Ip Classless

Unanswered Question
Mar 23rd, 2009
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guys can someone tell me what exctly Ip classless means....the word which is comonly used in books is discontiguous subnets .......can someone please tell me with easy example what discontiguous subnets or network means...thanks guys

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Giuseppe Larosa Mon, 03/23/2009 - 03:07
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Hello Khan,

in the old days of classful routing protocols discontigous major networks were not supported.


A major network is a Class A, B, C depending on the value of the first most significant byte:

10/8 is Class A network

172.16.0.0/16 is a Class B

192.168.2.0/24 is a Class C


A major network is partitioned if between two subsets of subnets there is a link but that link has an ip subnet taken from a different major network


example:

net 10/8 with a link in the middle belonging to 192.168.2.0/24


some 10 subnets --- R1 -- 192.168.2.0 --- R2 other 10 subnets


Classful routing protocols don't carry subnet masks and automatically summarize at major network boundary

a router connected to net 192.168.2.0/24 different from R1 and r3 thinks that:

net 10/8 is reachable via R1 or R2.


the no ip classless behavior means that a router internal to left cloud of 10 subnets if has to decide where to send packets destined for an unknown 10 subnet will discard the packet even if a default route is installed in the routing table.


the ip classless behaviour allows to use a default or other route to forward traffic for an unknown subnet of a connected major network


ip classless is default since IOS 12.0


Hope to help

Giuseppe




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