[aclug-L] Re: DSL

[Chris Owen <owenc@xxxxxxxxxx>]

On 25 Sep 2002, Clint Brubakken wrote:

> I admit, I'm kinda soft on networking what are the differences between
> a bridge and a router?

A bridge works at the layer 2 level (data link level).  It doesn't
understand anything about IP addresses (or any other protocal).  It just
works by sending packets from one MAC address to another.  There is no
real configuration usually.  The device just automatically learns of MAC
addresses on the network.

In a bridged model the box on your end is serving as a gateway between two
ethernet networks.  It basically extends the ethernet on the ISP side to
the box on the customer side.  The customer side can't really control what
goes over the box (everything does).  It is up to the ISP side to control
what it "listens to".  A bridge is closer to a switch than to a router.

A router works at the layer 3 level (network level).  It understands IP
addresses and can route data between different networks.  The box on the
customer side controls what goes out to the ISP and how it goes out (the
ISP side has to agree obviously before they talk to each other).

The difference for the purposes of this discussion is that with a bridge
if the ISP is putting one IP address down the pipe the bridge doesn't have
any clue about IP so it just passes those packets through and the IP
address is assigned to something (a computer) on the customer side.  That
means if there is one IP address there is one computer on the other side.

With a router the IP address is actually assigned to the router box.  It
then can route traffic from the customer side.  So in most cases with a
single IP address setup the box is assigned a single public IP address and
then box hands out private addresses on the customer side (using DHCP).
There can be a large number of hosts inside the network that all talk to
the router which then turns around and routes those packets out to the
world.  Since the addresses on the inside are private the box also does
network address translation (NAT) to allow the translation of the private
addresses to the single public address.

This also serves as a basic firewall.  To the outside world there is only
one box on the network.  Incoming requests all stop at the box (unless the
box is specifically configured to redirect them into the network).

Another advantage of having a router like this is that since it can talk
IP (and ATM in the DSL example) it can handle all the authentication.  So
for instance with SBC DSL if you have a bridge you have to install PPPoE
(PPP over ethernet) software on the computer in order to authenticate.
With the routers we distribute all the authentication is in the box so the
comptuers don't require any special software or anything like that.  In
fact a computer out of the box using DHCP can be put on the network and
work without any configuration.

Chris

--
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Chris Owen                ~ Garden City (620) 275-1900 ~  Lottery (noun):
President                 ~ Wichita     (316) 858-3000 ~    A stupidity tax
Hubris Communications Inc ~       www.hubris.net       ~
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