I installed DDWRT on a linksys router and changed the default ip for the admin login. I'm trying to get back into the router but forgot what ip I changed it to. What would happen if I were to try to do a factory reset on this router?
You can perform a hard reset and it will set everything back to the defaults that come with DD-WRT. Just unplug the router, push in the reset button on the back, plug it back in, wait at least 30 seconds, and release. (The instructions call for a few more steps but in my experience they're not necessary on WRT54G devices.)
This is commonly called a factory reset, but it really just resets the memory on the device to the defaults supplied with the current firmware. To restore the Linksys firmware, you'd have to perform the above steps and then upload a firmware image from Linksys via the admin interface.