basic but elusive question

I understand what hubs, switches, and routers are, what they do, and the
differences between them. I am not as certian on bridges. I don't have any
direct experience with a bridge. I realize 'bridging' may be one of many
functions a network device may perform. For example, as I understand it, a
Wireless Access Point is a 'transparent bridge'. Which leads me to a
definition like: connects 2 networks segments of dissimilar physical
topologies. Because it enables comunication both ways between the 'air
ethernet (802.11)' and 'wired ethernet (802.3)'

I don't know if that is completely accurate but it is my current
understanding of a 'bridge'. Can someone else correct me if I'm wrong, or
elaborate more? In particular, taking a switch operating at layer 2 for
example, I know when computerA comunicates with computerB that name to IP
resolution and then IP to MAC resolution takes place. Then at the switch,
the MAC is compared to entries in the switch's MAC table and sent out the
appropriate port. Or if no MAC entry is present, it gets broadcast out all
ports. So, in the same terms of 'what something does', what does a bridge
do?

any help is appreciated. Thanks.

http://www.microsoft.com/windowsxp/h...ing_bridge.asp

Good little explanation of a bridge. An example of a bridge that we use to
use, to maybe help you understand is this. We had a token ring network. By
itself the token ring worked flawlessly. Eventually when we got the
internet back in the early 90's we had to create a bridge from the token
ring network to the Ethernet backbone that was getting installed. This way
the machines on the token ring network could access the internet. Another
bridge we had was the Mac bridge. We took Mac's on a phonenet type network.
That required a bridge over to the Ethernet network. So apple talk would
travel the local phone net in that section of the building but we could pass
the packets over to the Ethernet side and supply tcp-ip access. The Mac's
on the phonenet were old SE's that didn't have Ethernet cards or support
Ethernet cards. Don't know if that helps any but when ever I've used a
bridge its mostly to connect dissimilar network media. Token to Ethernet.
Phonenet to Ethernet. Thomas Conrad to Ethernet. But now it can just be
connecting segments of Ethernet, so in a way now a days many routers are
also looked at as bridges because they bridge network segments of dissimilar
IP ranges.

--


Jeremy Kettelhohn


"djc" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I understand what hubs, switches, and routers are, what they do, and the
> differences between them. I am not as certian on bridges. I don't have any
> direct experience with a bridge. I realize 'bridging' may be one of many
> functions a network device may perform. For example, as I understand it, a
> Wireless Access Point is a 'transparent bridge'. Which leads me to a
> definition like: connects 2 networks segments of dissimilar physical
> topologies. Because it enables comunication both ways between the 'air
> ethernet (802.11)' and 'wired ethernet (802.3)'
>
> I don't know if that is completely accurate but it is my current
> understanding of a 'bridge'. Can someone else correct me if I'm wrong, or
> elaborate more? In particular, taking a switch operating at layer 2 for
> example, I know when computerA comunicates with computerB that name to IP
> resolution and then IP to MAC resolution takes place. Then at the switch,
> the MAC is compared to entries in the switch's MAC table and sent out the
> appropriate port. Or if no MAC entry is present, it gets broadcast out all
> ports. So, in the same terms of 'what something does', what does a bridge
> do?
>
> any help is appreciated. Thanks.
>
>

Keeping the explaination simple:

A bridge is the same thing as a switch but with only one port in and one
port out. The is no routing, and an IP# is not even required for the device
(just like a switch). The bridge does not cross subnets, each port (barring
VLANs) would be in the same subnet. In theory, you could take a screwdriver
and rip out all but two ports on a switch and you would have a "bridge".
Well, just kidding about the screwdriver,...don't let you kids try it at
home.

Unlike a switch however, one of the bridges ports can somtimes be a WAN
link. For example, we use a pair of bridges to connect a remote office over
an old "Nailed 56k" line. Since it is all the same subnet there is no
routing so a router is not needed and the bridges do fine.


--

Phillip Windell [MCP, MVP, CCNA]
www.wandtv.com


"djc" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I understand what hubs, switches, and routers are, what they do, and the
> differences between them. I am not as certian on bridges. I don't have any
> direct experience with a bridge. I realize 'bridging' may be one of many
> functions a network device may perform. For example, as I understand it, a
> Wireless Access Point is a 'transparent bridge'. Which leads me to a
> definition like: connects 2 networks segments of dissimilar physical
> topologies. Because it enables comunication both ways between the 'air
> ethernet (802.11)' and 'wired ethernet (802.3)'
>
> I don't know if that is completely accurate but it is my current
> understanding of a 'bridge'. Can someone else correct me if I'm wrong, or
> elaborate more? In particular, taking a switch operating at layer 2 for
> example, I know when computerA comunicates with computerB that name to IP
> resolution and then IP to MAC resolution takes place. Then at the switch,
> the MAC is compared to entries in the switch's MAC table and sent out the
> appropriate port. Or if no MAC entry is present, it gets broadcast out all
> ports. So, in the same terms of 'what something does', what does a bridge
> do?
>
> any help is appreciated. Thanks.
>
>

Thanks for the good explanation.

"Phillip Windell" <@.> wrote in message
news:%(E-Mail Removed)...
> Keeping the explaination simple:
>
> A bridge is the same thing as a switch but with only one port in and one
> port out. The is no routing, and an IP# is not even required for the
device
> (just like a switch). The bridge does not cross subnets, each port
(barring
> VLANs) would be in the same subnet. In theory, you could take a
screwdriver
> and rip out all but two ports on a switch and you would have a "bridge".
> Well, just kidding about the screwdriver,...don't let you kids try it at
> home.
>
> Unlike a switch however, one of the bridges ports can somtimes be a WAN
> link. For example, we use a pair of bridges to connect a remote office
over
> an old "Nailed 56k" line. Since it is all the same subnet there is no
> routing so a router is not needed and the bridges do fine.
>
>
> --
>
> Phillip Windell [MCP, MVP, CCNA]
> www.wandtv.com
>
>
> "djc" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
> > I understand what hubs, switches, and routers are, what they do, and the
> > differences between them. I am not as certian on bridges. I don't have
any
> > direct experience with a bridge. I realize 'bridging' may be one of many
> > functions a network device may perform. For example, as I understand it,
a
> > Wireless Access Point is a 'transparent bridge'. Which leads me to a
> > definition like: connects 2 networks segments of dissimilar physical
> > topologies. Because it enables comunication both ways between the 'air
> > ethernet (802.11)' and 'wired ethernet (802.3)'
> >
> > I don't know if that is completely accurate but it is my current
> > understanding of a 'bridge'. Can someone else correct me if I'm wrong,
or
> > elaborate more? In particular, taking a switch operating at layer 2 for
> > example, I know when computerA comunicates with computerB that name to
IP
> > resolution and then IP to MAC resolution takes place. Then at the
switch,
> > the MAC is compared to entries in the switch's MAC table and sent out
the
> > appropriate port. Or if no MAC entry is present, it gets broadcast out
all
> > ports. So, in the same terms of 'what something does', what does a
bridge
> > do?
> >
> > any help is appreciated. Thanks.
> >
> >
>
>

Thanks for the good explanation.

"Jeremy" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
>
http://www.microsoft.com/windowsxp/h...ing_bridge.asp
>
> Good little explanation of a bridge. An example of a bridge that we use
to
> use, to maybe help you understand is this. We had a token ring network.
By
> itself the token ring worked flawlessly. Eventually when we got the
> internet back in the early 90's we had to create a bridge from the token
> ring network to the Ethernet backbone that was getting installed. This
way
> the machines on the token ring network could access the internet.
Another
> bridge we had was the Mac bridge. We took Mac's on a phonenet type
network.
> That required a bridge over to the Ethernet network. So apple talk would
> travel the local phone net in that section of the building but we could
pass
> the packets over to the Ethernet side and supply tcp-ip access. The Mac's
> on the phonenet were old SE's that didn't have Ethernet cards or support
> Ethernet cards. Don't know if that helps any but when ever I've used a
> bridge its mostly to connect dissimilar network media. Token to
Ethernet.
> Phonenet to Ethernet. Thomas Conrad to Ethernet. But now it can just be
> connecting segments of Ethernet, so in a way now a days many routers are
> also looked at as bridges because they bridge network segments of
dissimilar
> IP ranges.
>
> --
>
>
> Jeremy Kettelhohn
>
>
> "djc" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
> > I understand what hubs, switches, and routers are, what they do, and the
> > differences between them. I am not as certian on bridges. I don't have
any
> > direct experience with a bridge. I realize 'bridging' may be one of many
> > functions a network device may perform. For example, as I understand it,
a
> > Wireless Access Point is a 'transparent bridge'. Which leads me to a
> > definition like: connects 2 networks segments of dissimilar physical
> > topologies. Because it enables comunication both ways between the 'air
> > ethernet (802.11)' and 'wired ethernet (802.3)'
> >
> > I don't know if that is completely accurate but it is my current
> > understanding of a 'bridge'. Can someone else correct me if I'm wrong,
or
> > elaborate more? In particular, taking a switch operating at layer 2 for
> > example, I know when computerA comunicates with computerB that name to
IP
> > resolution and then IP to MAC resolution takes place. Then at the
switch,
> > the MAC is compared to entries in the switch's MAC table and sent out
the
> > appropriate port. Or if no MAC entry is present, it gets broadcast out
all
> > ports. So, in the same terms of 'what something does', what does a
bridge
> > do?
> >
> > any help is appreciated. Thanks.
> >
> >
>
>

"Jeremy" <(E-Mail Removed)> wrote in message
> Ethernet cards. Don't know if that helps any but when ever I've used a
> bridge its mostly to connect dissimilar network media. Token to
Ethernet.

This describes a media conversion device, in some cases called a
"Gateway",not a bridge. The "gateway" in this context is not related to
Layer3 routing gateways although both can be built into the same device. The
idea is that it "gateways" between disimilar LAN Protocols like Token vs
Ethernet.

A bridge is used to create Layer 2 "collision domains" although I guess a
Protocol Gateway could be combined into the same physical device.

--

Phillip Windell [MCP, MVP, CCNA]
www.wandtv.com