The Patch that Saved the Internet
Deconstructing CIDR & Aggregation
1. The Death of the Classful System
In the beginning, the internet used a "Class" system. If you were a medium company, you got a "Class B" lease—65,534 IP addresses. If you only had 500 employees, you wasted over 65,000 IPs. By 1993, the internet was running out of space.
CIDR abolished these rigid classes. It allowed address space to be leased in exactly the size needed, using a variable prefix.
2. Slash Notation: The Variable Prefix
Before CIDR, you had to type 255.255.255.0. With CIDR, we use /24. This number tells the router exactly how many bits (from the start of the 32-bit address) are the "Network."
- /8: Large block (16 million IPs).
- /24: Common home/SMB network (254 usable IPs).
- /30: Point-to-point link between routers (2 usable IPs).
3. Route Summarization (Supernetting)
The most powerful feature of CIDR is Aggregation. Suppose an ISP owns 256 individual /24 networks. Instead of announcing 256 separate entries to the global internet, they can "summarize" them into a single /16 route.
4. Managing the Routing Table Explosion
If every small business announced their small network to the internet, the global routing table (the Default Free Zone) would contain millions of entries. Routers would crash under the weight of the data. CIDR allows regional hierarchies: ISPs group small routes into larger ones, providing a tiered structure that keeps the internet stable.
5. The "Longest Prefix Match" Rule
Modern routers are incredibly fast. When a packet arrives, the router might find multiple matches for that IP in its memory. CIDR dictates the rule: The specific route always wins. If a router sees 1.1.1.0/24 and 1.1.0.0/16, and the packet is going to 1.1.1.5, it will use the /24. It is searching for the "Longest Match" in binary bits.
Conclusion: The Infrastructure of Efficiency
CIDR is the unsung hero of the 1990s. It bought the world thirty extra years of IPv4 use by stopping waste and managing complexity. While users see only the slash notation, the engineer sees a hierarchical web of binary prefixes that keep the global routing tables lean and efficient.
Frequently Asked Questions
Does IPv6 use CIDR?
IPv6 is classless by design. It uses the exact same prefix logic as CIDR (e.g., /64). Because it was built after classes were abolished, it never had the "Classful" problem to begin with.
What is a /0 route?
A /0 means "zero bits match." This is the Default Route (0.0.0.0/0). It matches everything because it doesn't require any specific bits to be correct. It is the "last resort" for any router.
Can I summarize random IP blocks?
No. You can only summarize addresses that are Contiguous (next to each other in binary). If you have two networks at opposite ends of the IP space, they cannot be aggregated into a single route.