Besides allowing for a greater number of ip addressed, Classless Interdomain Routing (CIDR) was also intended to slowdown routing table growth in core routers.[1] I may understand how it expands the range of possible Internet Protocol (IP) addresses, but how it helped to slowing down routing table growth? [2][3][4][5]

Following I describe my understanding of IP, Core Routers, and routing tables:

IP address: a logical non-geographic address which is unique for each devidce coonnected to the Internet. It is composed by a netid (identifying the network) and a hostid (identifying the host in a given network) parts.

Core routers: a device for linking network devices. These routers need to keep a routing table with all netids they have heard of, and the identifier for appropriate outgoing link for each of the netids.

Routing table: is a table that shows the next hop ID for each Netid. If there is 900,000 netids assigned to networks around the world, there will be 900,000 entries in the routing table.

Maybe I'm getting those terms wrong, and because of it I can't understand the effects of CIDR on table growth.

I would really appreciate if someone could clear that up for me.

1 Answer 1


Im writing a new answer because on reflection I was initially incorrect. CIDR did reduce the routing table size.

In theory, classful routing has a maximum of about 2.1 million possible networks (all the possible A B C networks). With CIDR, today there about 900,000 routes with all of IPv4 addresses allocated.

So practically speaking, CIDR reduced the routing table by more than 50%.

  • Hi. Again, thanks for the reply! So, how did CIDR slow down the growth and significantly reduce routing table size? I mean how its implementation allowed for such an accomplishment?
    – Fb16455
    Jul 22, 2022 at 6:56
  • Simply by allowing flexible allocation of addresses.
    – Ron Trunk
    Jul 22, 2022 at 7:50
  • How allowing a more flexible allocation led to a reduced table? Could you provide an example with entries in a table before and after inplementing CIDR?
    – Fb16455
    Jul 22, 2022 at 17:48
  • 4
    If you need 1000 addresses, you would need 4 class C allocations -- that's 4 table entries. With CIDR you only need one /22
    – Ron Trunk
    Jul 22, 2022 at 19:10

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