strange wireless problem that I don't understand

Hello,
My interfaces file looks like this

auto
lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 192.168.2.4
netmask 255.255.255.0
gateway 192.168.2.1

auto wlan0
iface wlan0 inet dhcp

1 I connect the net cable and boot the computer,
I have network connection, I can ping 192.168.2.1 and I can connect to
the internet.

2 I disconnect the net cable and sudo /etc/init.d/networking restart
the system tells me that it receives a dhcp offer and got ip
192.168.2.5 for wlan0(the wireless card). After this, I try to ping
192.168.2.1(the router), I got unreachable
message. and it shows that my ip is still 192.168.2.4(static ip of
eth0).

3 I sudo ifdown eth0
and everything is ok, I can ping 192.168.2.1 and I can connect to the
internect.


What confuses me is that why do I have to ifdown eth0 to change the
computer ip to the wireless ip?

Thanks.
Zhengquan


.

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