IP subnetting basics

Subnetting can seem complex at first, but with a step-by-step approach, it becomes much more manageable. Let’s break it down.

1. Understanding IP Addresses and Classes

IP Address Structure

IPv4 addresses are 32 bits long, commonly written as four octets (e.g., 192.168.1.1).
Each octet represents 8 bits, ranging from 0 to 255.
Example (binary and decimal):11000000.10101000.00000001.00000001 → 192.168.1.1

Address Classes

Class A: 1.0.0.0 – 126.255.255.255 (/8 subnet mask)
Class B: 128.0.0.0 – 191.255.255.255 (/16 subnet mask)
Class C: 192.0.0.0 – 223.255.255.255 (/24 subnet mask)

2. Network vs. Host Bits

Network bits (left): Identify the subnet.
Host bits (right): Identify a specific device.
Subnetting takes bits from the host part and assigns them to the network, forming new (smaller) subnets.

Subnet Mask

Defines how many bits are network (1s) and how many are host (0s).
Example:255.255.255.0 = 11111111.11111111.11111111.00000000

CIDR Notation

Shorthand for subnet mask: /24 means 24 bits for network.

3. Subnetting Step by Step

Let’s walk through subnetting a /24 network (e.g., 192.168.1.0/24) into 4 subnets.

Step 1: How many subnets do you need?

4 subnets
Formula: 2^n ≥ number of subnets, where n = number of bits borrowed.

Step 2: Borrow Host Bits

From a /24 (255.255.255.0), the default has 8 host bits.
Need at least 2 borrowed bits:2^2 = 4 subnets

Step 3: New Subnet Mask

Original: /24 (255.255.255.0)
New: /26 (255.255.255.192), since 24 + 2 = 26 network bits.

Step 4: Calculate Subnet Ranges

8 host bits – 2 borrowed = 6 remaining host bits.
Host addresses per subnet (formula): 2^host bits - 2 (subtract 2 for network & broadcast addresses).
- 2^6 - 2 = 62 usable IPs per subnet.

Subnet Number	Network Address	First Host	Last Host	Broadcast Address
1	192.168.1.0	192.168.1.1	192.168.1.62	192.168.1.63
2	192.168.1.64	192.168.1.65	192.168.1.126	192.168.1.127
3	192.168.1.128	192.168.1.129	192.168.1.190	192.168.1.191
4	192.168.1.192	192.168.1.193	192.168.1.254	192.168.1.255

4. General Subnetting Formulas

Number of subnets: 2^n, where n = # of bits borrowed.
Number of hosts per subnet: 2^h - 2, where h = # of host bits left.
New subnet mask: Add n to the network mask.
Block size (interval between subnets): 256 - value in subnet mask's new octet.

Example Block Sizes

/26: 255.255.255.192 → Block size: 256 - 192 = 64 (subnets start at .0, .64, .128, .192)

5. Binary Math Trick

Third Subnet Calculation (binary):

192.168.1.128 (Subnet Starts)
192.168.1.191 (Broadcast = 128 + 63)
Hosts: 129-190

Write in binary if you’re stuck—borrowed bits help you spot boundaries!

6. Subnetting for Custom Host/Subnet Needs

Example:

You need at least 30 hosts per subnet on 192.168.10.0/24.

Hosts per subnet: 2^h - 2 ≥ 30 2^5 - 2 = 30 ; use 5 host bits → borrow 3 bits (since 8-5=3)
New mask: /27 (255.255.255.224)
Subnets: 2^3 = 8
Block size: 256 - 224 = 32

7. CIDR and VLSM

CIDR: Classless Inter-Domain Routing lets you use mask sizes that don’t fit default classes.
- Example: 10.1.1.0/28
VLSM (Variable Length Subnet Mask): Allows subnets with different sizes within the same network for efficient IP use.

8. Practice Problem

Subnet 10.0.0.0/16 into 16 subnets.

Host bits in /16: 16
Bits to borrow: 4 (2^4 = 16)
New mask: /20 (16+4=20; 255.255.240.0)
Block size: 256 - 240 = 16 in the third octet.
Subnets:
- 10.0.0.0/20
- 10.0.16.0/20
- 10.0.32.0/20
- … up to 10.0.240.0/20

Summary Table

Term	Description
Subnet Mask	Bitmask for dividing IP parts
Network Bits	Number of bits in subnet mask
Host Bits	Remaining bits for hosts
CIDR Notation	Shorthand (e.g., /27 for 255.255.255.224)
Block Size	Increment between subnets

Subnetting is about dividing, organizing, and optimizing address spaces. Practice binary math, use the above formulas, and you’ll become proficient!

If you want to practice with specific scenarios or want more advanced examples (like VLSM or real-world design), just let me know!