”. A /24 has 254 usable hosts, not 256. The network and broadcast addresses are reserved — always.
2. Assigning the network or broadcast address to a device. Putting 192.168.1.0 or 192.168.1.255 on a host breaks connectivity for the whole subnet.
3. Creating overlapping subnets. 192.168.1.0/25 (.0–.127) and 192.168.1.0/26 (.0–.63) overlap — the second must start at .64.
4. Using a /30 where a /31 would do. On point-to-point router links, /31 (RFC 3021) wastes zero addresses; /30 wastes two.
5. Mixing up subnet and wildcard masks in ACLs. In a wildcard mask, 0 means “must match” — the exact opposite of subnet-mask logic.

IP Subnet FAQs