when to use no ip classless

[matt_heff]

what are common situations for using no ip classless?

[abhishek_nandavat]

Hi!

You use the 'no ip classless' command when your network is not using subnetted IP addressing scheme.

Refer the following link for more -->

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml

Please rate all helpful posts.

Regards,

AbhisheK

[kthned]

NO IP CLASSLESS

the router makes decision as it is classfull if packet destined to 10.10.10.10/16 and 10.10.0.0 is not in it routing table but any 10.0.0.0/8 is in its routing table. the packet will not go to any next hop even if IT HAS DEFAULT ROUTE.

if IP CLASSLESS

the packet move becoz of default route.

This means "no ip classless" does not make decision on the basis of default route if it has to route subnetted network and the routing table contain the major network.

Hope this will help you.

Please rate this if it help you.

[gpulos]

ip classless is used to allow the router to forward packets to supernets. ie: 192.168.2.0/23

packets of ip networks on 'classful' boundries, ie: 10.1.1.0/24 or 172.31.1.0/16, do not require 'ip classless' forwarding.

(this is not true if the default or supernet route is learned via OSPF or ISIS; ip classless behavior is used)

[senglobe0907]

Hi Matt,

It will helpful when you use default route. That is when the router does't know the path for it uses default route. But if it knows the major class network but not all the subnet, it will not forward the packet to default route and it drops the packet. To avoide this problem we use ip classless, that is ip lookup in the routing table is classless.

Thanks.

C.Senthil

[Richard Burts]

Matt

I believe that Senthil has given a good answer with some detail focusing on default route. I will suggest a more general answer: unless you have a particular situation that needs classful behavior, it is best to use ip classless.

HTH

Rick

[vladrac-ccna]

I aggee with Rick...

and in fact the question should be :

When not to use ip classless?

This is a default command now (since ios 11.3), might be a reason for this, right?

Vlad