[Hampshire] LinkSys WRT54GL Router

I'm trying to add the above to my existing wired network.

The setup routine provides fields for two static IP addresses: one provided
by the ISP and one for the device.

For the first I've entered the IP address and subnet mask provided by my ISP
(Zen)

For the second I've selected the next IP address in sequence, and entered
the same subnet mask that everything else on the LAN uses.

When I try to save these settings, I get the message "Illegal subnet mask!"
Reverting to the router's default address and mask, (
192.168.1.1/255.255.255.0) I get the same message, so I deduce that it's the
one provided by Zen that it's not happy with.

Can anyone suggest what's wrong?

Chris.