Classless Inter-Domain Routing (CIDR) is a way to identify IPv4 networks with an explicitly sized address prefix instead of assigning them one of a few fixed network classes. Its slash notation shows the prefix length—for example, /24 means the first 24 bits identify the network.
What does CIDR mean?
CIDR stands for Classless Inter-Domain Routing. It replaced the old class-based approach to IPv4 network assignment with prefixes whose lengths can be chosen to fit the size of a network. IPv4 addresses are 32 bits long; a CIDR prefix specifies how many leading bits belong to the network portion. The remaining bits identify addresses within that block.
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RFC 4632 describes CIDR as a strategy for conserving IPv4 address space and limiting the growth of global routing state. It is an Internet standards document published by the IETF in August 2006, and it obsoletes the original CIDR strategy document, RFC 1519, published in September 1993. RFC 4632 · RFC 1519
How to read CIDR notation
A CIDR network is written as an IP address followed by a slash and a prefix length, such as 192.168.99.0/24. The number after the slash is a count of leading bits, not a count of addresses. In this example, 24 bits define the network prefix and the remaining 8 bits are available within the block. CIDR allows different prefix lengths rather than requiring networks to fit a fixed class size. RFC 4632
How CIDR differs from classful addressing
| Aspect | Classful IPv4 addressing | CIDR |
|---|---|---|
| Network sizing | Uses a few fixed network classes and their implied sizes. | Uses an explicit prefix length to describe the network boundary. |
| Route representation | Network boundaries are implied by the address class. | The prefix length is written directly, such as /24. |
| Allocation and aggregation | Fixed sizes make allocation and summarization less flexible. | Blocks can be sized to need and aggregated when allocation and topology align. |
This comparison is about IPv4. CIDR’s original standards context addresses IPv4’s 32-bit address space; the old IPv4 address classes should not be applied to IPv6.
Why CIDR matters to Internet routing
Address allocation can better match demand
Under classful addressing, a network had to fit one of a small set of standard sizes, which could waste addresses when the available class was larger than needed. CIDR makes the boundary explicit, allowing address blocks to be allocated in sizes that more closely fit actual requirements. RFC 4632
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Route aggregation can reduce global routing state
If a provider assigns customers smaller blocks from one larger, contiguous block, it can advertise a single aggregate route for that larger block instead of advertising a separate global route for every customer network. Other routers can use that aggregate to reach destinations within it. The benefit depends on the address allocation following the network’s provider hierarchy; CIDR creates the opportunity to aggregate routes but does not guarantee that every route can be combined. RFC 4632 · RFC 1519
How routers choose between overlapping routes
Route aggregation and route selection are related, but they are not the same operation. Aggregation combines routes for advertisement. When forwarding a packet, a router compares the destination address with the routes it knows and uses the matching route with the longest prefix—the one sharing the greatest number of leading bits with the destination. RFC 4632 · RFC 1812
For example, if both a broad aggregate and a more-specific route match a destination, the more-specific route wins. That lets a network make exceptions to a broad route, but it also means an incorrect or malicious more-specific announcement can divert traffic that would otherwise follow the aggregate. RFC 4632 identifies this as a routing security risk; it is not an automatic flaw in every CIDR network. RFC 4632
Why routes cannot always be aggregated
Aggregation works best when a network uses address space assigned from its provider and its routes follow that provider’s hierarchy. Multihoming, changing providers, traffic engineering, or routing policy can require a network to advertise more-specific routes that do not fit under one provider’s aggregate. These exceptions can increase the number of routes other networks need to consider. CIDR helps contain routing growth where aggregation is practical; it does not eliminate every cause of routing complexity. RFC 4632 · RFC 1519
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why CIDR was introduced
As the Internet expanded in the late 1980s and early 1990s, concerns included exhaustion of Class B address space and growth in routing tables. CIDR, deployed with BGP-4’s support for classless prefix routes, addressed those pressures by enabling more flexible IPv4 allocation and route aggregation. RFC 4632 presents this as historical standards context, not as a current measurement of routing-table size or a complete solution to Internet scaling. RFC 4632
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