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.
  • A default route is in the format.

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