I am running Debian testing. When I reboot and plug in my network cable, I
am able to ping external servers and machines on my LAN. Basically, it all
just works with the network cable plugged in.

When I reboot and turn on the wifi (ie there is a switch on my laptop to
turn on/of wifi) and have the network cable disconnected, I can ping
external sites but not internal machines. I get the message Destination
Host Unreachable. If  I then plug in the network cable (and turn off wfi) I
am able to ping both internal and external servers. If I now disconnect the
cable and turn on wifi again, I can ping both external and internal
servers.

/etc/network/interfaces
auto eth0
iface eth0 inet static
        address 192.168.25.150
        netmask 255.255.255.0
gateway 192.168.25.1
dns-search ph.cox.net
dns-nameservers 68.105.28.12 68.105.29.12 68.105.28.11
        gateway 192.168.25.1

allow-hotplug wlan0
iface wlan0 inet dhcp

I looked at resolv.conf and route for the two scenarios -

1. Reboot into cable connected network -
root@orca:/home/mark# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use
Iface
default         gateway         0.0.0.0         UG    0      0        0 eth0
link-local      *               255.255.0.0     U     1000   0        0 eth0
192.168.25.0    *               255.255.255.0   U     0      0        0 eth0

root@orca:/home/mark# cat /etc/resolv.conf
# Generated by NetworkManager
search ph.cox.net
nameserver 68.105.28.12
nameserver 68.105.29.12
nameserver 68.105.28.11

2. Reboot into wifi
root@orca:/home/mark# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use
Iface
default         gateway         0.0.0.0         UG    0      0        0
wlan0
link-local      *               255.255.0.0     U     1000   0        0
wlan0
192.168.25.0    *               255.255.255.0   U     0      0        0 eth0
192.168.25.0    *               255.255.255.0   U     0      0        0
wlan0

root@orca:/home/mark# cat /etc/resolv.conf
nameserver 192.168.25.1

I could change interfaces to have a static IP for wifi, and it would work
when I reboot. However, for that occasional visit to Starbucks my wifi
would not work.

What do I need to do to make my wifi work both at home and on the road?

Thanks,

Mark