Just to add to Tim's answer:
In Win2K, you can map a drive letter to a subfolder under a shared folder. So, you could map L: drive to \\MYSERVER\SHARED\subfolder\subfolder2\subfolder3
But in NT4, it does not support this. It can only map to the shared folder itself. So, L: drive could only be mapped to \\MYSERVER\SHARED
Now, if you need to map to a folder that is physically under another shared folder (like to the subfolder3 in my example above), you will have to share out subfolder3. Then you would map L: drive to \\MYSERVER\subfolder3
If you are looking to avoid having dozens of shared folders visible whenever anyone opens Network Neighborhood (it can be unsightly!), then share subfolder3 with a $ at the end of the shared folder name. Using a $ makes the shared folder a "hidden" share to normal browsing.
So, you could share out subfolder3, but make the Share Name: subfolder3$
You would then map L: to \\MYSERVER\subfolder3$
Now, just keep in mind that the hidden share is not visible at any time in Network Neighborhood OR in the Map Network Drive dialog box. When you go to map L: to this hidden share, and if you double-click \\MYSERVER in the attempt to map to subfolder3$, you won't see it. But you can just type in the full UNC path \\MYSERVER\subfolder3$ to do the mapping.
This also works in logon scripts.
hope this helps (I guess I was in a wordy mood today!)