CIDR inside

CIDR inside

From PING by APNIC

April 29, 2026 · 49 min · Season 6 · Episode 8

About this episode

Geoff Huston discusses the history and significance of Classless Inter-Domain Routing (CIDR) in the context of Internet growth and address allocation.

In this episode of PING, APNIC Chief Scientist Geoff Huston discusses the tortuous history of The CIDR report Classless Inter-Domain Routing or CIDR, is a mechanism defined in the 90s, to replace the former model of fixed sized networks defined in RFC791 called class-A class-B or class-C (there were actually class-D and class-E but for now we can ignore them) -the "Classless" part means no longer obeying the fixed bit-pattern at the "top" of the address (in the top 3 bits) which defined which class you were in, the classes defining how many addresses were in that block: a Class-A was 17 million, a Class B was 65,000 and a Class C was 256. This worked fine for the early life of the Internet, but under the stresses of exponential growth in the 1990s a new method for allocating addresses was defined, which exploited this "classless" model and allowed people to be given sizes between 17 million and 65,000 or between 65,000 and 256. -Which in turn fixed two problems: access to addresses into the future (through the Regional Internet Registry model of justified need for addressing) and the scaling problems of the routing mechanism. Routing has roots which reach back into the 1950s when…

People in this episode

Guest: Geoff Huston

Topics covered

  • CIDR
  • Internet history
  • networking
  • routing
  • address allocation

Keywords

  • CIDR
  • routing
  • address allocation
  • Internet growth
  • networking

Mentioned in this episode

Organizations: APNIC, DECnet, Digital

Books & works: RFC791

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