DHCP setup problems

[Doug]

Hello:

I switched from Verizon DSL to Comcast Broadband/Cable/Phone service
when I moved out to Acton.  This is what I thought would happen: both
of the modems have a DHCP server in them, so if I plug them in, they
work.  DHCP servers are in the modems, DHCP clients are on the
computers.

That worked for my Linux box.  I did not change a setting, and the box
was able to get on the Internet with no changes.  I tried to connect
both my Linux (Ubuntu) laptop, and an IMac, but neither one worked. 
Both the laptop and the IMac had been using wireless to connect, but
cannot do so now (I am not sure if that messes things up).

I thought the Linux box would need 2 things to be setup.  I set the
/etc/resolv.conf to be exactly like the one on the box that works:
search comcast.net
nameserver 68.87.71.226
nameserver 68.87.73.242

/etc/network/interfaces    has a line
iface eth0 inet dhcp

I made sure that line was "live" by switching the cable between my big
box and the laptop.  If I do sudo ifdown eth0; sudo ifup eth0, the
computer tries 6 times, but fails.  There are two clues in the output.
 First it complains that there is alread a pid file
/var/run/dhclient.eth0.pid with pid 1.  Second, the failed lines look
like this:

DHCPDISCOVER on eh0 to 255.255.255.255.255 port 67 interval 7

For a moment, I thought the 4 255 was suspicious, but I just checked
on the working machine, and ifup does the same thing, except it gets a
DHCPOFFER, and then it works.

So I am not sure where to trouble shoot next.  Is wireless failing an
issue? Are there other settings to change?

doug