You need to identify the WAN Address (the dynamic IP address from your ISP) and plug it in to LimeWire under "Force IP to..."

LINKSYS
(procedure - initial setup)
1. at your personal computer -- the ONLY one that will be running Limewire! -- run the WINIPCFG program and identify what your in-house IP address is, e.g. 192.168.1.100
2. Log into the Linksys (e.g. http://192.168.1.1 ) as <blank> and with your password
3. click on the Advanced "tab"
4. on the Advanced menu, switch to the 3rd tab, Forwarding
5. for the first setup line, enter 6346 and 6346 as the first/last ports, then enter your in-house IP as the forward-to address.
(optional)
Set the last port as 6699, i.e. set the port range as 6346-6699, so that you can run both Gnutella and Napster!

(procedure - check IP)
Everytime you change your Linksys configuration and/or reset your DSL(?) modem, you're going to be assigned a new WAN IP Address, so this 2nd/3rd procedure will have to be redone periodically.
1. Log in to the Linksys as <blank> and with your password
2. switch to the third tab, Status
3. make sure it says, "Status:Connected"
4. scroll down to the WAN section and record whatever is entered in the first data field, IP Address.

LIMEWIRE
(FYI - this applies to any Gnutella client)
1. go into the options screen
2. switch to the last(?) tab, Advanced(?)
3. enable the option for Force IP Address
4. enter the WAN IP Address you copied from the "check IP" procedure.

P.S. I have had two Linksys routers. The first was the 4-port switch model and it locked-up under heavy usage, e.g. Limewire, until I installed the most recent firmware. Currently, I'm using a new 4-port + Wireless model and, again, it is locking-up every couple of days...
P.P.S. If/when you upgrade the firmware, BE SURE to go into the Advanced settings and enable "Allow Remote Upgrades" first! If you don't -- at least in my experience... -- the upgrade utility will fight with the router and hose the firmware. BUG?! Afterwards, assuming it didn't get trashed, go back in and disable it again (which is the default).