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<H4>"The Linux Gazette...<I>making Linux just a little more fun!</I>"</H4>
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<H4>By James T. Dennis,
<a href="mailto:linux-questions-only@ssc.com">linux-questions-only@ssc.com</a><BR>
LinuxCare,
<A HREF="http://www.linuxcare.com/">http://www.linuxcare.com/</A>
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<H3 align="left"><img src="../../gx/dennis/qbubble.gif"
height="50" width="60" alt="(?) " border="0"
>Subnetting</H3>
<p><strong>From Berlin Tokugawa on Sun, 30 Jan 2000
</strong></p>
<!-- ::
Subnetting
~~~~~~~~~~
:: -->
<P><STRONG>
I have a LAN in our office connected to the Internet using a
subnet (240) for 16 IP numbers. Our office actually use only 4
computers to assign IP numbers from the said IP pool. One of
those computer in our office LAN is a Linux box configured as a
PPP server so I could dial-in from home (and get a static IP
number from the office IP pool assigned to the ppp interface) and
connect my small home LAN using a subnet of 248. I used eight(8)
IP numbers at home (using those unused, contiguous IP numbers from
the office IP pool) but I'm having problems.
</STRONG></P>
<P><STRONG>
The other computers at home (not the dial-out computer) can not
ping the dial-out computer when I'm PPP-connected to the office
LAN. Unconnected via PPP to the office LAN, all my home computers
can ping each other. All the other computers at home have the
home dial-out computer as their gateway to the outside world,
while the office computers have their gateway set to our office
router connected via leased-line to an ISP. I am wondering if the
cause of the problem is the re-use of the IP numbers at home that
are already subnetted in the office --regardless of their
assignment or non-assignment to working computers.
</STRONG></P>
<P><STRONG>
BTW, I do not want to use private IP addresses, IP aliasing,
firewalling, etc., as there is a need for my home computers at
home to be referenced by the outside world via valid IP numbers
directly. Any thoughts on this problem is greatly
appreciated. Thanks.
</STRONG></P>
<P><STRONG>
Berlin Tokugawa
</STRONG></P>
<BLOCKQUOTE><IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
HEIGHT="28" WIDTH="50" BORDER="0"
>
You should really draw an ASCII diagram of
your network and include the IP addresses (even
a fake set of <EM>consistent</EM> IP addresses) when you
ask a question like this.
</BLOCKQUOTE>
<blockquote><pre> +-----------------------+
| The Internet |
+-------+---------------+
| (A)
+---+----------+
| Your Office +-----------+
+--------------+ (B) | +-----------+
+--+ Home |
(C) +-----------+
</pre></blockquote>
<BLOCKQUOTE>
The routers here are:
</BLOCKQUOTE>
<BLOCKQUOTE>
A your office's end of your link from your ISP,
<br>B your offices end of the link between your
office and your home,
<br>C your home's end of the link to your office
</BLOCKQUOTE>
<BLOCKQUOTE>
So there are five routing tables you care about.
</BLOCKQUOTE>
<BLOCKQUOTE>
Let's assume that they've given you 123.45.67.176
through 123.45.67.192 (a.k.a. the 123.45.67.176/240
network).
</BLOCKQUOTE>
<BLOCKQUOTE>
Let's presume that you and your ISP have followed common
conventions and assigned the first usable IP address
in your block to your router (A). That means that
(A) is 123.45.67.177. Therefore it would make sense
for the office to use the lower subnet (from 177 to 182)
Thus that subnet will have a netmask of 255.255.255.248
and a broadcast address of 123.45.67.183 (add 7 to
176) Remember, you only get six usable addresses out
of that mask since one is reserved for the network
(the "zero offset from your base) and the last address
(your base net address with a trailing sequence of
binary 1's).
</BLOCKQUOTE>
<BLOCKQUOTE>
So you pick an IP address for (B): lets call that
123.45.67.182 (the last usable address in your
lower subnet). You'll also need an address for (C)
123.45.67.185 (the first usable address in the
upper subnet). Actually all of these routers
will have multiple interfaces. The PPP (exterior)
interface to your ISP at (A) will usually have one
of the ISP's addresses. You can actually use any
RFC1918 address for your PPP link from (B) to (C)
since only B and C will use those addresses in
their routing tables. Let's call those PPP
endpoints 10.1.1.1 and 10.1.1.2
</BLOCKQUOTE>
<BLOCKQUOTE>
(I'm not sure but I think there's a way around
that in some TCP stacks but this should work).
</BLOCKQUOTE>
<BLOCKQUOTE>
Now the home system have to have a default route that
points to (C). (C) has a default route that points
to (B), and a network route that point to eth0
(the home network). That network route corresponds
to our upper subnet so it looks like:
</BLOCKQUOTE>
<BLOCKQUOTE><BLOCKQUOTE><CODE>
route add -net 123.45.67.84 netmask 255.255.255.248 eth0
</CODE></BLOCKQUOTE></BLOCKQUOTE>
<BLOCKQUOTE>
(A) and (B) also each a route that look similar. In
(B)'s case it looks like:
</BLOCKQUOTE>
<BLOCKQUOTE><BLOCKQUOTE><CODE>
route add -net 123.45.67.84 netmask 255.255.255.248 gw 10.1.1.2
</CODE></BLOCKQUOTE></BLOCKQUOTE>
<BLOCKQUOTE>
... (B) is listing (C) as the gateway to the upper
subnet. (C) lists ppp0 as its default route.
</BLOCKQUOTE>
<BLOCKQUOTE>
Finally there's (A) which lists (B) as its gateway
to the upper subnet and the ISPs address as its
default route.
</BLOCKQUOTE>
<BLOCKQUOTE>
The only tricky part is that all of the machines on
the office subnet should also know about the
subnet route to (B).
</BLOCKQUOTE>
<BLOCKQUOTE>
This is simplified slightly if (B) is actually
not a separate router, but merely an extra interface
on (A).
</BLOCKQUOTE>
<BLOCKQUOTE>
Of course there are many ways to do all of this.
When asking questions about routing --- draw a picture
and then go to each network and router (connecting point)
and ask what the routing tables must look like
from that location!
</BLOCKQUOTE>
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<H5 align="center"><a href="http://www.linuxgazette.com/copying.html"
>Copyright &copy;</a> 2000, James T. Dennis
<BR>Published in <I>The Linux Gazette</I> Issue 51 March 2000</H5>
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