Laboratory experience is a key factor in technical and scientific education. Virtual laboratories have been proposed to reduce cost and simplify maintenance of lab facilities while still providing students with access to real systems. It is important to determine if such virtual labs are still effective for student learning. In the assessment of a graduate computer networks course, the author quantifies the amount of learning that is observed in lectures and labs. The results not only show that learning indeed occurs during lab sessions, but almost equally as much (45.9%) as in lectures (54.1%). Also, it is observed that even students who have prior experience in networking benefit from virtual labs.