What is an IPv4 address?
An IPv4 address is a 32-bit address written as four decimal octets, such as 192.168.10.14.
Beginner-friendly • IPv4-focused • Interactive
Learn subnetting step by step.
This site starts with the basics, teaches the vocabulary, and then gives students a full set of tools to practice CIDR notation, subnet masks, host counts, equal subnetting, VLSM planning, binary conversion, and quick self-check quizzes.
- Clear explanations for first-time learners
- Fast calculators and planners for practice
- Instant feedback on subnetting problems
1. Basics
Start with the core ideas
These cards explain the ideas students need before jumping into calculators.
What is subnetting?
Subnetting splits a larger network into smaller networks so addresses can be organized and used more efficiently.
What is CIDR notation?
CIDR uses a slash and a number like /27. The number tells you how many bits belong to the network portion.
What is a subnet mask?
A subnet mask shows which bits are network bits and which bits are host bits. Example: /26 = 255.255.255.192.
Why do we subtract 2?
On most normal IPv4 LAN subnets, one address is the network address and one is the broadcast address, so usable hosts are usually 2h - 2.
Why subnet at all?
Smaller subnets improve organization, reduce waste, keep broadcast domains smaller, and make address plans easier to manage.
How to solve a subnetting question
- Read the prefix. Example: /28 means 28 network bits.
- Find host bits. Compute 32 - prefix.
- Find block size. Use 2^(host bits) for total addresses.
- Find key addresses. Determine network, broadcast, and usable range.
Common student mistakes
- Mixing up total addresses and usable hosts
- Forgetting that /24 means 24 network bits, not 24 hosts
- Using a host address when the question asks for the network address
- Not counting the subnet block size correctly in the interesting octet
- Forgetting that /31 and /32 are special cases
2. Vocabulary
Subnetting terms students should know
Search a term below and use the cards as quick study notes.
3. Worked examples
See the process before you try it alone
These examples show how students can think through subnetting one step at a time.
Example 1: What does 192.168.1.0/26 really mean?
/26 means 26 network bits and 6 host bits.
Total addresses: 2^6 = 64
Usable hosts on a normal subnet: 64 - 2 = 62
Subnet mask: 255.255.255.192
Block size in the last octet: 64, so the /26 subnets start at 0, 64, 128, and 192.
Example 2: Find the network address of 192.168.10.77/27
/27 gives a block size of 32 in the last octet.
The /27 ranges are 0-31, 32-63, 64-95, 96-127, and so on.
Because 77 falls inside 64-95, the network address is 192.168.10.64/27.
The broadcast address is 192.168.10.95, so the usable range is 192.168.10.65 - 192.168.10.94.
Example 3: Why is /30 common on point-to-point style practice problems?
/30 has 2 host bits.
Total addresses: 2^2 = 4
Usable hosts: 2
That is why /30 often appears in older router-link examples.
Example 4: Borrowing bits to create more subnets
Start with 10.0.0.0/24.
If you borrow 2 host bits, the new prefix becomes /26.
That creates 2^2 = 4 equal subnets.
Each new subnet has 62 usable hosts on a normal LAN.
4. Tools
A full subnetting toolbox for students
Use these calculators to explore concepts, check homework, and build intuition.
IPv4 subnet calculator
Core toolEnter any IPv4 address and CIDR prefix to find the network address, broadcast address, usable range, mask, wildcard mask, host counts, and address type.
CIDR / mask / wildcard converter
Quick referenceConvert between a prefix length, subnet mask, and wildcard mask.
Hosts needed → smallest prefix
PlanningGive the tool a required number of hosts and it will suggest the smallest traditional LAN subnet that fits.
Equal subnet planner
Split networksSplit a larger network into equal-size subnets by count or by target prefix.
Simple VLSM planner
Advanced basicEnter a base network and a list of host requirements. The tool sorts the requests largest-first and builds a simple VLSM allocation plan.
Binary visualizer
Bit-level viewSee exactly which bits are the network part and which bits are the host part.
Address type checker
Quick checkCheck whether an address is private, public, loopback, link-local, documentation space, multicast, and more.
5. Practice
Practice problems and a self-check quiz
Generate random questions, check your answers, and then test yourself with multiple-choice review.
Random practice generator
Subnetting quiz
Answer all questions, then grade yourself.
6. Cheat sheet
Common prefixes at a glance
These are some of the most-used IPv4 prefixes students memorize.
| Prefix | Subnet mask | Total addresses | Usable hosts (normal LAN) | Block size |
|---|
Fast memory tips
- /24 is the classic 255.255.255.0
- The interesting octet is where the mask stops being 255 and starts being something else
- Block size in that octet = 256 - mask value
- Usable hosts usually = 2^(host bits) - 2
Important note
This site teaches the most common classroom approach to IPv4 subnetting. Some real-world cases use special rules, especially for /31 and /32.
The calculator explains those special cases when they appear so students can see both the basic rule and the exception.