NTL cable modem - why will it only talk to one PC?

A customer has an NTL cable modem with an Ethernet port. This is conneced to
a PC. Everything works fine: the PC gets an IP address from the modem.

But if I disconnect the PC and connect any other PC in its place, that new
PC times out while trying to get an IP address. As far as I can see, the
Ethernet cards on the PC the works and any of the PCs which fail seem to be
configured identically.

Could the modem be configured in some way to talk to one MAC only?

He wants me to connect a router (WAN port is Ethernet, not ADSL!) to the
modem so as to share the connection. I'm concerned that the router may
encounter the same problem - that the modem will refuse to talk to the WAN
port of the router.

He already has the router and is sure that it worked in the past before he
removed it because at the time he only had one PC, so maybe I'm worrying
about nothing. Unfortunately I couldn't test it yesterday because the
router's power-supply adaptor wasn't working - a frustrating day of
equipment failures and petty restrictions to mail servers (see my other
thread "Wanadoo SMTP server").

In MsgID<442a4f02$0$3593$(E-Mail Removed)> within
uk.comp.home-networking, 'Martin Underwood' wrote:

>A customer has an NTL cable modem with an Ethernet port. This is conneced to
>a PC. Everything works fine: the PC gets an IP address from the modem.
>
>But if I disconnect the PC and connect any other PC in its place, that new
>PC times out while trying to get an IP address. As far as I can see, the
>Ethernet cards on the PC the works and any of the PCs which fail seem to be
>configured identically.
>
>Could the modem be configured in some way to talk to one MAC only?

The DHCP server in one of my home routers can be, so I imagine the DHCP
server in the modem could be yes. Have a look through the configuration
pages with reference to DHCP ip assignation.

>
>He wants me to connect a router (WAN port is Ethernet, not ADSL!) to the
>modem so as to share the connection. I'm concerned that the router may
>encounter the same problem - that the modem will refuse to talk to the WAN
>port of the router.

I believe some routers can have their inbound mac reconfigured, so if you
can't find the settings on the modem then perhaps the router could be
persuaded to masquerade as the PC network card?

Dave J.
--
'nuther funny flash anim - why smoking's bad news..
http://www.antimult.ru/antimults/ant...mokekills/view

because that's the way they designed them.
on connection, it'll only talk to a single MAC address (in effect the
electronic serial number of your network card). to get around this,
unplug the NTL cable modem for 30 secs (not turn off: unplug) and then
power it back up with the new PC connected.
if you're doing a lot of this, use a router, and it'll see the MAC
address of the router and not care about the PCs.
ric

because that's the way they designed them.
the cable modem will note the MAC address of a network device connected
to it, and only work with that device. to get around this, unplug (not
power off, unplug) the cable modem for 30secs and reconnect with
network cable attached to second PC.
if you're doing a lot of this, get your router working.
ric

ric wrote in message
(E-Mail Removed). com:

> because that's the way they designed them.
> on connection, it'll only talk to a single MAC address (in effect the
> electronic serial number of your network card). to get around this,
> unplug the NTL cable modem for 30 secs (not turn off: unplug) and then
> power it back up with the new PC connected.
> if you're doing a lot of this, use a router, and it'll see the MAC
> address of the router and not care about the PCs.
> ric

When you say "unplug", is it necessary to unplug it from the incoming cable
feed as well as from the mains?

I'll give it a try.

The use of multiple PCs is probably a bit of a red herring: I was intending
to connect the router (if its PSU hadn't been broken) and in the absence of
this I plugged alternative PCs in, expecting them to work as for the
existing one.

But if the modem has this built-in behaviour, then I can see why I had
problems!

As a matter of interest, should the Ethernet cable between a modem and a
router, or between the output of a router/hub and the input of a second hub
that's daisy-chained off the first, be a normal patch cable or a crossover
cable. I've always wondered? Given that a lot of modern equipment is
auto-sensing, it probably doesn't matter, but my gut feeling is that in
these cases it should strictly speaking be a crossover.

In article <442a4f02$0$3593$(E-Mail Removed)>
Martin Underwood <(E-Mail Removed)> wrote:
> A customer has an NTL cable modem with an Ethernet port. This is conneced to
> a PC. Everything works fine: the PC gets an IP address from the modem.
>
> But if I disconnect the PC and connect any other PC in its place, that new
> PC times out while trying to get an IP address. As far as I can see, the
> Ethernet cards on the PC the works and any of the PCs which fail seem to be
> configured identically.
>
> Could the modem be configured in some way to talk to one MAC only?

Yes.
>
> He wants me to connect a router (WAN port is Ethernet, not ADSL!) to the
> modem so as to share the connection. I'm concerned that the router may
> encounter the same problem - that the modem will refuse to talk to the WAN
> port of the router.

Most routers have the ability to clone the MAC id from a NIC, I did mine
that way, but I think you can just reset the modem and it will connect
to the first device it sees.

"Martin Underwood" <(E-Mail Removed)> wrote in message
news:442a4f02$0$3593$(E-Mail Removed)...
> A customer has an NTL cable modem with an Ethernet port. This is conneced
> to a PC. Everything works fine: the PC gets an IP address from the modem.

Actually, no, it doesn't. A cable modem is a bridge; DHCP requests are
broadcast onto the cable network. In NTL's case at least, requests are
relayed by the CMTS to a DHCP server. This point has some relevance; see
later.

> But if I disconnect the PC and connect any other PC in its place, that
> new PC times out while trying to get an IP address.
[snip]
> Could the modem be configured in some way to talk to one MAC only?

When the cable modem boots up, it downloads a configuration file from the
ISP. Amongst other things, this configuration file may contain a setting for
the maximum number of CPE MAC addresses, which is normally one.

The modem maintains a list of MAC addresses it has seen, which is empty when
the modem first boots up. Whenever it sees a packet, it checks if the source
MAC address is in the list. If not, it is added, but only if the size of the
list would not then exceed the CPE addresses limit. If the address was not
in the list and could not be added, the packet is dropped. Otherwise, it is
handled normally (which typically means bridging it to the cable network).

With the typical CPE limit of one, the effect is that the modem "latches
onto" the first MAC address it sees after booting up. All you need to do to
swap computers is cause the modem to reboot and avoid it latching onto the
original computer.

Partly because of "media sense" in Windows, a suitable sequence is:
1. Power down the modem.
2. Unplug the Ethernet cable from the first computer.
3. Power up the modem.
4. Wait for the modem to enter the operational state. (How this is
indicated varies between modems.)
5. Plug the Ethernet cable into the second computer, triggering a DHCP
exchange.

The first two steps can be done in either order. Or even simultaneously, if
you really want to .

It is not necessary (or even desirable) to unplug the coax connection. It is
not typically necessary to wait any appreciable amount of time with the
modem powered off. The reason for step 4 is that the DHCP request triggered
in step 5 may time out before the DHCP server is reachable.

Alex

Alex Fraser wrote in message
(E-Mail Removed):

> "Martin Underwood" <(E-Mail Removed)> wrote in message
> news:442a4f02$0$3593$(E-Mail Removed)...
>> A customer has an NTL cable modem with an Ethernet port. This is
>> conneced to a PC. Everything works fine: the PC gets an IP address
>> from the modem.
>
> Actually, no, it doesn't. A cable modem is a bridge; DHCP requests are
> broadcast onto the cable network. In NTL's case at least, requests are
> relayed by the CMTS to a DHCP server. This point has some relevance;
> see later.
>
>> But if I disconnect the PC and connect any other PC in its place,
>> that new PC times out while trying to get an IP address.
> [snip]
>> Could the modem be configured in some way to talk to one MAC only?
>
> When the cable modem boots up, it downloads a configuration file from
> the ISP. Amongst other things, this configuration file may contain a
> setting for the maximum number of CPE MAC addresses, which is
> normally one.
>
> The modem maintains a list of MAC addresses it has seen, which is
> empty when the modem first boots up. Whenever it sees a packet, it
> checks if the source MAC address is in the list. If not, it is added,
> but only if the size of the list would not then exceed the CPE
> addresses limit. If the address was not in the list and could not be
> added, the packet is dropped. Otherwise, it is handled normally
> (which typically means bridging it to the cable network).
>
> With the typical CPE limit of one, the effect is that the modem
> "latches onto" the first MAC address it sees after booting up. All
> you need to do to swap computers is cause the modem to reboot and
> avoid it latching onto the original computer.
>
> Partly because of "media sense" in Windows, a suitable sequence is:
> 1. Power down the modem.
> 2. Unplug the Ethernet cable from the first computer.
> 3. Power up the modem.
> 4. Wait for the modem to enter the operational state. (How this is
> indicated varies between modems.)
> 5. Plug the Ethernet cable into the second computer, triggering a DHCP
> exchange.
>
> The first two steps can be done in either order. Or even
> simultaneously, if you really want to .
>
> It is not necessary (or even desirable) to unplug the coax
> connection. It is not typically necessary to wait any appreciable
> amount of time with the modem powered off. The reason for step 4 is
> that the DHCP request triggered in step 5 may time out before the
> DHCP server is reachable.

Thanks for an excellent explanation. I think I understand it perfectly now.

Now all I need is for the customer to get a new PSU for his router then I
can test it.