LINQ query listof returning multiple index references

Hi, I am pretty new to LINQ and google is failing me for VB specific answers..

I have listof ItemDetails, within this list I have property ItemNumber.. ItemDetails.ItemNumber. I want to query this to return all the index numbers where ItemDetails.ItemNumber = x (there will be duplicate hits and it is possible there may also be no hit so I need to handle null).

I then want to iterate through each of these indexes that get returned so guess I need the results into some new temporary listof. I imagine this is simple stuff but for the life of me all I can find is how to return a single index using vb.

Any help or a point in the right direction much appreciated.

Thanks!