I thought I had ip classless nailed - but maybe not

I set up 2 routers, A & B, connected by a WAN (192.168.2.0/24). The LAN attached to A is 192.168.1.0/24 and that attached to B is 172.16.16.0/24. I used RIPv1 and configured the following on B:

router rip
network 192.168.2.0
network 172.16.0.0

In the IOS, ip classless is the default and everything worked in terms of ping between hosts attached to the LANs attached to A & B.

I made the following changes on router B:

no ip classless
router rip
no network 172.16.0.0
network 172.16.16.0

I cleared both route tables and tried the ping between the hosts. I expected it to fail in both directions but it didn't I looked at both route tables and the networks were there, whether I had configured <ip classless> or <no ip classless>. I was confident that it would fail but obviously my understanding is flawed. Can anyone offer any advice? I'd like a configuration that works with <ip classless> but fails with <no ip classless> because that's the way that I'll learn what's happening. I can't post the running-config files for a couple of weeks because we've finished college until after Easter.

NB, this isn't a question set by our tutor, it's something that I'm doing "under my own steam"!

Thanks for your time.

RIP V1 does not support VLSM. When RIP V1 sees 172.16.16.0/24 it only looks at the first octet and thinks it's a /16 mask. If you turn V2 on it should work if everything else is configured correct since V2 includes subnet mask in the updates.

Perfect - many thanks.

So, just to clarify, if I change from RIPv1 to RIPv2 and use the second configuration above (network 172.16.16.0 rather than network 172.16.0.0), it will work with <ip classless> but fail with <no ip classless>?

Pity I'll have to wait until after Easter to see this in action!