Date   : Tue, 14 Jul 2009 17:11:58 +0100
From   : darren.grant@... (Darren Grant)
Subject: Is it a bridge? Was: Master Ethernet upgrade

The definition is a bit of a grey area but something that converts one
network type into another is a gateway not a bridge.

A bridge is something that connects two parts of the same network segment
over a different link.

For example if you have two offices with Ethernet networking and a private
link between them that runs on ATM or some other network transport then a
Bridge will encapsulate Ethernet packets using PPP or some other
encapsulation protocol and will have know knowledge of the protocols running
on top, it cold be TCP-IP or IPX it would make no difference to the bridge
as it works entirely on MAC addresses. The bridging is totally transparent
to the higher network level, so TCP does not need to specify the bridge as a
default gateway for example as as far as TCP-IP is concerned all devices are
on the same network and subnet. A bridge therefore typically consists of two
devices one at each end of a link.

A router understands the protocol in use so that it can make decisions on
where the packets must be sent using the addressing scheme of the protocol
and the router usually must be specified. So in the case of TCP-IP the IP
address determines if the packets stay local by comparing against the subnet
mask and non-local packets are then sent to the default gateway.

So the question is how does this device work ?

If it encapsulates Econet packets over TCP-IP and all devices connected
appear on the same network and the Econet network has no knowledge of the
separate nature of the systems then it is a bridge.

If Econet sees systems behind the device on a different segment then it is
acting as a router.