Windows 7: The Default Gateway is not Available

I encountered a problem affecting a Windows 7 Ultimate (64-bit) system,  where the Internet connection was randomly disconnected, often while downloading a large file or streaming media across the network.  The Windows network troubleshooting wizard was able to correct the issue temporarily by performing a reset of the Network Adapter (Manually doing the same also restored connectivity).  Oddly enough,  the Local Area Network connection remained active,  but the Internet link was show as disconnected.  The network troubleshooting wizard identified the problem as being that the “Default Gateway was not available” even though the Broadband router was online and working fine for other computers.

After running some searches on the issue, I discovered that there were a large number of Windows 7 users who had been experiencing a similar problem.  The common factor was often the use of an NVidia NForce onboard network adapter,  however in this case the computer in question was using an Atheros 10/100/1000base-t controller (Onboard an ASUS motherboard).

The initial recommendation provided was that affected users should acquire updated NIC drivers from the manufacturer of their network card,  however there were no update drivers available for the Atheros card.

Luckily I came across information posted by a user suffering from the same problem on a similar system using the same onboard NIC (Atheros).  The recommendation that corrected the problem permanently was to:

Open Device Manager

Locate the Network Adapter

Right click, Properties

Go to Advanced tab

Locate the Task Offload (aka TCP offload) property on the list and set it to Disabled

After setting this property and applying the changes,  the Internet connection remained stable and no longer dropped out when transferring large amounts of data.

For more information see:
http://www.sevenforums.com/network-sharing/5787-problems-atheros-l1-ethernet-adapter-win7-x64.html
http://social.answers.microsoft.com/Forums/en-US/w7hardware/thread/59bcb7f5-fcca-44a4-b1db-787b1d269825