A simplified model of a communications network is a probabilistic graph in which each edge operates with the same probability. The all-terminal reliability, or probability that all nodes are connected, can be expressed as a polynomial in the edge operation probability. The coefficients of this polynomial are obtained from an interval partitioning of the cographic matroid, and only the first few coefficients can be computed efficiently. One of the best sets of efficiently computable reliability bounds is the Ball-Provan bounds. These bounds are obtained using the efficiently computable coefficients and can be improved substantially if additional coefficients are known. In this paper, we develop a Monte Carlo method for estimating additional coefficients by randomly sampling over spanning trees of the network. Confidence intervals for all-terminal reliability are obtained by using these estimates as additional constraints in the Ball-Provan bounds. This approach has some advantages over conventional Monte Carlo point estimate methods. In particular, the computational complexity does not depend on the reliability of the network.The communication links of a large computer or communications network can be unreliable, affecting node-to-node communications. A simplified model of the topology of such a network is a probabilistic graph G = ( V , E ) consisting of a set V of n nodes, representing communication centers, and a set E of m edges, representing communication links. In addition, each edge e E E has an independent probability p e of being operational. In this paper, we consider only the case in which each edge operates with the same probability p .Since links are unreliable, a link may at any instant be operating or failed. A state of the network is a set S E of edges that represents the operating edges. The probability, P ( S ) , that the network is in state S is n e E s p rIeEE-S (1 -p ) . A state S is operational if all the nodes in the graph G' = ( V , S) are connected, that is, if G' contains at least a spanning tree. Define the binary functions r ( S ) to be 1 if state S is operational, and 0 otherwise.