1. CIDR Classless Inter-Domain Routing
Computer Network System
Sirak Kaewjamnong

2. Address Allocation Problem
Exhaustion of the class B network address space
The lack of a network class of size which is appropriate for mid-sizes organization
class C, with a max of 254 hosts, too small
While class B, with a max of hosts, too large
Allocate block of class C instead and downside is more routes entry in routing table

3. Routing Table Problems
Issue multiple block class C addresses (instead single class B address) solves a running out of class B address
Introduces problems of routing table
By default, a routing table contains an entry for every network
How large a routing table should be for all class C networks?
Growth of routing table in the internet routers beyond the ability of current software and hardware manage

4. Size of the Routing Table at the core of the Internet
Source:

5. Prefix Length Distribution
70000
60000
50000
40000
Number of Prefixes
30000
20000
10000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Prefix Length
Source: Geoff Huston, Oct 2001

6. How to solve Topological allocate IP address assignment
We divide the world into 8 regions (RFC 1466)
Multi regional
Europe
Others
North America
Central/South America
Pacific Rim
Others
Others
IANA Reserved

7. Classless Interdomain Routing
Class C address’s concept becomes meaningless on these route between domain, the technique is call Classless Interdomain Routing or CIDR or Supernet
Kay concepts is to allocate multiple IP address in the way that allow summarization into a smaller number of routing table (route aggregate)
CIDR is supported by BGP4 and based on route aggregation
16 class C addresses can be summarized to a single routing entry (router can hold a single route entry for a main trunks between these areas)

8. Supernetting
An organization has been allocate a block of class C address in 2n with contiguous address space
archive by using bits which belongs to the network address as hosts bits
class C example : altering the default class C subnet mask such that some bit change from 1 to 0
(Super) netmask
4 class C networks appear
to network outside as a
single network

9. Supernetting Sample An organization with 4 class C
, , ,
mask
net
net
net
net
Bit wise AND results :
This organization’s network has changed from 4 net to a single net with 1,022 hosts

10. The longest Match Supernetting
Europe has with mask
A case of one organization ( mask ) needs different routing entry
datagrams matches both Europe’s and this organization. How to do?
Routing mechanism selects the longest mask ( is longer than ), then route to the organization

11. Summary
Routing decisions are now made based on masking operations of the entries 32 bits address, hence the term “classes”
No existing routes is changed
CIDR slows down the growth of routing tables (current 130K entries in core routers)
Short term solution to solve routing problem
limitation: not all host/router software allows supernet mask