CIDR (Classless Inter-Domain Routing)

CIDR replaced the old classful addressing system to use IP addresses more efficiently.

Why CIDR?

• Old classful system (Class A, B, C) was wasteful. Need 300 hosts? You get Class B = 65,534 hosts.
• CIDR allows variable length subnet masks — allocate exactly what you need.
• Also helps with route aggregation (reduces routing table size).

CIDR Notation

Written as IP address / prefix length. Example: 192.168.1.0/24

192.168.1.0/24
   /24 → first 24 bits are the network part
   Remaining 8 bits are the host part

Subnet Mask Reference Table

CIDR	Subnet Mask	Network Bits	Host Bits	Hosts Available
/8	255.0.0.0	8	24	16,777,214
/16	255.255.0.0	16	16	65,534
/24	255.255.255.0	24	8	254
/25	255.255.255.128	25	7	126
/26	255.255.255.192	26	6	62
/27	255.255.255.224	27	5	30
/28	255.255.255.240	28	4	14
/30	255.255.255.252	30	2	2
/32	255.255.255.255	32	0	1 (host route)

Formula: Hosts Available = 2ⁿ - 2 (subtract 2 for network address + broadcast address)

How Subnetting Works

Take 192.168.1.0/24:

Network address  → 192.168.1.0   (first address, not assignable)
Broadcast address→ 192.168.1.255 (last address, not assignable)
Usable hosts      → 192.168.1.1 to 192.168.1.254 = 254 hosts

Splitting a Network — Example

Split 192.168.1.0/24 into 2 subnets → use /25

Subnet 1: 192.168.1.0/25
  → Hosts:     192.168.1.1 – 192.168.1.126 (126 hosts)
  → Broadcast: 192.168.1.127

Subnet 2: 192.168.1.128/25
  → Hosts:     192.168.1.129 – 192.168.1.254 (126 hosts)
  → Broadcast: 192.168.1.255

Route Aggregation (Supernetting)

192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24

→ All summarized as 192.168.0.0/22

This reduces routing table entries — routers work faster.

Quick Trick — Host Bits to Hosts

Host bits	1	2	3	4	5	6	7	8
Hosts	0	2	6	14	30	62	126	254

Formula: 2ⁿ - 2, where n = host bits

CIDR vs Classful

	Classful	CIDR
Subnet mask	Fixed per class	Variable (any prefix)
Flexibility	Low	High
IP wastage	High	Low
Route aggregation	No	Yes