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See: 1519 on datatracker.ietf.org

The title of this RFC is “ An Address Assignment and Aggregation Strategy.”

RFC 1519 introduces Classless Inter-Domain Routing (CIDR), a revolutionary strategy for managing IP address allocation and route aggregation in IP networks. Prior to the introduction of CIDR, the internet used classful addressing, where IP addresses were allocated based on predefined classes (Class A, Class B, and Class C). This method often resulted in inefficient address allocation and the rapid depletion of IP address space. To address these challenges, RFC 1519 proposed the use of CIDR, which allows for more flexible address assignment by eliminating the fixed boundaries of classful addressing. The related RFC is RFC 1519, which outlines the framework for the transition from classful addressing to classless routing and its implications for IP networks. https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing https://tools.ietf.org/html/rfc1519

The main benefit of CIDR is its ability to aggregate multiple IP routes into a single, summarized route, which significantly reduces the size of routing tables. By aggregating routes, routers can advertise fewer entries, minimizing router memory usage and router processing requirements. This aggregation helps alleviate the burden on Border Gateway Protocol (BGP), which handles routing between autonomous systems on the global internet. Without route aggregation, the sheer number of individual routes would overwhelm routers, leading to inefficient network operations. The related RFC is RFC 4271, which defines the operation of BGP-4 and its reliance on CIDR for managing route aggregation. https://en.wikipedia.org/wiki/Border_Gateway_Protocol https://tools.ietf.org/html/rfc4271

In addition to improving route aggregation, CIDR also addresses the problem of IP address exhaustion. Under the classful system, large blocks of addresses were often assigned to networks that did not fully utilize them, resulting in wasted IP address space. CIDR enables the allocation of IP address blocks in a more granular manner, allowing network operators to assign only the number of addresses actually needed. This more efficient use of IP address space has been crucial in slowing the depletion of IPv4 addresses, which are a finite resource. The related RFC is RFC 4632, which provides further guidelines on the allocation of CIDR blocks for IP address conservation. https://en.wikipedia.org/wiki/IP_address_exhaustion https://tools.ietf.org/html/rfc4632

CIDR works by using a variable-length subnet mask (VLSM), which allows for the division of an IP address space into subnets of varying sizes. This flexibility allows networks to be allocated address space that closely matches their actual needs, preventing the waste associated with classful addressing. For example, instead of being restricted to class A, B, or C blocks, networks can be assigned prefixes like /24, /22, or /30, depending on their requirements. The related RFC is RFC 950, which originally introduced the concept of subnetting and paved the way for the implementation of CIDR in modern networks. https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing https://tools.ietf.org/html/rfc950

One of the key challenges that RFC 1519 addresses is the need for backward compatibility with older classful systems. To ensure a smooth transition to CIDR, routers and other network devices had to be capable of understanding both classful and classless routing information. This dual compatibility was critical during the early stages of CIDR deployment, allowing networks to gradually adopt the new addressing scheme without disrupting existing operations. The related RFC is RFC 1812, which defines the requirements for IP routers, including the ability to process classless routing information. https://en.wikipedia.org/wiki/Router_(computing) https://tools.ietf.org/html/rfc1812

RFC 1519 also emphasizes the importance of route aggregation for improving the scalability of the internet. As the number of devices and networks connected to the internet continues to grow, the ability to aggregate routes becomes increasingly important for managing routing tables and ensuring efficient data transmission. Aggregation allows multiple smaller networks to be represented by a single route, reducing the complexity of routing tables and making it easier for routers to process traffic. The related RFC is RFC 1058, which defines the Routing Information Protocol (RIP) and its limitations in handling large routing tables without route aggregation. https://en.wikipedia.org/wiki/Routing_Information_Protocol https://tools.ietf.org/html/rfc1058

Another critical aspect of RFC 1519 is its role in supporting the hierarchical structure of the internet. By aggregating routes at different levels of the network hierarchy, CIDR enables more efficient inter-domain routing. For example, ISPs can aggregate the routes for all of their customer networks into a single route advertisement, reducing the number of routes that need to be propagated to other autonomous systems. This hierarchical approach helps contain the size of routing tables within each domain, improving scalability and reducing processing overhead. The related RFC is RFC 1771, which describes how BGP implements route aggregation and CIDR to manage inter-domain routing more effectively. https://en.wikipedia.org/wiki/Internet_hierarchy https://tools.ietf.org/html/rfc1771

In addition to its technical benefits, RFC 1519 also represents a significant shift in the way IP addresses are allocated and managed. Before CIDR, IP address assignments were often inefficient, with large blocks of addresses going unused. CIDR introduced the concept of IP address block leasing, where organizations could obtain address space that closely matched their actual needs, rather than being forced to take an entire classful block. This more flexible approach has been critical in extending the lifespan of IPv4 addresses and delaying the need for a transition to IPv6. The related RFC is RFC 2460, which defines IPv6, the next-generation IP addressing protocol that was developed in response to IPv4 exhaustion. https://en.wikipedia.org/wiki/IPv6 https://tools.ietf.org/html/rfc2460

Another key advantage of CIDR is its ability to support route filtering, which allows network administrators to control which routes are advertised to other networks. By filtering out unnecessary or redundant routes, BGP routers can reduce the amount of routing information exchanged between autonomous systems. This technique helps improve the security and stability of the internet by preventing the propagation of incorrect or malicious routing information. The related RFC is RFC 7454, which provides best practices for securing BGP operations, including the use of route filtering alongside CIDR to protect the internet from routing misconfigurations and attacks. https://en.wikipedia.org/wiki/Route_filtering https://tools.ietf.org/html/rfc7454

The adoption of CIDR has also been critical in enabling the deployment of IPv6, which uses a similar approach to address aggregation and routing. Although IPv6 addresses are much larger than IPv4 addresses, the principles of CIDR still apply, allowing for flexible and efficient allocation of address space. This continuity between IPv4 and IPv6 addressing schemes has made it easier for network operators to transition to IPv6 while maintaining compatibility with existing routing practices. The related RFC is RFC 1883, which describes the architecture of IPv6 and its reliance on CIDR principles for efficient address management. https://en.wikipedia.org/wiki/IPv6 https://tools.ietf.org/html/rfc1883

The flexibility of CIDR also enables more efficient multi-homing, where networks are connected to multiple ISPs for redundancy and load balancing. By aggregating routes from multiple providers, multi-homed networks can advertise a single route to the global internet, reducing the complexity of routing tables and ensuring more stable connectivity. This is especially important for organizations that rely on multiple connections to maintain service availability in the event of a network failure. The related RFC is RFC 1998, which describes techniques for multi-homing and how CIDR helps facilitate this by allowing flexible route advertisements. https://en.wikipedia.org/wiki/Multihoming https://tools.ietf.org/html/rfc1998

The title of this RFC is “ An Address Assignment and Aggregation Strategy.” RFC 1519 introduced CIDR, a transformative approach to IP address allocation and route aggregation that has dramatically improved the scalability and efficiency of the internet. By enabling flexible address assignments and route summarization, CIDR has helped reduce routing table sizes, conserve IP address space, and extend the lifespan of IPv4. Through its support of route aggregation, hierarchical routing, and multi-homing, CIDR continues to play a critical role in managing the global internet's routing infrastructure.