IP Addresses And Classless Interdomain Routing (CIDR)
Each IP address is composed of two portions:
the network prefix: defines the administrative domain (e.g. MIT, Princeton)
the host portion
Before CIDR: ip addresses were allocated in blocks of /8 (class A blocks), /16 (class B blocks) or /24 (class C blocks).
Class A ip addresses : begin with binary 0
Class B ip addresses begin with binary 10
Class C ip addresses begin with binary 110
Classless InterDomain Routing (or simply CIDR) allows to create blocks of any number of bits and not be restricted to /8, /16 or /24 blocks.
When we talk about a CIDR block, we refer to its netmask. For example, a CIDR block of /20 means we have 20 bits set to binary “1” for the netmask and 212 host addresses. The shorter the CIDR block, the larger the block of host IP addresses
Today, IPv4 addresses are managed in CIDR blocks
IPv4 address allocation is delegated by ICANN to IANA. IANA is a department in ICANN.
IANA assigns a /8 CIDR block to each RIR (Regional Internet Registry). Examples of RIRs: ARIN, APNIC,…
All routers look up at their Forwarding table to determine the route to use to forward a packet. The algorithm they use is the Longest Prefix Match (LPM) algorithm.
The forwarding table in a router contains a list of network prefixes (network addresses) in a CIDR notation, followed by the link they are attached to.