Subnet Routing &
Communications
Introduction
So we understand all (almost !) about subnetting,
but there are few questions/topics which we haven't
talked about as yet. Experience shows you can never
know everything 100% ! Routing and Communication
between subnets is the main topic here. We have
analysed subnetting and understood how it works, but
haven't yet dealt with the "communication" side of
things. These, along with a few other things I would
like to bring to your attention, are going to be
analysed here ! It's an easy and very interesting
page, so sit back and read through it comfortably.
Communication Between Subnets
So, after reading all the previous pages about
subnetting, let me ask you the following:
Do you think computers that are on the same physical
network but configured to be on separate subnets are
able to communicate ?
The answer is "no". Why ? Simply because you must
keep in mind that we are talking about the
communication between 2 different networks !
Looking
at our example of the Class C network on the
previous page, the fact is that one computer is part
of the network
192.168.0.0
and the other one part of
network 192.168.0.32,
and these are two
different networks. In our
example, from
the moment we modified the
default subnet mask
from
255.255.255.0
to
255.255.255.224,
we split that one network to 8 smaller ones.
Let's try
it !
And because we just have to prove it..... we are
going to try it on my home network ! In the worst
case I'll have to spend all night trying to figure
out what went wrong but it will be worth it ! :)
Without complicating things, here is a diagram of my
home network (I've excluded any computers we are not
going to be using, in order to save space):
Well, that's the network we
have to play with. I have put on the diagram the
results of a few simple pings from each host and as
you can see, they all came out nice:
PASS.
So in order to proceed to
phase 2 of our experiment, I modified the
Subnet mask
of
my workstation
to
192.168.0.35 / 255.255.255.224
, my
Slackware Linux Firewall
to 192.168.0.1 /
255.255.255.224
(internal Network Interface Card) and my
NetWare 6 Server
to
192.168.0.10 / 255.255.255.224
as shown in the diagram below:
As you can see, the results for my workstation were
devastating ... alone and totaly unaware that the
other two servers are still there ! When my
workstation tries to actually ping the Linux
Firewall, it will get no reply, because its Gateway
is a host which belongs to another network,
something that we knew would never work.
So, we have concluded that
there cannot be any sort of communication between
the computers of
Network 1 and
Network 2.
So how can two hosts in two different subnets talk
to each other ? That's what we are going to have a
look at right now !
Building
The Bridge
There is a way to allow the communication between my
workstation and my servers and the Internet.
Actually there are a few ways to achieve this and
I'm going to show you a few ways, even though some
might seem silly or impractical. We are not
interested in the best solution at the moment, we
just want to know the ways in which we can establish
communication between the two subnets.
Considering that subnets are smaller networks, you
would remember that we use routers to achieve
communications between two networks. This example of
my home network is no exception to this rule.
We need a router which will route packets from one
network to the other. Let's have a look at the
different ways we can solve this problem:
Method 1:
Using a Server with 2 Network Cards
Our first option is to use one of the Servers, or a
new Server which has at least 2 network cards
installed. By connecting each network card to one of
our networks and configuring the network cards so
that each one belongs to one subnet/network we can
route packets between them:
The above diagram shows pretty
much everything that's needed. The 2nd network card
has been installed and it's been assigned an IP
Address that falls within our
Network 1
range and therefore can communicate with
my workstation
. On the other hand the
NetWare
server now acts as a Gateway
for Network 1,
so my workstation is reconfigured to use it as its
Gateway. Any packets from
Network 1
to
Network 2
or the Internet
will pass through the NetWare
server
Method 2: Binding 2 IP Addresses to the same network
card
This method is possibly the best and easiest way
around our problem. We use the same network card on
the NetWare server and bind another IP Address to
it.
This second IP Address will
obviously fall within the
Network 1 IP
range so that my workstation can communicate with
the server:
As noted on the diagram, the only problem we might
encounter is the need for the operating system of
the server to support this type of configuration,
but most modern operating systems would comply.
Once configured, the Server takes care of any
routing between the two networks.
Method 3: Installing a router
The third method is to install a router in the
network.
This might seem a bit far fetched but remember that
we are looking at all possible ways to establish
communications between our networks ! If this was a
large network, then a router could possibly be the
ideal solution, but given the size of my network,
well... let's just say it would be a silly idea :)
My workstation in this setup
would forward all packets to its Gateway, which is
the router's interface and is connected to
Network 1
and it will be able to see all
other servers and access the Internet. It's a
similar setup to Method
1 but instead of a
Server we have a dedicated router. Oh, and by the
way, if we would end up using such a configuration
in real life.. the hub which both of the router's
interface's connect to, would be replaced by some
type of WAN link.
That completes our discussion on Subnet routing and
communication.
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