Classless Inter-Domain Routing (CIDR)

• CIDR is an IP address allocation method that improves data routing efficiency on the internet.
• An IP address is composed of two parts
• The network address is a series of numerical digits pointing to the network's unique identifier
• The host address is a series of numbers indicating the host or individual device identifier on the network
• An IPv4 address consists of 32 bits

Classful Notation

Class A

• 8 network prefix bits
• 255.0.0.0/8: 11111111.00000000.00000000.00000000
• Support 16,777,214 hosts
• Example: 44.0.0.1

Class B

• 16 network prefix bits
• Support 65,534 hosts
• 255.255.0.0/16: 11111111.11111111.00000000.00000000
• Example: 128.16.0.2

Class C

• 24 network prefix bits
• 255.255.255.0/24: 11111111.11111111.11111111.00000000
• Support 254 hosts
• Example: 192.168.1.100

CIDR Notation

• The classful arrangement was inefficient when allocating IP addresses and led to a waste of IP address spaces
• For example, an organization with 300 devices couldn't have used a Class C IP address, which only permitted 254 devices. So, the organization would've been forced to apply for a Class B IP address, which provided 65,534 unique host addresses
• CIDR addresses use variable length subnet masking (VLSM) to alter the ratio between the network and host address bits in an IP address. Therefore it is not limited to the classes A, B, C (classful), in fact the proportion is adjusted as needed

• First bits defined by /nn are dedicated to the network portion of the address while the remainin bits are the host portion of the address

• 255.255.240.0/12: 11111111.11110000.00000000.00000000

• 12 network prefix bits
• 255.255.255.240/28: 11111111.11111111.11111111.11110000
• 28 network prefix bits