Hoping someone can shed some light on this weird problem.

I have a wireless router ( http://www.dynamode.net/upload/br6004wg1.pdf
). It is connected to my cable modem, and has been configured to
"spoof" the network card the modem was originally configured to. It
has been set to act as a DHCP server, and to assign IP addresses
192.168.1.2 - 192.168.1.100

Machine A is wired into it, and I have told the router to always
assign it's MAC an IP address of 192.168.1.100 This machine can access
the internet no problem. Both in windows and ubuntu, the network
interface has been set to "automactically obtain IP address".

Machine B is wireless connected. It is being assigned an IP address of
192.168.1.99 (which I find suspicous. What happened to 192.168.1.2 ?)
It too can happily access the internet.

So what can't machine A ping machine B, or machine B ping machine A ?

On both machines, after an unsuccessful ping, "ARP - A" shows the MAC
address of the other machine.

I tried running "Wireshark" under ubuntu to see what was happening,
but not being a network guru didn't really understand it.

The most annoying thing is that I've previously had 3 machines happily
connecting to each other, both wired and wireless. So something has
changed.

I'm convinced something is cached in my router (which is why it
insists on passing out .99 as the first assigned IP address). But I've
tried restoring the factory defaults, and it's still not working.

Can anyone suggest how to fix this please ?