The Gateway to the Internet is 10.151.240.1!
No internet connection...
root@bt:~# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.151.248.0    0.0.0.0         255.255.248.0   U     0      0        0 eth0
root@bt:~# route add default gw 10.151.240.1
SIOCADDRT: No such process

But when I do:

route add -net 10.151.240.1 netmask 255.255.255.255 dev eth0
route add default gw 10.151.240.1
I get:

route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         10.151.240.1    0.0.0.0         UG    0      0        0 eth0
10.151.240.1    0.0.0.0         255.255.255.255 UH    0      0        0 eth0
10.151.248.0    0.0.0.0         255.255.248.0   U     0      0        0 eth0

... Everything seems alright!
Why can't I do
route add default gw 10.151.240.1
on the first step?