CIDR
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Definition
Classless Inter-Domain Routing. Eine Methode zur Zuweisung von IP-Adressen unter Verwendung variabler Subnetzmasken (z.B. 10.0.0.0/8) anstelle fester Klassengrenzen, die eine effizientere Nutzung des IPv4-Adressraums ermöglicht.
The Problem CIDR Solved
Before CIDR, IP addresses were allocated in rigid classes: Class A (/8), Class B (/16), and Class C (/24). An organization needing 500 addresses had to take a Class B (65,536 addresses), wasting over 65,000 addresses. This classful system accelerated IPv4 exhaustion and caused explosive growth in BGP routing tables.
CIDR, introduced in 1993 via RFC 1519, allows address blocks of any power-of-two size using prefix notation. A /22 block gives 1,024 addresses; a /28 gives 16. ISPs can allocate exactly what an organization needs and aggregate multiple blocks into a single BGP announcement, a process called route summarization.
Reading CIDR Notation
A CIDR block like 10.0.0.0/20 means the first 20 bits are the network prefix and the remaining 12 bits identify hosts. The Subnet MaskA 32-bit number (e.g., 255.255.255.0) that divides an IP address into network and host portions. It determines which part of the address identifies the network and which part identifies individual devices. equivalent is 255.255.240.0. The block contains 2^12 = 4,096 addresses (4,094 usable, subtracting network and broadcast).
The prefix length directly determines the Subnet MaskA 32-bit number (e.g., 255.255.255.0) that divides an IP address into network and host portions. It determines which part of the address identifies the network and which part identifies individual devices.: /24 = 255.255.255.0, /16 = 255.255.0.0, /8 = 255.0.0.0. Prefixes shorter than /8 are super-nets; prefixes longer than /24 are fine-grained subnets used inside enterprise networks.
CIDR and Routing
CIDR enables longest-prefix match routing. When a router has overlapping routes (e.g., 10.0.0.0/8 and 10.1.0.0/16), it forwards packets to the most specific matching prefix. This allows hierarchical routing — upstream routers carry aggregate summaries while downstream routers maintain detailed internal routes.
Use Subnet Calculator to explore CIDR block boundaries and host counts interactively. CIDR Aggregator can merge overlapping or adjacent prefixes into optimal summaries.