Cisco routers by default are still classfull, even though the internet has
long since gone classless.
For a router to effective understand CIDR routes that don't fall on
classfull boundrys it is necessary to turn off the default by executing the
command ip classless
If for some reason you live in a time warp, and your network is fully
classfull, and you just got a used router from someone who was using it on
a classless network, and you wanted to convert it back to being classfull
you would execute the command
no ip classless.
Why Classless?
Lets say you have a large network, and you happen to own a class A network.
Lets say the 5.0.0.0 network. Lets say that one of the interfaces on your
router connects to another router which connects to your larger network.
Let say that on your end the interface address is 5.0.0.1 /30 and on the
other end the address is 5.0.0.2 /30. Lets say you have a default route
pointing out the serial interface that has the 5.0.0.1 interface.
If you then tried to reach something else in the 5.0.0.0 network, say
5.1.2.3, the packet would go to your router. (remember our router is
configured for classfull, the default). Then your router would say to itself
"hey I have an interface in the 5 network, that means that all of 5.0.0.0
/8 must be connected to me, but I don't see the specific network I'm trying
to reach (5.1.2.3) sop I guess it doesn't exist so I'll throw the packet
away"
That's what happens if your router is set to classful
SO to recap
Classfull cisco default BAD
Classless need ip classless command GOOD
hope this helps
Tom
At 12:31 PM 06/04/2001 -0400, [EMAIL PROTECTED] wrote:
>In what situation would you use the command "no ip classless"?
>
>Cisco's site says
>ip classless --- This command allows the software to forward packets that
are
>destined for unrecognized subnets of directly connected networks. The
packets
>are forwarded to the best supernet route.
>
>no ip classless --- When this feature is disabled, the software discards the
>packets when a router receives packets for a subnet that numerically falls
>within its subnetwork addressing scheme, if there is no such subnet number
in
>the routing table and there is no network default route.
>
>When would you use this in the real world?
Tom Pruneau
Trainer Network Operations
GENUITY
3 Van de Graff Drive Burlington Ma. 01803
24 Hr. Network Operations Center 800-436-8489
If you need to get a hold of me my hours are 8AM-4PM ET Mon-Fri
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