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New Member

24 bit subnet mask under a 30 bit masked interface

Greetings. I have a lab that now requires a routed interface. The setup works like this. A router with a class A address and 24 bit mask (ex. 21.6.225.254/24) has an attached Catalyst 4006 with a L3 card. All lab nodes were configured on the same network, same subnet mask. Static routing. If I change the router mask to 30 bits (21.6.225.254/30) and assign 21.6.225.253/30 to one subinterface on the catalyst - what happens?

That is, I'd like to avoid changing the subnet masks on each lab node. So if they remain at a 24 bit mask, is this a problem? I would need to give the lab vlan an interface, so I'd be adding a 21.6.225.x/24. So the catalyst would have, for example, int Gig3.1 21.6.225.253/30 and int Gig3.2 21.6.225.250/24. Would this work?

4 REPLIES
New Member

Re: 24 bit subnet mask under a 30 bit masked interface

"So the catalyst would have, for example, int Gig3.1 21.6.225.253/30 and int Gig3.2 21.6.225.250/24. Would this work?"

No. The 21.6.225.0/24 subnet, and the 21.6.225.252/30 subnet overlap.

I am also not quite sure what you need to do. Do you want to split the 21.6.225.0/24 network? Perhaps 21.6.225.0/25 and 21.6.225.128/25?

New Member

Re: 24 bit subnet mask under a 30 bit masked interface

Trouble is, the upstream router isn't mine, and neither are the lab nodes. So I have to try and fix their problems in my limited sphere. I know the 24 and 30 bit networks overlap, I just didn't know if I could trick the box into letting me do it..

Cisco Employee

Re: 24 bit subnet mask under a 30 bit masked interface

on the same box, you can not have address/mask overlapping. The device won't accept it and will give you an error message.

New Member

Re: 24 bit subnet mask under a 30 bit masked interface

you would still need to change the default gateway of all the lab nodes, wouldn't you?

Is this what you have:

router(.254)-------/24-------cat4006-------/24-------(.x)labnodes

and labnode's default gateway is the router right?

so the catalyst 4006 is working only at layer 2.

now you want the cat4006 to route, right?

solution 1:

change router ip

router(y.z)-------/30-------(y.z-1)cat4006(.254)-------/24-------(.x)labnodes

if you can't change the router ip then you could:

router(.254)-------/30-------(.253)cat4006(.126)-------/25-------(.x)labnodes

but restricted to 0 < x < 126

is that clear???

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