The first reply is correct, if that's where your D2 is installed. Also, it may not be clear to many how to apply it if they've never added switches to shortcuts before. Here's how to put it to use (directions are for Windows operating systems. Mac may be similar but with Command-Click instead of Right-Click? I've been out of the Mac loop for a long time):
1. Right-click the icon you use to start Diablo II
2. Select 'Properties' from the context menu that pops up
3. In the 'Target:' box, click to position cursor at the end, add a space, and "-skiptobnet" (without the quotes)
Mine looked like this: "D:\_Games\Diablo II\Diablo II.exe"
and was changed to this: "D:\_Games\Diablo II\Diablo II.exe" -skiptobnet