[LUGSB] Home networking problem

[James Crasta]

> Any suggestions?
>
> (Cable modem was supplied by the ISP and there are no user configurable
> options in its web interface. It has only information)

If the ISP in question is Cablevision, most of their modems give the
first DHCP client a routable IP in the 24.x.x.x. range or 68.x.x.x
range, and then any subsequent clients connected to the cable modem
will get an RFC 1918 IP in the 192.168.100.x range.  The problem
therein is exactly as you're seeing, that the first PC cannot talk to
any of the others (but if you had a third and fourth PC, PC's 2, 3, 4
will all be able to talk to each other).

Assuming at least one of the machines is a linux box, there is an easy
solution:  make the linux box get the routable IP, and any other
machines get the nonroutable IP's.   Then go to the linux box, become
root and run something like:
$ ifconfig eth0:1 up 192.168.100.3 netmask 255.255.255.0
This assumes eth0 is your ethernet interface and 192.168.100.3 is in
the correct subnet and is available.

What this will do is create an alias interface on a different subnet
and IP.  Now you can talk to the other machines on your lan, but still
get internet through the routable IP.   This will not persist through
reboots, but most distros have a config file where you can set the
alias interface to come up on boot.   The caveat of this is that if
some other machine grabs the routable IP first, then you'll have to
manually release the IP and make sure the linux machine grabs it.

In the end, it is a quirky solution (due to it possibly being
dependent on the order machines reboot), but it will be free at least.
 I'd just invest the $40 to $50 in a decent router honestly and not
have to deal with the issue.  You also get firewalling and
configurable NAT port forwarding as a bonus.