As a guy who gets 100 calls a day about leased gateways from comcast:
Don't call us about it, its so easy to do 10x over during the hold wait alone.
Don't even think about having more than one xbox on open nat. The closest is open on xb1, and moderate/open on a 360 at the same time.
Xfinity Gateway Open NAT Guide for one xb1:
1:Go to connected devices, hit the edit button to the right of "XboxOne" in the device list (after connecting to the wifi of course)
Select "reserved IP" radio button, below DHCP (theres only those two so its pretty simple). It will fill in the fields and keep the same ip (10.0.0.x) Don't worry if its in the lower range, since we are doing this on the router side of things no other device will be assigned to the same ip after it is reserved. REMEMBER THIS NUMBER FOR LATER
Hit save or save settings.
Go to Advanced on the left hand side, then the submenu pops down to show "DMZ"
Enable DMZ, it starts disabled and red, hit the green enable button
Depending on which comcast/xfinity Wireless gateway (modem router combo box) you may see both ipv4 and ipv6 adress. Ignore ipv6 if it is there.
Put the IP Adress (10.0.0.x) of the now static xbox into the field and hit save.
Sometimes you have to uplug the WG for ten seconds and plug it back in for the DMZ to take effect, sometimes not, but it cant hurt.
Unless you want to get your own router, that is all that can be done. I recommend dmz over port forwarding as some games such as madden and blops3 require additional ports to be opened that are not on the standard xbl list.
Final note: erase any port forward settings you had for the xbox before, disable it entirely if you have nothing else using it.