Two elementary questions on LANs

[Timothy Murphy]

1) I was trying to change a LAN on my home-system
from 192.168.1.0 to 192.168.2.0
but I found it surprisingly difficult for example
to change my laptop from 192.168.1.7 to 192.168.2.7 .
I made the change in /etc/sysconfig/network-scripts/ifcfg-eth1
and deleted all the files in /var/lib/dhclient with the old address.
But when I re-booted the old address came up,
as given by ifconfig and route (or ip route).
I was running NetworkManager, but I didn't see
the IP address specified in /etc/NetworkManager .
I may say the laptop was WiFi connected to a LinkSys router running dhcp,
but the Local IP Address in this had been changed to 192.168.2.1 .

Who decides the IP address of a device?

2) I also had a problem with the default gateway,
but this is more a matter of understanding an elementary networking issue.
I guess what I need is a short primer on networking for simpletons.
Is there anything along those lines online?

Suppose machine A has default gateway machine B,
and suppose machine B has default gateway machine C.
Now suppose that on machine A I ping, or ssh to, an address not on the LAN.
Shouldn't this go to machine B, and then automatically get forwarded to C?

Concretely, 
my laptop 192.168.2.7 has default gateway 192.168.2.2 (a desktop)
and the desktop has default gateway 192.168.1.254 (my ADSL modem/router).
It seems to me that if I now ping www.google.co.uk (for example)
this should go to the internet and be responded to by google.
But it doesn't seem to be.

Incidentally, someone (I guess NM) keeps emptying my /etc/resolv.conf .
However, I fill it up each time.

Incidentally, I notice that my laptop, running Fedora-15,
seems to behave slightly differently to my desktop, running CentOS-5.6 ,
Changes to the routing table on the latter, eg changing the default gateway,
do not seem to come into force until I re-boot.

Any and all elucidation gratefully received and studied.



-- 
Timothy Murphy  
e-mail: gayleard /at/ eircom.net
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College, Dublin 2, Ireland