HI,
Does this device (and the firmware on it) have UPnP ?
UPnP allows multiple devices use the same ports (Port Forwarding does not)
This means you can leave the modem/router doing DHCP to Issue IP addresses and it will not matter if the computer gets a new IP now and then.
In a one computer set up it is likely to always get the same IP when you start it up.
1) because you probably restart it before the lease time runs out
2) because there is nothing else to "take" the number.
However with Smart Phones, Games Consoles and multiple computers you may find that with out setting up IP address that don't change, either by Static routing or Address reservation that computers (And other devices) will swap IP addresses and set ups like Port Forwarding will not work - they tend to list which IP to send stuff to.
The Port Forward site lists only a CG3100 (plain, no suffixes)
Presumably you have used the info on the Port Forwarding bit to learn the access User ID and Password.
I linked to the AIM talk set up (Portforward.com > Chose Brand > Chose Model > dismiss advert page > chose App > set up instructions)
Using the link at the bottom to see the Router's screen Shots I went to the Basic set up one
http://screenshots.portforward.com/routers/Netgear/CG3100/Basic_Settings.htm
Near the bottom of the menu is UPnP
On most Netgears it is enabled by default. Specific Screen Shot.
On some devices setting up Port Forwarding or Port Triggering and having UPnP on can cause conflicts.
UPnP is needed for Screen Sharing in Messages or iChat.
9:31 PM Tuesday; July 16, 2013
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously