CIDR (Classless Inter-Domain Routing)

An IP addressing method that replaces classful addressing, allowing flexible allocation of IP addresses using variable-length subnet masking.

What is CIDR?

Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and routing that replaced the older classful addressing system. CIDR enables more efficient use of IP address space and reduces routing table sizes.

CIDR Notation

CIDR uses slash notation to indicate network size:

/8: 16,777,216 addresses (Class A equivalent)
/16: 65,536 addresses (Class B equivalent)
/24: 256 addresses (Class C equivalent)
/32: Single host address

How CIDR Works

Instead of fixed Class A, B, and C boundaries, CIDR allows networks of any size using variable-length subnet masks (VLSM). This flexibility means organizations receive appropriately-sized address blocks rather than oversized classful allocations.

Benefits

Efficient IP address allocation
Reduced global routing table size through aggregation
Flexible network design
Enables supernetting (combining multiple networks)
Slowed IPv4 address exhaustion