Understanding IP addressing with broadband modem

Henry Law
Henry Law used Ask the Experts™
on
I'm trying to understand the assignment of IP addresses when the broadband modem (for example Draytek 130, which in the UK only operates in bridge mode) is separate from the router.

My understanding is that a bridge operates at level 2; it passes packets from one side to the other, converting between the different technologies at levels 2 and 1 (in this case between Ethernet on the inside and some-telephone-stuff (which I don't really understand) on the outside.  Therefore it has no IP address of its own, IP being a level 3 thing.

So if the ISP assigns an IP address to the subscriber of (say) 81.152.10.12, then that's what the WAN port of the router is, and the modem is transparent at the IP level.

But the manual for the Draytek modem says that if you connect a PC to the ethernet port it can get a DHCP address in 192.168.2.x.  That's the same ethernet port that bridges 81.152.10.12 to the WAN port of the router.  How can this be? Does the modem run a web server on some 192.168.2.x address at the same time as bridging the external IP address to the router?

Like this?  Have I got that right?
=====broadband==== MODEM ====81.152.10.12=== router
                     |
		     |--- 192.168.2.x ---- PC

Open in new window

Comment
Watch Question

Do more with

Expert Office
EXPERT OFFICE® is a registered trademark of EXPERTS EXCHANGE®
JohnBusiness Consultant (Owner)
Most Valuable Expert 2012
Expert of the Year 2018

Commented:
The WAN interface of the Draytek connects to the Modem. That is 81.152.10.12.  That is independent of the inside interface.

The router does NAT Translation and the inside ports are on 192.168.2.x subnet. You can use any subnet here. I use .100 to avoid VPN conflicts.

This is a very standard approach independent of router make or modem manufacturer (as long as it uses bridged mode.

Author

Commented:
John, thanks for your reply.  I may have confused you by mentioning a specific model because the Draytek 130 (at least in the UK) is a modem, not a router.  (Extract from the quickstart: "The Vigor 130’s primary function is as a transparent bridge modem for xDSL services – not as a router/firewall in itself."). In the configuration I'm looking at there is on the RJ45 port of the modem another router (make and model unimportant) which is doing all that NAT and DHCP stuff on the internal LAN, which is in fact in 10.x.x.x and not really what I'm asking about.

So the WAN port of that router is (in my example) 81.152.10.12, which is what the modem is bridging from the telco's xDSL environment into Ethernet.  The part that i don't understand is that the modem, which being a bridge doesn't have an IP address of its own, seems, according to the manual, also to provide an administrative web server on the same RJ45 port that the router plugs into, on a completely separate RFC1918 subnet.
JohnBusiness Consultant (Owner)
Most Valuable Expert 2012
Expert of the Year 2018

Commented:
If the Draytek is a modem, then what I said applies if thinking of one box. The modem has a WAN port connecting to the Internet on 81.152.10.12.  The NAT Translation occurs inside.

So the WAN port of that router is (in my example) 81.152.10.12, which is what the modem is bridging from the telco's xDSL environment into Ethernet   That does not happen the way you wrote. There is NAT circuitry to isolate the WAN from the Inside Ethernet and inside is separate from the WAN except the packets go back and forth by NAT. In this way, they are connected, of course.

You should be able to access the Draytek GUI interface from a LAN port because you need the WAN port connected while you are managing the setup.
Distinguished Expert 2018

Commented:
My guess is that any device that isn't able to complete a PPPoE connection then get that 192.168.2.x addresses. Try connecting a router that is programmed to get a DHCP address, but not use PPPoE. I bet that would get the 192.168.2.x address. Sounds like there is some VLAN-type logic built into that particular Draytek. Either that or all devices get addresses in that range, and devices that compete PPPoE connections get new IP addresses once logged on. I've seen ports where WAN and LAN IPs both work, but not when both utilize DHCP (Comcast business gateways in the US are an example of this).

Author

Commented:
The combination of the two of you has given me a much greater insight into what's going on.  I'll see if I can find a router which I can use to play with, and switch off PPPoE and so forth as an experiment; but at the very least I can see how the modem might do both bridging and web serving on the same socket.  Thank you both.

Do more with

Expert Office
Submit tech questions to Ask the Experts™ at any time to receive solutions, advice, and new ideas from leading industry professionals.

Start 7-Day Free Trial