Bridge mode

[Michael H. Phillips]

I have been provided with a Cisco EPC3925 cable modem/router. I have set it
to bridge mode. It is connected to an Airport Express. My Mac Pro is
connected to the Airport Express by ethernet and all is working well.

One thing is puzzling me. If I connect the Mac Pro to the EPC3925 by
ethernet, I still have an internet connection. As the EPC3925 is in bridge
mode and therefore not functioning as a router, shouldn't I fail to get an
internet connection from its ethernet ports?

Or have I misunderstood 'bridge mode'?

--
Michael

mhphillips at gmail dot com

[Graham J]

Presumably you mean when you connect ONLY the Mac Pro to the Cisco?

What happens is that the Cisco in bridge mode connects to the cable
network and passes TCP/IP traffic through itself, unaltered. Any
authentication for the connection is done in the cable router itself.

So the Mac Pro communicates through the router to whatever is on the
cable network, gets an IP address from it, and can see everything on the
internet. It follows that everything on the internet can see your Mac Pro.

The Cisco has several LAN ports. If you were to connect two devices
(say the Mac Pro and the Airport Express) with the Cisco in bridge mode
then both devices would try to get an IP address from the DHCP server on
the cable network; and neither would work properly.

When not in bridge mode, the Cisco functions as a router and applies NAT
(Network Address Translation) and some sort of firewall; so you could
connect several computers. But as you know, you don't want this mode
when your primary router is the Airport Express.

--
Graham J

[Michael H. Phillips]

On Apr 29, 2013 Graham J wrote:

> Presumably you mean when you connect ONLY the Mac Pro to the Cisco?

The Mac Pro and the Airport Express.

> The Cisco has several LAN ports. If you were to connect two devices
> (say the Mac Pro and the Airport Express) with the Cisco in bridge mode
> then both devices would try to get an IP address from the DHCP server on
> the cable network; and neither would work properly.

Both seem to be working properly when connected as above. Whether the Pro is
connected to the Airport Extreme or to the Cisco I get a steady 50Mbps. I
have no way, at the moment, of measuring the wireless connection.

Sorry for the delay in replying, by the way. I was trying to resurrect an old
Powerbook G4 to see what would happen if it too was connected to the Cisco,
but the Powerbook is dead as a doornail.

[Graham J]

It would be instructive to check the IP addresses issued to the AE WAN
port and to the MacPro when both are connected to the Cisco.

Are you sure you have the Cisco in bridge mode?

If the Cisco is not in bridge mode then you would not know the
difference from a computer connected to the AE - at least, not without
running traceroute.


--
Graham J

[Michael H. Phillips]

On Apr 29, 2013 Graham J wrote:

> It would be instructive to check the IP addresses issued to the AE WAN
> port and to the MacPro when both are connected to the Cisco.

AE: 89.101.44.*
Pro: 89.101.44.***

Connected correctly:

AE: 10.0.1.1
Pro: 10.0.1.2

>
> Are you sure you have the Cisco in bridge mode?

In the Administration section of the DOCSIS page it says "Working mode:
bridged only" so it should be. However, I know that UPC (my ISP) have been
playing silly buggers and had disabled bridge mode in the past only to
re-enable it recently because of user complaints. So, to answer your
question, I think so but I'm not sure.

[Graham J]

Michael H. Phillips wrote:
> On Apr 29, 2013 Graham J wrote:
>
>> It would be instructive to check the IP addresses issued to the AE WAN
>> port and to the MacPro when both are connected to the Cisco.
>
> AE: 89.101.44.*
> Pro: 89.101.44.***

So your ISP is happy to issue two separate IP addresses. Are they
consecutive? Your very reasonable obfuscation suggests not. Are you
actually paying for a range of IP addresses? If so, why?

--
Graham J

[Michael H. Phillips]

You're losing me here. The word 'numpty' used to be popular here. In this
case it applies to me.

No, they're not consecutive and no, I'm not paying for a range of IP
addresses. I wouldn't know what to do with them if I had.

To check if the Cisco is actually in bridge mode I suppose I'll have to
borrow a laptop, plug it and the Pro into the Cisco and see if I have an
internet connection to both. Is that a good plan?

[Graham J]

You can look up teh IP address at:
http://whois.domaintools.com/89.101.44.1

I also am confused. Normally an ISP issues you with one IP address. If
you ask, they will issue you with a small consecutive range; and
probably charge more. If you wanted more than one IP address you would
know why ...

So far as I can tell you've already proved you have two simultaneous
internect connections and two public IP addresses: The AE and the Mac Pro.

You could go on connecting computers to the Cisco and see how many IP
addresses you get.

But I don't understand either!

--
Graham J

[Richard Kettlewell]

When router is connected it issues a DHCP request and gets an IP
address. When it is put into bridging mode it releases that address.
Subsequently the Mac makes the DHCP request and happens to get a
different IP address. The ISP is still only providing one address at a
time.

--
http://www.greenend.org.uk/rjk/

[Richard Kettlewell]

Wait, the AE, not the router. I guess they are issuing two addresses
concurrent after all. That’s likely not to be the ISP’s intent but if
everything involved is connected to the same broadcast domain then it’s
not a particularly surprising outcome.

--
http://www.greenend.org.uk/rjk/

[Graham J]

My experience is with ISPs that issue static IP addresses. I certainly
would not expect such an ISP to issue two or more different IP addresses.

However if the addresses are dynamic, as would be the case for a
domestic-grade ISP service, then two DHCP requests might well get two
separate IP addresses.

But I'm sure that this would not be the ISP's intent.

--
Graham J

[Jaimie Vandenbergh]

On Tue, 30 Apr 2013 17:21:51 +0100, Graham J <graham@invalid> wrote:

>Richard Kettlewell wrote:
>> Richard Kettlewell <r...@greenend.org.uk> writes:
>>
>>> Graham J <graham@invalid> writes:
>>>> Michael H. Phillips wrote:
>>>>> On Apr 29, 2013 Graham J wrote:
>>>
>>>>>> It would be instructive to check the IP addresses issued to the AE WAN
>>>>>> port and to the MacPro when both are connected to the Cisco.
>>>>>
>>>>> AE: 89.101.44.*
>>>>> Pro: 89.101.44.***
>>>>
>>>> So your ISP is happy to issue two separate IP addresses. Are they
>>>> consecutive? Your very reasonable obfuscation suggests not. Are you
>>>> actually paying for a range of IP addresses? If so, why?
>>>
>>> When router is connected it issues a DHCP request and gets an IP
>>> address. When it is put into bridging mode it releases that address.
>>> Subsequently the Mac makes the DHCP request and happens to get a
>>> different IP address. The ISP is still only providing one address at a
>>> time.
>>
>> Wait, the AE, not the router. I guess they are issuing two addresses
>> concurrent after all. That’s likely not to be the ISP’s intent but if
>> everything involved is connected to the same broadcast domain then it’s
>> not a particularly surprising outcome.
>>
>My experience is with ISPs that issue static IP addresses. I certainly
>would not expect such an ISP to issue two or more different IP addresses.

I've had a /29 and a couple of /30's in the past. The lowest (or
rarely, highest) available IP in your range would normally get
allocated to your router's external interface by the ISP's DHCP, or
you can set it manually to any of your IPs of course. You will be
responsible for allocating any other IPs.

I've never heard of:

>However if the addresses are dynamic, as would be the case for a
>domestic-grade ISP service, then two DHCP requests might well get two
>separate IP addresses.

the ISP's DHCP giving out other ones in your range, that'd just be
odd. There might be managed services that'll do this, but you'd have
had to order it so you'd know.

>But I'm sure that this would not be the ISP's intent.

Indeed!

Cheers - Jaimie
--
A problem shared is a problem halved, so is your
problem really yours or just half of someone else's?

[Michael H. Phillips]

Excuse the long delay in following up. I didn't manage to borrow a laptop
until today.

To recap, I was worried that the newly supplied Cisco EPC3925 cable
modem/router wasn't functioning properly in bridge mode. Now that I have
connected the Mac Pro and a Windows laptop to it by ethernet, I see that the
Mac has an internet connection and the laptop doesn't. So it *is* in bridge
mode and behaving as it should.

I was also concerned that, although the EPC3925 has wireless connectivity
turned off, the "wireless link" LED was lit. Despite that, the laptop does
not connect wirelessly to it, so all is well.

Thanks all for the help and for dealing so patiently with a networking
numpty.