Digital Subscriber Line Re: Telnet to Modem/Bridge

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Subject Author Date
Re: Telnet to Modem/Bridge en.nui 12-07-06
Posted by on December 7, 2006, 3:08 am
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Hi JF,

Thanks for the reply.

The scenario you just described is applicable when the modem is
configured as a router.
But if I understand correctly, the scenario will be different if the
modem is configured as a bridge since the PC will be the one to
initiate the PPPoE connection. After PPPoE has been established, the PC
will be assigned a new IP address that might not belong to the subnet
of the modem.

My question now is:
How can the PC access the telnet interface of the modem? If the PC
sends packet to the modem's IP address, will the modem/bridge be able
to identify the packet and send response to the PC?


Thanks,
Ina


JF Mezei wrote:
> en.nui@hotmail.com wrote:
> > What I can't understand is this:
> > If the modem acts as a bridge, the PC connected to it will initiate the
> > PPPoE connection. After the PPPoE has been established, the PC will
> > have a new IP address. How can it still be able to access the telnet
> > interface of the modem?
>
>
> If you setup your router to be a NAT/PAT device, here is how it works:
>
> Your router has two distinct IP interfaces. One facing the internet, and
> one facing your LAN. Both are independant.
>
> [INTERNET]------- [ 204.206.149.21]-ROUTER-[10.0.0.1]------[LAN]
>
> Your Router negotiates via PPPoE (or DHCP) an IP with the ISP. It uses that
> IP on its WAN facing interface. (WAN= internet)
>
> It has a fixed IP that you've configured on your LAN. That IP doesn't
> change. Your Router can either supply an IP (via DHCP) to your PC or your
> PC can use a fixed IP, in both cases, the PC would have an IP that is
> within the same subnet as the Router's LAN-facing IP (in the above case a
> 10.* IP)
>
>
> Say your PC is 10.0.0.2 . It connects to 72.14.205.103 (google).
>
> Your Router converts the packets sent by your PC, replacing the "from" IP
> (10.0.0.2) with the IP of its WAN-facing interface (204.206.149.21). So
> Google sees a call coming in from 204.206.149.21. So responses from google
> go to 204.206.149.21. When it reaches your router, the router has smarts in
> it that know that the response goes to 10.0.0.2 and your pC then gets the
> packet.
>
>
> If your pC wants to connect to any host on the internet, it sends it to its
> normal internet IP address. If it wants to talk to the router, it connects
> to the LAN side IP address of the router 10.0.0.1. Also, you need to tell
> your PC that the default route (gateway) is 10.0.0.1 (your router's LAN
> side IP). This tells the PC that any packet destined outside the LAN's
> subnet is to be forwarded thorugh the router.


Posted by JF Mezei on December 7, 2006, 3:58 am
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en.nui@hotmail.com wrote:
> But if I understand correctly, the scenario will be different if the
> modem is configured as a bridge since the PC will be the one to
> initiate the PPPoE connection. After PPPoE has been established, the PC
> will be assigned a new IP address that might not belong to the subnet
> of the modem.

Correct.

Not sure if Windows is able to do this. But on a real operating system, you
could configure multiple interfaces on the same ethernet device.

So, you would have a 10.0.0.2 with subnet of 10.0.0.0/8 and then the
interface configured by the PPPoE software with some random IP address.

If you try to talk to 10.0.0.1, the computer should then use the first
interface and send out real TCPIP packets to the modem destined for the
modem's IP address.

If you try to talk to the internet, you will be sending packets out to a
pseudo interface that converts the packets to PPPoE stuff and sends it out
to the modem which then forwards those packets over to the DSLAM/BAS.

If your computer is unable to have multiple interfaces, then you can't
really do this. You might be able to if you insert a second ethernet card
and configure a new IP interface on that one.

try "ifconfig -a" from a command mode in windows. It should list its
internet interfaces.

Posted by on December 7, 2006, 5:06 am
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Hi JF,

Thanks for the explanation. I'll check if Windows can support multiple
interfaces on the same ethernet device.

Thanks,
Ina

JF Mezei wrote:
> en.nui@hotmail.com wrote:
> > But if I understand correctly, the scenario will be different if the
> > modem is configured as a bridge since the PC will be the one to
> > initiate the PPPoE connection. After PPPoE has been established, the PC
> > will be assigned a new IP address that might not belong to the subnet
> > of the modem.
>
> Correct.
>
> Not sure if Windows is able to do this. But on a real operating system, you
> could configure multiple interfaces on the same ethernet device.
>
> So, you would have a 10.0.0.2 with subnet of 10.0.0.0/8 and then the
> interface configured by the PPPoE software with some random IP address.
>
> If you try to talk to 10.0.0.1, the computer should then use the first
> interface and send out real TCPIP packets to the modem destined for the
> modem's IP address.
>
> If you try to talk to the internet, you will be sending packets out to a
> pseudo interface that converts the packets to PPPoE stuff and sends it out
> to the modem which then forwards those packets over to the DSLAM/BAS.
>
> If your computer is unable to have multiple interfaces, then you can't
> really do this. You might be able to if you insert a second ethernet card
> and configure a new IP interface on that one.
>
> try "ifconfig -a" from a command mode in windows. It should list its
> internet interfaces.


Posted by Mike on December 7, 2006, 1:49 pm
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> en.nui@hotmail.com wrote:
>> But if I understand correctly, the scenario will be different if the
>> modem is configured as a bridge since the PC will be the one to
>> initiate the PPPoE connection. After PPPoE has been established, the PC
>> will be assigned a new IP address that might not belong to the subnet
>> of the modem.
>
> Correct.
>
> Not sure if Windows is able to do this. But on a real operating system,
> you could configure multiple interfaces on the same ethernet device.
>
Windows is a real operating system.



Posted by Tegger on December 7, 2006, 4:00 pm
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@read2.cgocable.net:

>
>> en.nui@hotmail.com wrote:
>>> But if I understand correctly, the scenario will be different if the
>>> modem is configured as a bridge since the PC will be the one to
>>> initiate the PPPoE connection. After PPPoE has been established, the PC
>>> will be assigned a new IP address that might not belong to the subnet
>>> of the modem.
>>
>> Correct.
>>
>> Not sure if Windows is able to do this. But on a real operating system,
>> you could configure multiple interfaces on the same ethernet device.
>>
>
>
> Windows is a real operating system.
>
>


Do you know if Windows will accept more than one interface per ethernet
device?

--
Tegger


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