Steps to setup a block of Static IP on RT-N66U router using Shibby build...
A quick note for users who are looking for a way to setup a block of static IP on their ASUS RT-N66U router (you will be using Shibby's build):
1. Download Shibby's TomatoUSB firmware "tomato-K26USB-1.28.RT-N5x-MIPSR2-104-AIO-64K.trx" (google that filename for download link)
2. Set your computer's ip (make it static) between 192.168.1.2 and 192.168.1.254 (ASUS RT-N66U router defaults to having 192.168.1.1 in "rescue mode").
3. Disconnect all ethernet cables from RT-N66U router and turn it off
4. Connect an ethernet cable between your computer and one of RT-N66U LAN ports (pick any yellow colored port between 1 ~ 4, do NOT connect to WAN port which is blue and labelled "e")
5. Press "Reset" button and keep it pressed, and then turn ON the power to RT-N66U router
6. Wait for 10 seconds or so and make sure that power LED light very slowly starts blinking - now you're in "rescue mode"
7. On your computer web-browser go to address "http://192.168.1.1"
8. You will see an option to upload new firmware to the router on that page, along with an option to Erase NVRAM. Click on the button for erasing NVRAM (I did it twice just to make sure)
9. Once NVRAM is erased in above step, select the Shibby build firmware file (the one you downloaded in step#1) and click on the button to upload it to the router. It will be quick to upload but you'll have to wait at least 90~120 seconds before the router programs properly and reboots.
10. Do NOT disturb the router while step #9 is in progress. Once 90 seconds pass, on your web-browser type "http://192.168.1.1" again. You will see Shibby build main page come up from the router.
11. I setup my router with one of the static IP (under "Basic" setup menu) from my AT&T assigned block of static IP's (in the block they assign you, I believe the first IP is public IP for broadcast, next 5 IP are usable by you, and the last IP is used as gateway IP. You can leave the router for DHCP configuration which is its default in Shibby build)
12. Go to menu "Administration->JFFS", and enable JFFS+format that partition (it's all done within that same page, it will take a few seconds to do so).
13. Once step#12 is done, turn off your router and disconnect the ethernet between your computer and the router
14. Temporarily connect an ethernet cable from your computer straight to LAN port of your AT&T router, go to its setup page (I access my AT&T router through "192.168.1.254", usually it's stated in your service provider or dsl/modem router manual). Setup your AT&T router for "IP-PASSTHROUGH" (under Firewall). Once done with that, disconnect the ethernet cable from it.
15. Connect an ethernet cable between one of the LAN ports of AT&T router and WAN (blue) port of ASUS RT-N66U router. Also connect another ethernet cable between LAN port (yellow) of RT-N66U router and your computer such that your setup has the following Router-behind-router (where AT&T router is in bridge mode) network wiring:
Internet<-->AT&T Router<-->RT-N66U<-->LAN (your computer)
16. On your computer, set the networking back to DHCP instead of static IP that you did in step#2.
17. Turn ON RT-N66U (assumption is that your AT&T router is already ON, if not then turn it ON first and wait for 90 seconds before turning ON RT-N66U router).
18. Wait for 90 seconds after you turned ON RT-N66U router in the above step.
19. Check network properties in your computer to get a new dynamically assigned IP address (it is now assigned dynamically by DHCP server in RT-N66U router). Replace the last digit on that IP address with a "1" and enter that address on your web-browser to access RT-N66U. (NOTE:: My computer gets assigned a dynamic IP address in range 192.168.2.2~254, in which case my RT-N66U is on 192.168.2.1 - this setting is all dependent upon what you have in AT&T router settings).
20. Go to "Administration->Scripts->Firewall" and enter the follow three lines for each static IP that you want to setup (don't forget to click on Save afterwards):
/usr/sbin/ip addr add 99.XXX.XXX.105/30 dev vlan1
/usr/sbin/iptables -t nat -I PREROUTING -d 99.XXX.XXX.105 -j DNAT --to-destination 192.168.2.20
/usr/sbin/iptables -t nat -I POSTROUTING 1 -p all -s 192.168.2.20 -j SNAT --to 99.XXX.XXX.105
Where 99.XXX.XXX.105 = one of the static IP addressese from the block of 5 static IP assigned by AT&T, and 192.168.2.20 = LAN IP of my computer to which I wanted the traffic routed. I then repeated above three lines for three more static IP's and left one free (assigned to RT-N66U router so that all other computers can go through that).
21. Final step is a reboot... you should be now setup with your block of static IP, with each of them getting routed to whichever computer you assigned in the above step.
This looks like a long process but hopefully simple and if you follow it properly then it won't take you more than 15~20 minutes to setup your network.
Good luck!
(Please feel free to correct me if you see any mistake(s) in my steps. I've tried to be as detailed and accurate as possible).