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.
Next - Subnetting
Guidelines
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