I think I finally figured out how to use Netinstall. There were a couple of things that were preventing Netinstall from working. First, the MikroTik Wiki is incorrect in saying that the Client IP Address needs to be in CIDR notation (ie. 192.168.88.1/24). You only need to put the IP address in the Client IP Address box without the slash subnet part. I'm guessing the MikroTik assumes a /24 subnet. Netinstall won't let you put a slash subnet in the Client IP Address box, but it is confusing when the Wiki says that you are supposed to.
On the computer, you need to set a static IP address in the same /24 subnet. In my case, I set the Netinstall Client IP Address to 192.168.88.1 and my computer to 192.168.88.110. I don't think there is anything special about those addresses, but I'm just notating that here in case there is
. I also made sure that the firewall on my computer was disabled to be sure that Netinstall could properly communicate with the Routerboard 951G.
It is important that the network cable is connected to Ethernet port 1 on the router (at least that was the case for the 951G).
With all of those things set like that, my computer (which is a Lenovo W530 running Windows 8.1) was still not able to see the Routerboard 951G. I decided to try a different computer (Lenovo R500 running Windows 7) with all the settings I specify above. I was then able to see the Routerboard 951G in Netinstall. I'm not sure if Netinstall is incompatible with Windows 8.1 or what.
On the Windows 7 computer, after the RB showed up in Netinstall, I selected the installation package to install it and Netinstall I clicked "Install". Netinstall would sit on the status of "Sending offer" for about 10 seconds and then silently fail to send the updated OS. After some troubleshooting, I realized that I needed to wait until Windows quit trying to detect the type of network the computer was connected to before Netinstall could actually send the update package. (You can tell Windows has stopped trying to detect the connection when the spinning circle goes away from the top of the network icon in the task tray in Windows. It takes up to a minute for Windows to give up trying.)
After Windows quit trying to detect the connection, I tried sending the package again from Netinstall. Again it failed after a few seconds of having the status of "Sending offer". I found that I needed to close Netinstall and re-open it before it would finally send the updated firmware package to the router. As long as the router is in the Netinstall mode, Netinstall will find the router when it's re-opened.
Let me add some RB951G-specific tips. The manual says to hold the reset button during boot until the LED turns off, then release it to make the RouterBOARD look for Netinstall servers. My problem was that the LED was blinking sporadically and I didn't know at what point it "turned off". I finally just held it down (for probably 30 seconds) and didn't let up until the device did a single beep, indicating that it was entering Netinstall mode. If it does the normal multiple beeps after you hold the reset button down, then it couldn't communicate with the Netinstall application on the computer and was defaulting to boot to the internal memory.
Hopefully this will save someone from unnecessarily loosing time.