This error results from attempting to reach a location on the network that is not accessible. This connectivity problem is usually associated with computers running Microsoft Windows Vista. Your computer is not able to connect to the Internet, and you cannot ping your IP or default gateway once the error has occurred.
When ERROR_NETWORK_UNREACHABLE' you shoud do following
To address this error code, attempt a routine network troubleshooting session. See Windows Help to initiate. Since this error code is usually associated with Microsoft Windows Vista, if it is occurring with regularity, this may be a good opportunity to upgrade to the latest version of Windows. If that is not an option however, you can remedy the situation while still running Windows Vista. The first thing to do is to check your cable or port configuration. Ping your local host and the assigned IP address. If you get a response, then attempt to ping the gateway. If no response, check the duplex and speed settings on the card, and the subnet mask setting. Another alternative is to set up another DHCP server and attempt to connect again.
It may be that your TCP/IP stack has become corrupted. In some cases, if you are running Vista and are also running TCP/IPv6, this may be the cause of the problem, as V4 seems to work better with Vista. Deselect TCP/IPv6 and select TCP/IPv4, and try to connect again. If still no luck, remove and re-install TCP/IP. If all else fails, then chances are it is a problem with your network interface card—remove the NIC and install a new one, then attempt to connect again.
TipFor more information, please see: How to fix Access error #1231