ADSL Router vs Cable modem - IP address question

Jimbo Mahoney

Jimbo Mahoney

Jimbo Mahoney

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18 Oct 2002
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Hey guys,

Just changed from NTL Cable to BT ADSL and have a question about how the IP addresses and stuff work, in relation to having a Dynamic DNS hostname.

I'm using a Smoothwall box, and when the cable modem was connected to my 2nd NIC (Red interface), it would get assigned an external IP address.

Now that the ADSL Router is connected to it, it gets assigned an internal IP address (192.168.1.2, with the router being 192.168.1.1). Presumably the router is acting as the external IP address now.

How do I get my Smoothie box to tell the Dynamic DNS people my external IP address?

:confused:

/sorry, ADSL newb...
 
ADSL Modems and router are two entirely different devices.

A modem will act as a passthrough device which simply acts as an endpoint for a broadband connection, as such it is not a true network device and has no IP address, the result is that the machine connected to it shall receive the DHCP address issued by your ISP.

An ADSL router has a modem built in but has a lot more functionality, it can be assigned an IP address and is a true network device, it shall be assigned the ISP assigned DHCP address, in turn it will utilise NAT to share your internet connection between all machines connected behind it IE: Your network whom will be issued private IP addresses using the routers own DHCP server. This provides good basic security as it does not expose your internal network to the internet.

If you wanted to use dynamic dns you would have to install a client on one of your machines which would periodically poll the dynamic dns server with updated IP address information.

Ps: There is no difference between ADSL Modems and Cable Modems just the connectivity used. If you wanted you could get yourself an ADSL modem and scrap the router so that you can also direct access to your firewall again.
 
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Smoothwall does the periodic reporting, but since it has an internal IP address, it reports that.

So I'll have to get an ADSL modem if I want to use Dynamic DNS?

Is there any way for my ADSL Router to act as a modem, or to tell my Smoothwall box its external IP address?

Thanks.

EDIT - Smoothwall forums say the ADSL Router could have a 'Bridging' mode that would do the trick?
 
Jimbo Mahoney said:
EDIT - Smoothwall forums say the ADSL Router could have a 'Bridging' mode that would do the trick?
Click to expand...

Yeah it depends what router you've got ... which one is it? If it's a Netgear then you can forget it unless you get a multiple IP block from your ISP.
 
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