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Classless Inter-Domain Routing (CIDR) is a way to describe IPv4 networks with explicitly sized address prefixes instead of fixed Class A, B, and C network sizes. Its slash notation shows how many leading bits identify the network, allowing more flexible address allocation and route aggregation.
What the slash number means
An IPv4 address contains 32 bits. In notation such as 192.168.99.0/24, the number after the slash is the prefix length: 24 leading bits identify the network prefix, and the remaining 8 bits are outside that prefix. The prefix length makes the network boundary explicit rather than implying it from an address class. See the IETF’s RFC 4632.
A prefix describes a block of addresses; it does not by itself say how a network must use every address in that block. CIDR’s key change was to let address blocks be sized to better match actual needs rather than restricting assignment to a few class-based sizes.
Why CIDR matters to Internet routing
CIDR connects address allocation with route aggregation. If a provider assigns customers portions of one larger, contiguous address block, the provider can advertise a route for that aggregate rather than a separate route for each customer network. Other routers can use that less-specific route to reach destinations within the block. This can limit growth in the global routing state; RFC 1519 introduced the original CIDR strategy, and RFC 4632 later updated it: RFC 1519 and RFC 4632.
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Aggregation works best when address allocation follows provider and network topology. A customer using provider-assigned space through one provider is comparatively easy to include in that provider’s aggregate. Allocation flexibility makes this possible, but does not guarantee that every route can be combined.
How routers choose between overlapping routes
Aggregation and forwarding are related but separate operations. Aggregation combines routes for advertisement. When forwarding a packet, a router compares the destination address with its available routes and chooses the matching route with the longest prefix—the one with the most matching leading bits. RFC 1812 sets out IPv4 router requirements and longest-match forwarding: RFC 1812.
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For example, if a router has both an aggregate route and a more-specific route that covers the destination, it uses the more-specific route. The aggregate can still cover other destinations in its block that have no more-specific match.
When routes cannot be summarized cleanly
Multihoming, changing providers, traffic engineering, and routing policy can require more-specific routes that do not fit neatly into one provider’s aggregate. These exceptions reduce the routing benefits of aggregation, even though CIDR still allows flexible prefix sizes.
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A more-specific route also has a security consequence: if an incorrect or malicious route is accepted, it can attract traffic that would otherwise follow a less-specific aggregate. RFC 4632 discusses this risk. It is a potential routing issue, not an automatic vulnerability in every CIDR network.
How CIDR differs from classful addressing
| Aspect | Classful IPv4 addressing | CIDR |
|---|---|---|
| Network sizing | Uses a few fixed Class A, B, and C sizes | Uses explicitly sized prefixes |
| Network boundary | Implied by the address class | Shown by the prefix length, such as /24 |
| Route aggregation | Less flexible because of fixed network sizes | Can aggregate routes when address allocation and provider topology align |
| Overlapping routes | Not defined by a slash-prefix scheme | Forwarding selects the longest matching prefix |
Why CIDR was introduced
The Internet Engineering Task Force’s August 2006 RFC 4632 describes CIDR’s purpose as conserving IPv4 address space and limiting growth in global routing state. It recounts late-1980s and early-1990s pressures, including depletion of Class B space and routing-table growth, and describes CIDR with BGP-4 support as a way to ease those short-term difficulties while longer-term work continued. That is historical standards context, not a current measurement of routing-table size.
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The original CIDR standards context concerns IPv4. It should not be read as saying IPv6 uses the old IPv4 address classes.
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