ip masquerading/subnets

bruce bedouglas at earthlink.net
Mon Nov 26 23:43:26 UTC 2007

Hi mike..

here's my system:


    dsl box
     dlink router (
       switch  ->>>>>>>>>>>>>>+
         ^                    V
         ^                    V
         ^                    V
        box1           wireless access point (
          (eth0)               |
                                            box3 (future) (

 router - (gateway)
 wireless access point -
 box1 (eth0)

    ath0 -
    eth0 -

    eth0 -

box1 connects to the switch, which is connected to the access point via the
lan (eth) connection.

so, everything is on the 192.168.1 subnet, except the eth0 nic of box2, and
the box3 that'll be added to interface with box2/eth0...

box2 has ip_forwarding set.

so my basic questions:
 -what do i need to do on box2 to allow a user to
  be able to do a "ping"
 -what do i need to do on box2 to allow a user on
  box1 to be able to "ping" and have a
  successful reply.

bear with me !!


-----Original Message-----
From: fedora-list-bounces at redhat.com
[mailto:fedora-list-bounces at redhat.com]On Behalf Of Mike Wright
Sent: Monday, November 26, 2007 2:22 PM
To: For users of Fedora
Subject: Re: ip masquerading/subnets

bruce wrote:
> Hi Mike..
> and adding this route to the box2, will allow me to ping box2/eth0 from
> box1?
> or are you saying that i'd have to add the route to box1. if i have to add
> route to box1, then i'd have to add a route to every box in the system...
> which isn't what i'd want to do.
> again.. thanks for the basic understanding..

I'm not so sure I have that basic understanding ;)

Blaaargh!  The command I gave you was a linux command that can't be
executed on the windows box, ergo: bad advice.

You mentioned two boxes with one to come, then you mentioned that there
is also another router (to the internet? connected to box1?).  If so,
then is this what you have?

internet                     (router's default gateway)
router wan   xxx.xxx.xxx.xxx (your outside IP)
router lan   yyy.yyy.yyy.??? (box1's default gateway)
box1's eth?  yyy.yyy.yyy.??? (box1's other IP)
box1's eth0     (box2's default gateway)
box2's ath0     (box1's route to box2 and beyond)
box2's eth0     (box3's default gateway)
box3 eth0    192.168.2.???/24

Note that each box further from the internet has a default route that
points to the network device of the next closer box.

Now, in order to have box1 be able to access box2 and further box1 will
have to have a route added to its routing table that points to box2.
This is where I gave the wrong command.  You'd have to use whatever is
the appropriate windows command to add an additional route (ipconfig???).

The only other consideration (ignoring iptables) is that box2 must have
forwarding enabled or it will not pass traffic between its two interfaces:

     /etc/sysctl.conf -> net.ipv4.ip_forward = 1

Hope that is a little clearer.  :m)

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