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Command "ip classless' on Cisco

Posted on 2004-07-31
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Last Modified: 2008-05-30
Hi!
I see an comman "ip classless" on Cisco router, and I don't know what does it use for? How can you help me?
Thanks
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Question by:itman04
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sunnycoder earned 125 total points
ID: 11683173
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by:sunnycoder
ID: 11683178
itman04,

In short

ip classless command in global configuration mode makes the Cisco IOS software forward packets destined for unrecognized subnets to the best supernet route possible

Sunnycoder
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by:krazieintent
ID: 11705906
Ip classess is used to tell the router not to route to the classfull boundary of the Ip address subnet mask.

the classful boundard are.
for class a 255.0.0.0
class b 255.255.0.0
class c 255.255.255.0

It works in conjunction with the routing protocal used on the router.

for example if you use a classless routing protocol such as eigrp and have subnets

and have subnets 192.168.1.16

192.168.1.32
192.168.1.64

when you type router eigrp 100

and put in your networks

instead of putting the classfull network number such as 192.168.1.0

yiou would put as your networks 192.168.1.16
192.168.1.32 and 192.168.1.64

this will help routing if the subnets are all not connected to one router.

however is you use a classful protocol such as RIP or igrp

even if you type in your networks as 192.168.1.16, .32, and .64

when you do a show run command

the only network address you will see under the protocol would be 192.168.1.0, the classfull address.

if your network was subnetted on diferent routers, the classfull protocol would not work.

reason being both routers think they are connected to the same network and when routing updates are sent to eachother, one router is saying i am connected to the 192.168.1.0 network while the other one is say no, i am connected to the 192.168.1.0 network because even when you put in your subnet address it will automatically trunkate the address to its classfull boundary

ip classless stops that from happening to allow routing between subnets.

hope this was a good explaination of IP classless

Kevin
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