192.168.0.1 gives you access through the Router portion of the modem; while 192.168.100.1 is an address specifically for the modem side. Of course with your gateway combo, it still gets you to the diagnostics/options either way.
If you had a separate modem & router, you would only have 1 address for access each.
Thanks Johkal. Thought it might be something like that. I was trying to set up reserved IP addresses for devices in my dhcp. My Ooma voip phone, that I had plugged in to one of the 4 ports on the back, didn't show up. Any idea why it's invisible? It used to show up on my old router.