A CIDR block represents a collection of IP addresses that share a common network prefix and a specific number of bits.
Understanding CIDR Blocks
A CIDR (Classless Inter-Domain Routing) block is essentially a way to represent a range of IP addresses. It's a fundamental concept in networking that helps in efficiently allocating and managing IP address space. Instead of relying on traditional class-based addressing (Class A, B, and C), CIDR allows for more flexible allocation, preventing address exhaustion and simplifying routing.
Key Components of a CIDR Block
A CIDR block is written as an IP address followed by a slash and a number (e.g., 192.168.1.0/24). Let's break down what each part means:
- IP Address (e.g., 192.168.1.0): This is the starting IP address of the block.
- Slash (/): This separates the IP address from the network prefix length.
- Network Prefix Length (e.g., /24): This number indicates how many bits of the IP address are part of the network address. The remaining bits represent the host addresses within that network.
How CIDR Works
The network prefix length determines the size of the CIDR block. A smaller number after the slash means a larger block of IP addresses, while a larger number means a smaller block. For instance:
/24indicates that the first 24 bits of the IP address are the network address, and the remaining 8 bits are for host addresses. This results in 28 (256) IP addresses, typically ranging from 192.168.1.0 to 192.168.1.255./16indicates that the first 16 bits are the network address, and the remaining 16 bits are for host addresses. This results in 216 (65,536) IP addresses.
CIDR Block Size and Prefix Length
Here’s a table illustrating some common CIDR prefixes and their corresponding number of IP addresses:
| CIDR Prefix | Number of IP Addresses | Use Case |
|---|---|---|
| /32 | 1 | Single host |
| /24 | 256 | Small to medium-sized LAN |
| /16 | 65,536 | Larger networks, small organizations |
| /8 | 16,777,216 | Very large networks, large organizations |
Importance of CIDR
- Efficient IP Address Allocation: CIDR allows for more granular allocation of IP addresses compared to classful addressing. Organizations can be assigned IP address blocks that closely match their actual needs, preventing wastage.
- Route Aggregation: CIDR simplifies routing by allowing multiple routes to be summarized into a single route advertisement. This reduces the size of routing tables and improves routing efficiency.
- Prevents IP Address Exhaustion: By efficiently allocating IP address blocks, CIDR helps to conserve the IPv4 address space, which is a finite resource.
According to the provided reference, a CIDR block is a collection of IP addresses that share the same network prefix and number of bits. The Internet Assigned Numbers Authority (IANA) assigns large CIDR blocks to regional internet registries (RIR).
