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Port Forward Modem and Separate Router

Posted on 2007-11-22
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Last Modified: 2013-12-27
Hi, I am not too good at networking (seems to confuse me).

Can someone please provide a step by step guide to port forwarding through a router and separate modem. I need to know things like, what IP do I give the modem etc...

My setup is;

D-Link DI-624S Router and Netcomm NB1300 Modem.

Thanks
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Question by:jset_expert
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by:Andrew Davis
Andrew Davis earned 100 total points
ID: 20336516
have a read of the manual here ftp://files.dlink.com.au/products/DI-624S/REV_A/Manuals//v1.00/DI-624S_Manual_v1.00.pdf
what you are looking for is virtual server section on page 35

Cheers
Andy
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Expert Comment

by:moorhouselondon
ID: 20338329
This is pretty well the ultimate guide to port forwarding.  The S suffix on your router - not sure what that means, but if you scroll down the list to DLink on the following link you will know by going into the instructions whether the screenshots match your Router.  

http://www.portforward.com/english/routers/port_forwarding/routerindex.htm

The next question it will ask is what application you wish to port forward.  If you want an overview of how to do it select SMTP, but the principles are the same for most purposes.
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by:jset_expert
ID: 20340948
My issue is not port forwarding through the router or modem. If I hook up my PC directly to the modem, I have no problems, and I am pretty sure I can get through the router. If I have modem built into the router then  I know I would have no problems.

My issue is trying to port forward a PC through both the router and the modem together.
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moorhouselondon earned 250 total points
ID: 20341147
As its' name implies, a modem simply converts a signal from a communications type signal into one recognizable by a network card (and vice versa).  It has no routing or filtering capabilities of its' own.  (If it does have this functionality then it is not simply a modem).

Plugging a Modem directly into a pc means that the pc will operate on the same public network address that is fed to you from the Service Provider - do an ipconfig at the dos prompt and you will see that.  

Every port that comes through the modem is directly accessible to the pc, and vice versa - so a hacker could easily infiltrate your pc by directly probing the ports on your pc.

Sticking a router between the two segregates the networks - the public IP address goes into the router and gets converted into your Private IP range by means of a routing table.  

Any activity on any port on any pc attached to the Router (on its' Private IP address) will be fed into the Routing Table and sent out on the Public IP to the Service Provider (provided the Routing Table does not recognize the destination IP as being local, in which case it will route it internally).  Many Routers are setup by default with no Outgoing Port Blocking, so all outgoing activity on all ports will be unrestricted: web access, DNS requests, etc.

Activity on any port going into the Router from the Service Provider (on the Public IP address) will be routed according to the Firewall Rules setup in the Router for Port Forwarding.  Many Routers are setup by default to block incoming activity on all ports, so you have to explicitly direct activity on the incoming IP address, on a designated port, to the appropriate Private IP Address.  So packets coming in on Port 25 (SMTP), for example, are directed solely to the pc hosting your Mail Server.  Any probing that a hacker would do would reveal only the ports that you had intentionally left open.  Many routers have Port Scan protection, which means that anyone cycling sequentially through the Ports will detect this potential hacking activity and Stealth themselves away from view.

www.grc.com

the Shields Up facility on this website is a good tool to see what is happening with your different configurations.  To make sure you are not scratching your head over the Router "stealthing", it is best to use the Custom port probe to check one particular port to see whether it is open or closed, rather than doing a full Port Scan.
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by:jset_expert
ID: 20341262
thanks moorhouselondon, I will look into it.

My modem is specified as a Modem Router, the reason I have the separate router is purely for a wireless network.
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by:jset_expert
ID: 20341278
solved...I was using the wrong IP when port forwarding on the modem.

Thanks for your help.
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