classless inter-domain routing, cidr, ip address ion

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Classless Inter-Domain Routing(CIDR)

Introduction

IP addresses have been running out so moves have been made to find a solutionto this. One solution is to use private addresses in conjunction with NAT (seelater), another long term solution is to develop a new version of IP, IPv6 that uses

128 bits for the address rather than the current rather limiting 32 bits. A thirdsolution is CIDR. CIDR was first mooted in 1993 in RFC 1517 and modified in

RFC 1518, RFC 1519 before being finalised in RFC 1520.

Class C Addresses

CIDR has been designed to allow us to make the best use of the IPv4 addresses

that exist. If you have a requirement for 2000 IP addresses then you would not

have enough if you were just given a Class C address (which gives 28

- 2 = 254hosts). If you were given a Class B address you would then have enough

addresses for 216

- 2 = 65534 hosts, however you are never going to need allthese and because you are the owner of these addresses no one else can usethem. This makes for inefficient use of the IPv4 address space which is rapidly

running out of room.

Nowadays there are very few free Class B addresses, but there are some Class

C addresses. It is common for ISPs to be issued Class C addresses in blocks sothat they can issue them in contiguous address blocks to companies that requiremore than 254 IP addresses. For the above example, if you were given 8 Class C

addresses such as 199.103.104.0, 199.103.105.0 .... up to 199.103.111.0, thenyou would have 8 x 254 = 2032 IP addresses. If you look at the third octet youwill see the following:

Decimal Binary

104 01101000

105 01101001

106 01101010

107 01101011

108 01101100

109 01101101

110 01101110

111 01101111

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In bold, the bits do not change, you could therefore group these 8 Class Caddresses together as the network address 199.103.104.0/21, since 21 bits formthe network mask that encompasses the 8 Class C addresses. Normally, when

you see the 199 in the first octet, you expect to see this as one Class C addresswith a 24 bit mask. What has happened in effect is a summarisation, but insteadof a proper summarisation in a variable subnetted environment where the

summary mask sits somewhere between the natural mask and the subnet mask,this summarisation has occurred beyond the 'class boundary' and is called

Supernetting.

Officially, Class C address ranges such as 192 and 193 have been given to ISPsin the US as 'Classless'. Although these were originally Class C addresses, theycan now be treated as Class A address ranges and organisations can now be

issued with more flexible amounts of addresses rather than be restricted to theclass boundaries. Europe has been assigned the address ranges 194 to 204.

The following table details the CIDR address allocation by region:

Region Class C subnets

Multiregional 192 & 193

Europe 194 & 195

Others 196, 197, 204 - 207

North America 198 & 199

Central/South America 200 & 201

Pacific Rim 202 & 203

By regionalising these Class C addresses into blocks different sized CIDR blockscan be summarised up the Internet hierarchy. So blocks with prefixes /23, /22 etc.can be summarised into larger blocks such as /19, /18 etc. In fact is common forNAPs to not accept blocks that are less than /19 in size in order to minimise

administration and maximise route stability.

Summarisation

In the above diagram, address ranges A, B and C are in sequence. ISP 2 has tobe careful when summarising, not to include address range A with address rangeC, they must be kept separate. The problem is that packets bound for address

range B would think that they need to go to ISP 2 when in fact ISP 1 owns thatrange and the only route to B is via ISP 1. This is known as a Black Hole.

See RFC 2050 for more information on address assignment.


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Classful routing protocols such as RIPv1, EGP and IGRP will default to thenatural mask (or classful mask) for a network in it's routing table or will use theinterface configured mask (this requires the mask to be the same throughout the

IGP AS however!).

More information on CIDR can be obtained from the www.internic.net website.

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classless inter-domain routing, cidr, ip address ion

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