Subnet Calculator

What Is a Subnet Calculator?

A subnet calculator divides an IP network into smaller subnetworks (subnets) and shows the key facts about each one: the network address, broadcast address, subnet mask, and the range of usable host addresses. Network engineers use it when designing office networks, VLANs, and cloud VPCs, and students use it to check their networking homework. It works for IPv4, the 32-bit address system still dominant in most LANs.

How Subnet Math Works

An IPv4 address is 32 bits. CIDR notation like /24 says how many of those bits identify the network; the rest identify hosts:

• Subnet mask: the first n bits set to 1. A /24 mask is 24 ones then 8 zeros → 255.255.255.0. (Each "255" is eight 1-bits; "0" is eight 0-bits.)
• Network address: IP address AND subnet mask (bitwise) — the first address of the subnet, reserved as its identifier.
• Broadcast address: network address with all host bits set to 1 — the last address, reserved for broadcasts.
• Usable hosts: 2^{(32 − n)} − 2. The minus two removes the network and broadcast addresses, which can't be assigned to devices.
• Splitting a network: borrowing b extra bits from the host portion creates 2^b subnets. A /24 split into /26s borrows 2 bits → 4 subnets.

How to Use This Calculator

1. Enter an IP address and a subnet mask — either as dotted decimal (255.255.255.0) or CIDR (/24).
2. The calculator returns the network address, broadcast address, usable host range, total and usable host counts, and the wildcard mask.
3. To split a network, choose the new prefix length; it lists every resulting subnet with its ranges.

Worked Example

Take the network 192.168.1.0/24:

• Subnet mask: 255.255.255.0 (24 network bits, 8 host bits).
• Network address: 192.168.1.0; broadcast: 192.168.1.255.
• Usable hosts: 2⁸ − 2 = 256 − 2 = 254, ranging from 192.168.1.1 to 192.168.1.254.

Now split it into /26 subnets (borrowing 2 bits → 4 subnets), each with 2⁶ − 2 = 62 usable hosts:

Tips

• /31 is special: per RFC 3021, a /31 has no network/broadcast reservation — both addresses are usable, ideal for point-to-point links. The "minus two" rule doesn't apply there.
• Private ranges: for internal networks use 10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16 — these are reserved for private use and never routed on the public internet.
• Plan for growth: size subnets for the devices you'll have in 2–3 years, not today. Renumbering a live network is painful.
• Document everything: keep a simple IP plan (which subnet serves which VLAN or department) — future-you will be grateful.