Published work on computer network reliability frequently uses availability as a dependability measure. However, although several ways of defining availability have been proposed, none capture the overall level of service obtained by client hosts in a modern n-tier client/server architecture. We propose such a measure by calculating the fraction of client hosts receiving complete services from the network. We also extend a published efficient heuristic method for calculating availability to take into account our new proposed measure. The end result is a procedure of polynomial complexity O(n _t ⁴), where n _t is the total number of components (hosts, links and interconnection equipment) in the network.