Subnetting calculator
Enter an IP and CIDR to see the network instantly, then follow the Magic Number steps to check your own math. You can also paste 192.168.30.22/25 into the IP box.
Step by step Magic Number dissection
RFC 1918 and special addresses
Fill every blank from memory, then check yourself. Ranges or CIDR both count.
Example: Multicast
Also accepted: 224.0.0.0 to 239.255.255.255
Subnet Reps
20 subnetting practice problems from the workbook, graded as you go. Click Check Answer to grade yourself.
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What is subnetting?
Subnetting splits one IP network into smaller networks called subnets. Each subnet gets its own block of addresses, which keeps traffic organized and limits how far broadcasts travel. You do it by borrowing bits from the host part of an address, and the subnet mask records where the network part ends.
The Magic Number method
The Magic Number is a decimal shortcut that replaces binary conversion. Halve 256 once for each borrowed bit to get the block size, then count up by that block size to find network IDs and broadcast addresses.
Subnetting example: 192.168.14.77/27 step by step
- 1. Borrowed bits: /27 borrows 3 bits in the 4th octet (27 − 24 = 3).
- 2. Block size: Halve 256 three times: 256 ÷ 2 ÷ 2 ÷ 2 = 32 (Magic Number). Last octet mask: 256 − 32 = 224 → 255.255.255.224.
- 3. Network ID: Multiples of 32: .0, .32, .64, .96. .77 sits in .64 block → 192.168.14.64.
- 4. Broadcast & host range: Next block starts at .96 (broadcast .95). Usable range: 192.168.14.65 to 192.168.14.94 (30 hosts).
Want to see it in the tool? . Ready to try one yourself? , graded as you go.
Subnetting FAQs
What is a subnet mask?
A subnet mask is a 32 bit number that marks which part of an IPv4 address identifies the network and which part identifies the host. In dotted decimal, 255.255.255.0 means the first 24 bits are the network and the last 8 bits are for hosts.
What does /24 mean in CIDR notation?
The number after the slash is how many bits of the address belong to the network. A /24 means 24 network bits, which matches the mask 255.255.255.0 and leaves 8 host bits, or 254 usable addresses. A bigger number means a smaller subnet.
What is the difference between subnetting vs supernetting?
Subnetting divides a single network address into smaller subnetworks by borrowing host bits for network bits. Supernetting combines multiple contiguous network addresses into a larger single network by borrowing network bits for host bits.
What is classful subnetting?
Classful subnetting is a legacy IP addressing method that divides IPv4 addresses into fixed classes known as Class A, Class B, and Class C based on predefined leading bit patterns and default subnet masks.
How do you calculate subnets without binary math on the N10-009 exam?
The Magic Number method calculates subnets in decimal by subtracting the mask value in the active octet from 256. Halving 256 once for each borrowed CIDR bit yields the block size and defines network boundaries instantly.
Why are 2 IP addresses subtracted from total subnet hosts?
The first address in a subnet is reserved for the Network ID and the final address is reserved for the Broadcast ID. Neither address can be assigned to host interfaces, leaving total addresses minus 2 usable hosts.
What is the interesting octet in subnetting?
The interesting octet is the specific octet where the subnet mask transitions from 255 to 0 or contains a custom mask value. It is the exact octet where block size increments occur.
Subnetting table: CIDR, mask, block size, and hosts
| CIDR | Subnet mask | Block | Hosts |
|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |