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CIDRClassless Inter-Domain Routing

Computer Network SystemSirak Kaewjamnong

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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 65534 hosts, too large

• Allocate block of class C instead and downside is more routes entry in routing table

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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

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Size of the Routing Table at the core of the Internet

Source: http://www.telstra.net/ops/bgptable.html

0

20000

40000

60000

80000

100000

120000

140000

Aug-87 May-90 J an-93 Oct-95 J ul-98 Apr-01 J an-04

Num

ber

of

prefi

xes

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Prefix Length Distribution

0

10000

20000

30000

40000

50000

60000

70000

1 2 3 4 5 6 7 8 9 10 11 1213 14 15 1617 18 19 2021 22 2324 25 26 2728 29 3031 32

Prefix Length

Nu

mb

er o

f P

refi

xes

Source: Geoff Huston, Oct 2001

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How to solve• Topological allocate IP address assignment• We divide the world into 8 regions (RFC 1466)

Multi regional 192.0.0.0 - 193.255.255.255Europe 194.0.0.0 - 195.255.255.255Others 196.0.0.0 - 197.255.255.255North America 198.0.0.0 - 199.255.255.255Central/South America200.0.0.0 - 201.255.255.255Pacific Rim 202.0.0.0 - 203.255.255.255Others 204.0.0.0 - 205.255.255.255Others 206.0.0.0 - 207.255.255.255IANA Reserved 208.0.0.0 - 223.255.255.255

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Classless Interdomain Routing• Class C address’s concept becomes mea

ningless on these route between domai n, the technique is call Classless Interd

omain Routing or CIDR or Supernet• Kay concepts is to allocate multiple IP ad

dress in the way that allow summarizatio n into a smaller number of routing table (

route aggregate)• CIDR is supported by BGP4 and based o

n route aggregation– 16 class C addresses can be summarized

a aaaaaa aaaaaaa aaaaa aaaaaaa aaa aaaa a ( singler out e ent r y f or a mai n t r unks bet ween t h esear eas)

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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 011111111 11111111 11111100 00000000

255.255.252.0

(Super) netmask4 class C networks appearto network outside as a single network

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Supernetting Sample

• An organization with 4 class C193.0.32.0 , 193.0.33.0 , 193.0.34.0 , 193.0.35.011111111 11111111 11111100 00000000 mask 255.255.252.011000001 00000000 00100000 00000000 net 193.0.32.011000001 00000000 00100001 00000000 net 193.0.33.011000001 00000000 00100010 00000000 net 193.0.34.0 11000001 00000000 00100011 00000000 net 193.0.35.0Bit wise AND results 193.0.32.0: 11000001 00000000 00100000 00000000

• This organization’s network has changed from 4 net to a single net with 1,022 hosts

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The longest Match Supernetting• - 194000 1952Europe has . . . .

55255255 254. . with mask . 000. .

• - aaaa(195.0.16.0 195.0.36.0 255.255.254.0)a aaaaaaaaa aaaaaaa aaaaa

• a aaaaaa a1 950201

othEurope’sandt hi s or gani zat i on. Howt o do?• aaaa aaaaa aa (255.255.254.0 254.0.0.0),

t he or gani zat i on

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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