[plug] network routing -- cannot see outside world

[Denis Brown]

Dear PLUG members,

 From the "must be something obvious" department...

Debian GNU/Linux Woody installed on a laptop with Xircom Realport combo 
(net+modem) card.  Because it's a laptop I decided to set up the networking 
by hand and when I did this a month or so ago, I managed to get it working 
without too much bother.  Today, apart from a failed (human) memory, it 
refuses to play the game, specifically it refuses to see the outside world.

Some numbers:
Host address is a valid class C address, no conflicting hosts out 
there.  Call it X.Y.Z.73
Netmask is 255.255.255.0
Gateway is X.Y.Z.1
Broadcast address is X.Y.Z.255

Procedure:
Boot into Linux, log in as root.
ifconfig eth0 X.Y.Z.73 netmask 255.255.255.0 up
ifconfig eth0 broadcast X.Y.Z.255
ifconfig        at this point all the numbers look fine and I can ping 
within the X.Y.Z network both to other boxes on X.Y.Z and from other X.Y.Z 
boxes to the problem child.   But of course I cannot see addresses outside 
of X.Y.Z

At this point a route -n shows
Dest        GW        Netmask        Flags   Metric   Ref  Use   Iface
X.Y.Z.0   0.0.0.0   255.255.255.0     U          0        0     0       eth0
Fair enough, no way to the outside world, so...

route add default gw X.Y.Z.1 netmask 255.255.255.0 eth0

And now a route -n shows
Dest        GW        Netmask        Flags   Metric   Ref  Use   Iface
0.0.0.0    X.Y.Z.1  255.255.255.0     UG        0       0     0       eth0
X.Y.Z.0   0.0.0.0   255.255.255.0     U          0        0     0       eth0

But I still cannot reach beyond the X.Y.Z subnet.   It must be blindingly 
obvious.   What *am* I missing?

Cheers,
Denis