> When I have installed ethereal, I thought that it will show me all
> packets send to or from my machine.
It will - but it may also show you packets that weren't explicitly sent
to your machine. Ethereal will show you all packets that your machine
receives on the interface on which you're capturing packets.
> But when I do a capture on my network (my IP is 201.0.0.115, network
> is 201.0.0.255, we have a proxy with external IP 212.208.229.250 and
> internal IP 201.0.0.2) I can see lines like this one :
>
> NO TIME SOURCE DESTINATION PROTOCOL INFO
> 59 17.007931 212.208.229.39 212.208.229.255 BROWSER Host Announcement blablablablabla
>
> What does it mean ?
It means that your machine sees broadcast packets.
The packet in question is a broadcast packet - note that the destination
IP address ends in .255, which, given that I infer that your netmask is
255.255.255.0, means it's a broadcast IP address - and therefore *every*
machine on that LAN segment will see it, as it's probably sent to the
broadcast Ethernet address (ff:ff:ff:ff:ff:ff).