Wow, what a puzzle. So far it all looks right, but clearly you should be able to ping any system by IP from any other system. The fact that you can get to the Internet from the wireless notebook makes me dismiss any of the usual Linux/Knoppix USB NIC issues, as that is clearly working. The same holds true for the many networking problems often seen after installing to HD.

You didn't mention the OS on C1 and C2, nor talk about firewalls. Are both C1 and C2 running XP? Did you install the recent SP2 so-called upgrade? If so, did you have this problem before installing SP2, or have you only tried setting up the wireless connection since accepting SP2? SP2, in addition to having some "compatability issues", is said to turn on the XP firewall by default (it was there but disabled by default previosly). If all of this is the case you might want to try disabling the XP firewall (it's not well regarded anyway) and seeing if the problem persists. In any case, I would (and do) use a different firewall than the MS firewall, there are several better free ones out there. By the way, the firewall I use would need a rule to let the incoming ping in. So if my computer was C1 you could not ping me unless I allowed it (I do allow it for "trusted" computers in the 192.168.xxx.xxx range), but the MS firewall very well might not. However I don't see why this would prevent the ping that originates at c1 from working, particularly if c1 can ping c2 (but maybe Linux and XP are using different types of ping packets and MS is only letting their own through).

You mentioned the switch. I'm guessing it's only being used because there is a long run to the room where c1 and c2 live, as your SMC router should have a 4 port switch built in. I doubt there is any problem with the switch, but you might want to try bypassing it by plugging c1 and.or c2 into the SMC just to be sure.

I have not used your particular SMC router. The SMC router I have used is somewhat limited on it's ability to filter. For example, there is MAC filtering but it applies to everything, not just the wireless section (makes setting up the MAC table a lot more of a pain, and something you have to fool with every time you change anything on the network. There is no comment field that lets you track which MAC is for which computer either!) You're not trying to use MAC filtering, are you? I don't see how I could create the problem you are seeing by MAC filtering, but it seems worth asking. Same for any other filtering you might be setting up in your router. You have no MAC filtering, access control rules or scheduling rules that we should know about, do you?

By the way, you're sure of the IP addresses and have checked them when you did this. right? I expect you know enough to recheck this, but I ask only because the SMC router I have used has real problems in fixing an IP to a computer; if the system is restarted different internal IP addresses can be given out, which makes for real problems in the routers table for forwarding services. If you just note what IP address a computer was assigned, you can't be 100% certain that it will get that address by DHCP again. Since you seem to have the IP addresses scattered across the range of available addreses, rather than given out in a logical order, I'm guessing your SMC router works much like the one I used. For me this is a real downside to the SMC routers, as I have been unable to give out IP adresses in a logical and sequential order and keep them assigned where I want them, even after playing tricks like limiting the range of valid IP addresses, starting the next computer, then enlarging the IP pool by one address and start the next computer. Once computers are restarted SMC will not put them on the same IP addresses (mucking up the forwarding tables in the router for what SMC calls "virtual servers") even with lease time set to forever.

If your SMC firewall works like the one I have used it has a section for firewall/intrusion detection where there is a radio button for "Discard ping from WAN". I doubt very much that this is the issue (it shouldn't be as it should in no way affect the lan), but it might be worth turning this off. It's worth noting here that with your switch in the network the C1 <-> c2 pings never get to the router, so it might be something other than just a wireless issue (another good reason for doing the tests without your switch if that's physically reasonable).

After the above stuff is looked at, unless someone else can join in with some good suggestions, it seems like the next thing to do might be to sniff the wire. Do you have access to a hub (a switch will not work for this)? I would be interested in seeing what was on the wire between the switch and the router as c1 tried to ping NB and as NB tried to ping C1 (you could run ethereal on C2 and watch this traffic if you could insert a hub between the switch and the SMC, or even between the switch and C1). At the very least this would tell us if the ping isn't getting to the other system or if the reply isn't going back. It would rule out a lot of maybes and point us in the right direction.

OK, one last though. If you are running XP on C1 and C2, then let us get Microsoft out of the equation. Boot your Knoppix CD on C1 or C2 and see if that changes things. Once you boot Knoppix, first confirm that the network is woring on that system by getting to the Internet, then do the ping tests again. Can you now send or receive pings between NB and C1/C2?