Hello Anne,

This nest looks like a bald faced hornet nest. These hornets can be quite aggressive. If this nest is out of the way and you do not come in contact with them often, it may be safest to simply leave the nest alone - it will die out this winter when we get freezing temperatures. 

I have attached a publication on controlling wasps, hornets and yellowjackets. It states in the publication that it is often safest to have a company kill the nest, but it also gives you directions on how to kill the nest yourself. 

Good luck!

Robby