Modem Mode Network

Do Virgin put you onto a separate network if you activate Modem Mode?

I'm not having any issues but I did notice something odd when I recently had my old 20Mb connection upgraded to 100Mb. Obviously I had the old modem swapped for a Superhub which the installer left in normal mode.

When I checked my public IP it had, not surprisingly, changed. I was expecting it too since a different device was now connecting to the VM network. However, on switching the Superhub into Modem mode I got my old public IP back!

Obviously I don't care what my public IP actually is (that's what DynDNS is for) but I was surprised at this behaviour. Can anyone explain it? My guess is that Virgin want those who use Modem mode to be on a separate network segment, but I have no idea why that may be.

Im not sure, but im on VM 100Mbit and still using the VMG300 modem even tho EVERY time i call them, they tell me that it cant get 100Mbits, even though i have speed tested and received 106.9 Mbits peak, and double speed(up to 120 mbits) isn't even active yet

I hate the way my IP has change while i was away on holiday, since i have been back the connection has been less than its usual self and all because the IP address has changed

In reply to a post by Daemon66:

Can anyone explain it?

Yes, if you're using the same older router that you used with the old cable modem.

Its the hardware address (MAC address) of the router that is being passed to Virgin's DHCP server, and that looks up in its list, finds its given you an IP before, and returns the same one.

You're lucky the area you're in there are plenty of spare IP addresses, so yours wasn't re-used in that time.

Yes as that's the way it works I really can't believe that the SH got the same IP as the modem it replaced. I imagine it was just a similar IP - I've had mine change once in the past but didn't spot it had until I noticed my TBB monitor had stopped working - the last bit was unchanged ie n.n.n.17 became m.m.m.17 and maybe some of the n/m were the same too.

The old modem probably wasn't on DOCSIS3 but the Superhub will be. The change in performance probably only demonstrates different congestion levels between the old and new networks locally.

In reply to a post by kwikbreaks:

Yes as that's the way it works I really can't believe that the SH got the same IP as the modem it replaced. I imagine it was just a similar IP - I've had mine change once in the past but didn't spot it had until I noticed my TBB monitor had stopped working - the last bit was unchanged ie n.n.n.17 became m.m.m.17 and maybe some of the n/m were the same too.

The old modem probably wasn't on DOCSIS3 but the Superhub will be. The change in performance probably only demonstrates different congestion levels between the old and new networks locally.

The old modem was on the DOCSIS 3 network, same CMTS hence same IP address when the same CPE address appeared via modem mode. Address pools are assigned per CMTS, it's not possible to move CMTS and retain the same IP address unless the DHCP pool is moved with you, which doesn't happen.

Nearly all 20Mb legacy customers are on the DOCSIS 3 network running in non-bonded mode.

In reply to a post by jchamier:

In reply to a post by Daemon66:

Can anyone explain it?

Yes, if you're using the same older router that you used with the old cable modem.

Its the hardware address (MAC address) of the router that is being passed to Virgin's DHCP server, and that looks up in its list, finds its given you an IP before, and returns the same one.

You're lucky the area you're in there are plenty of spare IP addresses, so yours wasn't re-used in that time.

Virgin's IP leases are 7 days long. To lose an address in the space of a morning would be incredibly unlucky and require VM to be running incredibly close to the wire on addresses, which they don't do.

Post deleted by kwikbreaks

I'd misread the inital post as saying the IP hadn't changed when the SH was first installed in normal mode.

Edited by kwikbreaks (Thu 14-Jun-12 09:48:22)

In reply to a post by jchamier:

In reply to a post by Daemon66:

Can anyone explain it?

Yes, if you're using the same older router that you used with the old cable modem.

Its the hardware address (MAC address) of the router that is being passed to Virgin's DHCP server, and that looks up in its list, finds its given you an IP before, and returns the same one.

Thanks, yes that does explain it, I thought the modem would be the device to which the IP was allocated rather than my own router.
(Though VM using a MAC address that is totally under the customer's control to do their IP allocation sounds like a major security hole to me.)

In reply to a post by Daemon66:

(Though VM using a MAC address that is totally under the customer's control to do their IP allocation sounds like a major security hole to me.)

Why?

As it happens you can only have a few at any given time. Not sure of precise details but it's something to do with the CMTS storing the MAC/IP for each user and there only being a few slots available per user. If you fill them all you either have to wait til the oldest "times out", use an old MAC, or call TS to sort it because you will not get an IP.

Maybe ignition will feel like correcting/expanding on that.

Yeah it'll only remember a very few client PC/router MAC addresses for each modem MAC, these take a week to expire.

So you are saying that another user on a different modem would not get the same IP address if they used the same MAC? i.e. the scope of the MAC used for DHCP is per account?
My upgrade experience shows that the modem itself is not part of the calculation, since I got the same address back after switch from the old Ambit 256 to the Superhub, but if the scope for the MAC is limited to the account then no, I guess there isn't a security hole.

As was said the IP is issued according to the MAC of the router or PC connected to the modem and not the actual modem.

I initially thought you'd said you had the same IP with the Superhub in normal mode as you'd had with your old modem/router and that certainly shouldn't happen (at least not immediately but could just by chance once the IP had been released from its association with your old router). Putting the Superhub in modem mode presents the CMTS with your old router MAC once again so it got the same IP again as the old reservation hadn't expired.

Once an IP you have been using has been released then it will get allocated to somebody else eventually. Why do you see that as a security loophole?

I have no idea what would happen if somebody cloned your router MAC while connected to the same CMTS but I ceratinly can't believe they would get the same IP allocated as you. If it was a different CMTS then I don't expect any error would get registered.

In reply to a post by Daemon66:

So you are saying that another user on a different modem would not get the same IP address if they used the same MAC? i.e. the scope of the MAC used for DHCP is per account?
My upgrade experience shows that the modem itself is not part of the calculation, since I got the same address back after switch from the old Ambit 256 to the Superhub, but if the scope for the MAC is limited to the account then no, I guess there isn't a security hole.

If there are two devices with the same MAC address on a single DHCP scope it causes problems, the two devices will knock each other offline repeatedly.

This isn't a security risk any more than the ability to clone a MAC address on any shared media network. There are extremely dynamic IP address ISPs where addresses roll over every few days, it's not an issue unless hosting services at home, in which case dynamic DNS should be used on residential cable as it's a sticky, not static, IP service.

In reply to a post by Ignitionnet:

If there are two devices with the same MAC address on a single DHCP scope it causes problems, the two devices will knock each other offline repeatedly.

I have no idea how the VM network works internally but if you had two devices with the same MAC on the same Ethernet hub then both devices will receive the same IP from the DHCP server and both will receive all the data intended for each other. They certainly would not 'knock each other offline' since the DHCP server and everything else will think they are the same device.
In other words the potential security risk is probably about the same as having an Open Wi-Fi connection, anyone can use it for nefarious purposes, anyone can see the traffic and since the IP is registered on my account I'll be the one getting the infringement notices.
Of course, as previously said, there may be no risk if the scope of the MAC is limited to the account somehow and my guess is that this must be the case or VM would have no way of tracking who had what IP when, as they would be required to, to satisfy a court order.

It doesn't work that way with cable. When there are 2 devices with the same MAC address on the same scope they argue over the IP addresses.

Remember that cable uses DHCP helpers, the routers you connect to, which keep track of MAC address assignments, and indeed will query the DHCP server for information when they need it. When a new device takes the same MAC address of an existing device on the scope it then receives all its' traffic, the old entry is ignored. The CMTS knows which cable modem the DHCP discover came from and differentiates.

The only way to steal traffic is to use a modified cable modem that decodes the entire channel throughput.

Edited by deleted (Mon 18-Jun-12 16:27:03)

In reply to a post by Ignitionnet:

The CMTS knows which cable modem the DHCP discover came from and differentiates.

Many thanks, that's the bit I didn't know.

Surely in the early days of Blueyonder, the IP address was 'tied' to the cable modem's MAC address, but some years ago this was changed so that the IP address was 'tied' to the MAC address of the router, or the PC if there was no router?

At least that was what the Virgin Media engineer told me!

So that might mean the the IP address might stay the same if the old Scientific Atlanta or Motorola cable modem was replaced by a brand-new shiny "Super" Hub, in modem mode.

In reply to a post by JohnGray7581:

So that might mean the the IP address might stay the same if the old Scientific Atlanta or Motorola cable modem was replaced by a brand-new shiny "Super" Hub, in modem mode.

Not might, will.

Well, it didn't for me!

On my much-loved Scientific Atlanta I had 82.33.27.138,
but, once activated, on today's shiny Super Hub (modem mode) I have 82.40.162.xxx

In reply to a post by John_Gray:

Well, it didn't for me!

On my much-loved Scientific Atlanta I had 82.33.27.138,
but, once activated, on today's shiny Super Hub (modem mode) I have 82.40.162.xxx

Well yes, you moved from the DOCSIS 1 to the DOCSIS 3 network. Moving DOCSIS 3 to DOCSIS 3 you'll keep the same IP. Nothing to do with client MAC addresses the configuration files for the two devices are different, the Superhub lives on a different network to the SA modem in your area.

You moved from Staverton 2, a Cisco uBR 7246VXR, to Staverton 13, a Motorola BSR. Address ranges are per CMTS, so you're the exception that proves the rule.