Ensure that access is not blocked to the device. Preferable steps for Netinstall to work:

  1. Directly connect the Ethernet port of the device to the PC.
  2. Both devices on the same subnet.
  3. Disable the Windows firewall.
  4. Disable any antivirus programs.
  5. Make sure you press the reset button correct to bring the device to the Netinstall state.

No success? Other things to try:
  1. Disable all other interfaces on the PC leaving only the one connected to the device;
  2. Run netinstall.exe as administrator;
  3. Make sure the connection in Windows is set to Private;
  4. A fixed IP address;
  5. Win7 set compatibility mode for netinstall.exe to XPSP3;
  6. Try different Ethernet cable;
  7. Set parameter: System > Routerboard > Settings >Boot Device "try-ethernet-once-then-nand;
  8. Enable NetBIOS over TCP/IP.
  9. Have the device connected through a switch to the computer, in rare cases, this can help to bypass hardware incompatibility.