Step 2 — IP Address Fundamentals
⚠️ Prerequisite Required
This step assumes you're comfortable with binary conversion. If you haven't completed Step 1 – Binary Fundamentals, do that first. Subnetting builds directly on binary understanding.
Before you can subnet, you need to deeply understand how IP addresses work. This isn't just memorization — it's building intuition that will make subnetting feel natural.
Binary Confidence Check
Quick check: Can you convert these without hesitation?
• 192 to binary: ?
• 255 to binary: ?
• 128 to binary: ?
If these took more than 2 seconds each, review Step 1. Binary fluency is non-negotiable for subnetting.
What is an IP Address?
An IP (Internet Protocol) address is a unique identifier assigned to every device on a network. Think of it like a street address for computers.
IPv4 addresses are 32 bits long, written as four decimal numbers separated by dots. Each number represents 8 bits (one octet).
The Four Octets
Each IPv4 address has exactly four octets. Each octet is 8 bits, which means each can range from 0 to 255.
| Octet | Decimal | Binary |
|---|---|---|
| 1st | 192 | 11000000 |
| 2nd | 168 | 10101000 |
| 3rd | 1 | 00000001 |
| 4th | 100 | 01100100 |
Full binary: 11000000.10101000.00000001.01100100
Binary Conversion (The Key Skill)
Understanding binary is essential for subnetting. Here's the binary place value chart for one octet:
| 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
|---|---|---|---|---|---|---|---|
| 2⁷ | 2⁶ | 2⁵ | 2⁴ | 2³ | 2² | 2¹ | 2⁰ |
Example: Convert 192 to binary
- 192 ≥ 128? Yes → write 1, remainder = 192 - 128 = 64
- 64 ≥ 64? Yes → write 1, remainder = 64 - 64 = 0
- 0 ≥ 32? No → write 0
- 0 ≥ 16? No → write 0
- 0 ≥ 8? No → write 0
- 0 ≥ 4? No → write 0
- 0 ≥ 2? No → write 0
- 0 ≥ 1? No → write 0
192 = 11000000
Quick Binary Practice
Apply what you learned in Step 1. Convert each octet to binary.
Convert each octet of 192.168.1.100:
192 → 11000000
168 → 10101000
1 → 00000001
100 → 01100100
If this feels slow or uncertain, you're not ready for subnetting yet. Review Step 1.
Network Portion vs. Host Portion
Every IP address has two parts:
- Network portion: Identifies which network the device is on (like a street name)
- Host portion: Identifies the specific device on that network (like a house number)
The subnet mask determines where the split occurs. You'll learn this in the next step.
IP Address Classes (Historical Context)
Originally, IP addresses were divided into classes. While we now use CIDR (classless addressing), understanding classes helps explain default subnet masks.
| Class | First Octet | Default Mask | Example |
|---|---|---|---|
| A | 1–126 | 255.0.0.0 | 10.0.0.1 |
| B | 128–191 | 255.255.0.0 | 172.16.0.1 |
| C | 192–223 | 255.255.255.0 | 192.168.1.1 |
Note: 127.x.x.x is reserved for loopback (localhost). Class D (224-239) is multicast. Class E (240-255) is experimental.
Private IP Address Ranges (RFC 1918)
These ranges are reserved for private networks and are not routable on the public internet. You'll see these constantly in enterprise networks.
| Class | Range | CIDR | Common Use |
|---|---|---|---|
| A | 10.0.0.0 – 10.255.255.255 | 10.0.0.0/8 | Large enterprises |
| B | 172.16.0.0 – 172.31.255.255 | 172.16.0.0/12 | Cloud VPCs |
| C | 192.168.0.0 – 192.168.255.255 | 192.168.0.0/16 | Home/small office |
Practice Exercises
Convert these decimal values to binary (answers below):
- Convert
255to binary - Convert
128to binary - Convert
64to binary - Convert
224to binary - Convert
10to binary
Show Answers
- 255 = 11111111
- 128 = 10000000
- 64 = 01000000
- 224 = 11100000
- 10 = 00001010
Checkpoint
Before moving on, make sure you can:
- Convert any decimal (0-255) to 8-bit binary
- Convert 8-bit binary back to decimal
- Identify which class an IP address belongs to
- Recognize private IP address ranges
- Understand that IPs have network and host portions