Welcome to the Routers section. Here we will analyse routers quite some depth; what
they do and how they work. I point out to you that you should have some knowlege on
the OSI model and understand how data is sent across the network medium. If you find
the information a bit too confusing or don't quite understand it, I would suggest you go
back to the networking section and do some reading on the OSI model and Protocols.
You will find information on Cisco routers at the end of this page.
What are they and what do they do ?
Routers are very common today in every network area, this is mainly because every
network these days connect to some other network, whether it's the Internet or some other
remote site. Routers get their name from what they do.... which is route data from one
network to another.

For example, if you had a company which had an office in Sydney and another one in
Melbourne, then to connect the two you would use a leased line to which you would
connect a router at each end. Any traffic which needs to travel from one site to another
will be routed via the routers, while all the other unecessary traffic is filtered (blocked),
thus saving you valuable bandwidth and money.
There are two type of routers: 1) Hardware routers 2) Software routers.
So what's the difference ?
When people talk about routers, they usually don't use the terms "hardware" or
"software" router but we are, for the purpose of distinguishing between the two.
Hardware routers are small boxes which run special software created by their vendors
to give them the routing capability and the only thing they do is simply route data from
one network to another. Most companies prefer hardware routers because they are faster
and more reliable, even though their cost is considerably more when compared with a
software router.
So what does a hardware router look like? Check the picture below, it displays a Cisco
1600 and 2500 series router along with a Netgear RT338 router. They look like a small
box and run special software as we said.

Software routers do the same job with the above hardware routers (route data), but they
don't come in small flashy boxes. A software router could be an NT server, NetWare
server or Linux server. All network servers have built-in routing capabilities.
Most people use them for Internet gateways and firewalls but there is one big difference
between the hardware and software routers. You cannot (in most cases) simply replace
the hardware router with a software router.Why? Simply because the hardware router has
the necessary hardware built-in to allow it to connect to the special WAN link (frame
relay, ISDN, ATM etc), where your software router (e.g a NT server) would have a few
network cards one of which connects to the LAN and the other goes to the WAN via the
hardware router.
I have seen a few cards in the market which allow you to connect an ISDN line directly
into them. With these special cards, which retail from $5000 to $15000 depending on
their capacity, you don't need the hardware router. But as you can understand, it's a much
cheaper solution to buy a hardware router. Plus, the hardware routers are far more
advanced and faster than the software routers since they don't have to worry about
anything else but routing data, and the special electronic components they have in them
are developed with this in mind.
The Flash image below shows us what a router does when it receives packets from the
LAN or the Internet. Depending on the source and destination, it will pass them to the
other network or send them to the Internet. The router is splitting the below network into
2. Each network has a hub to which all computers on that network connect to. Futher
more, the router has one interface connected to each network and one connected to the
Internet, this allows it to pass the packets to the right destination:

The picture below illustrates a router's place in the Local Area Network (LAN):


In the example shown, the workstations see the router as their "gateway". This means that
any machine on this LAN that wants to send a packet (data) to the Internet or anywhere
outside its Local Area Network (LAN) will send the packet via the gateway. The router
(gateway) will know where it needs to send it from there on so it can arrive at its
destination.
This explains the reason you need to add an Internet Protocol (IP) number for a gateway,
when you have a LAN at home or in the office, in your TCP/IP network properties on
your windows workstation.
The above figure shows only one example of how routers connect so the LAN gets
Internet access. Let's have a look how 2 offices would use routers to connect them.

The routers in the above picture connect using a particular WAN protocol, e.g ISDN.
In reality, there would be a cable (provided by your service provider) which connects to
the "WAN" interface of the router and from there the signal goes straight to your service
provider's network and eventually ends up at the other router's WAN interface.
Depending on the type of router you get, it will support one of the most commonly used
WAN protocols: ISDN, Frame Relay, ATM, HDLC, PPP. These protocols are discused
in the protocols section.
It's important to note down and remember a few of the main features of a router:
• Routers are Layer 3 devices
• Routers will not propagate broadcasts, unless they are programmed to
• Most serious routers have their own operating system
• Routers use special protocols between them to exchange information about each
other (not data)