DDNS from behind a router and cable modem

When setting up DDNS, does the cable modem and router have to support DDNS?

I'm on a laptop attached to a router, which is attached to a cable modem.  When I browse to the DDNS URL, I get connected to the admin page for the cable modem... so it seems that the DDNS service is forwarding requests to the external IP address of this network... NOT to my laptop within the network.

Is that supposed to happen?  Do I have to setup DDNS within the cable modem and router?  

Or are there DDNS services that can transparently traverse routers and modems?

I'm using dns2go.com
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Is that router performing NAT?  If you've got a private address on the laptop, there's no way DDNS can refer to it.  (Well, technically it could, but it wouldn't do you any good.)
kenshawAuthor Commented:
well... skype is a server application... and it transparently allows other users to send information to it despite being behind a NAT router...

I was hoping that DDNS could work somethign like that.

What your saying is that to use DDNS from behind a NAT router, I must ALWAYS configure port forwarding on the NAT router?
Yep, I'm afraid so.  Public DNS can only point to public IP addresses, as records pointing to private addresses on a public DNS server would never direct traffic anywhere, since those addresses aren't routable on the Internet.  Forwarding the port(s) that Skype uses is therefore necessary to get the traffic from the public network to the correct destination on the private one.

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Applications that initiate traffic from within the LAN don't normally need port forwarding (e.g. ftp, POP...), but to be able to handle incoming calls you need port forwarding to be active. An incoming call to port 80 (as you describe) must be forwarded to your http server (inside the LAN), otherwise you get the router's page. (which, incidetally, you should perhaps disable for requests from the outside.... for safety reasons).
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