IP Subnetting Guide

IP Subnetting

Subnetting divides a network into smaller, manageable segments for better organization and security.

CIDR Notation

192.168.1.0/24

/24 = 24 network bits = 255.255.255.0 subnet mask
     = 8 host bits = 256 addresses (254 usable)

Subnet Mask Reference

Calculating Subnets

Example: Divide 192.168.1.0/24 into 4 subnets

Original: 192.168.1.0/24 (256 addresses)
Need: 4 subnets
Solution: Borrow 2 bits (2^2 = 4)
New mask: /26

Subnet 1: 192.168.1.0/26   (1-62)
Subnet 2: 192.168.1.64/26  (65-126)
Subnet 3: 192.168.1.128/26 (129-190)
Subnet 4: 192.168.1.192/26 (193-254)

Private IP Ranges

Class A: 10.0.0.0/8       (10.0.0.0 - 10.255.255.255)
Class B: 172.16.0.0/12    (172.16.0.0 - 172.31.255.255)
Class C: 192.168.0.0/16   (192.168.0.0 - 192.168.255.255)

VLSM (Variable Length Subnet Masking)

Allocate different subnet sizes based on need:

Network: 10.0.0.0/24

Dept A (100 hosts): 10.0.0.0/25   (128 addresses)
Dept B (50 hosts):  10.0.0.128/26 (64 addresses)
Dept C (20 hosts):  10.0.0.192/27 (32 addresses)
Links (2 hosts):    10.0.0.224/30 (4 addresses each)

Linux Network Configuration

# View current configuration
ip addr show

# Add IP address
sudo ip addr add 192.168.1.100/24 dev eth0

# Calculate subnet info
ipcalc 192.168.1.0/24

# Check routing table
ip route show

Subnetting Formula

Hosts per subnet = 2^(32 - CIDR) - 2
Subnets available = 2^(CIDR - original_CIDR)

Example /26 from /24:
Hosts = 2^(32-26) - 2 = 64 - 2 = 62
Subnets = 2^(26-24) = 4

Best Practices

1. Plan ahead - Account for future growth
2. Document everything - IP allocation spreadsheet
3. Use VLSM - Efficient address utilization
4. Reserve addresses - For network devices
5. Separate by function - Servers, users, management