Subnetting Basics

01Why subnet?

1. Efficiency — IP addresses are handed out to match need, not wasted
2. Security — segments are isolated from each other (accounting ↔ guest Wi-Fi)
3. Performance — broadcast traffic stays inside small areas
4. Management — which range belongs to which department is obvious at a glance

02IP address structure

An IPv4 address is 32 bits:

192.168.1.100
↓
11000000.10101000.00000001.01100100

Network and host parts

[  Network part  ][ Host part ]
 192 . 168 . 1   .    100
 "which network?"     "which device on it?"

03The subnet mask

The subnet mask says where the boundary is: 1s mark the network part, 0s mark the host part.

04CIDR notation

192.168.1.0/24
            ↓
   first 24 bits: network
   remaining 8 bits: host

The /24 example

Mask:   255.255.255.0
Binary: 11111111.11111111.11111111.00000000
        ←──── 24 bits ────→←─ 8 bits ─→
        Network              Host

05Counting hosts

Formula: 2ⁿ − 2 = usable hosts (n = number of host bits)

06The special addresses

192.168.1.0/24

Network address: 192.168.1.0    (unusable)
First host:      192.168.1.1
Last host:       192.168.1.254
Broadcast:       192.168.1.255  (unusable)

07Example: splitting into four with /26

Let's divide 192.168.1.0/24 into four subnets:

08Summary

• Subnetting = slicing a network into logical parts
• The subnet mask draws the network/host line; CIDR writes it as /24
• Usable hosts = 2ⁿ − 2 (network + broadcast are reserved)
• Next lesson: the practical method for doing these sums by hand