Isolating Networks

I use 2 network connections, one UMTS for the internet and the built in
ethernet for work, now the network security people have mentioned that they
see eroneous packets from my PB on the ethernet network. What I want to do is
configure so that my work network which is 10.1.99.xxx is isolated from the
Internet, so have a default route as the UMTS but a specific route to
10.1.99.xxx through my ethernet.

My Ethernet connection is configured as 10.1.99.21

I have tried this on my PB, but it fails with

route -n add -net 10.1.99.0 255.255.255.0 10.1.99.21

the error I get back is
"route: invalid mask: 10.1.99.21"

I just want it so ALL traffic other than 10.1.99.xxx will go
through the UMTS. No DNS requests or anyhting like that can
be seen on the 10.1.99.xxx network

Thanks in advance

Regards

Jed