A method of assessing a network uses a model (450) having nodes (100, 110) to represent parts of the network infrastructure and the application services, and having links to represent how the nodes influence each other. Dependencies or effects of the application services are found by determining paths through the nodes and links of the model (530). Such assessment can be useful for design, test, operations, and diagnosis, and for assessment of which parts of the infrastructure are critical to given services, or which services are dependent on, or could have an effect on a given part of the infrastructure. The dependencies or effects can encompass reachability information. The use of a model having links and nodes can enable more efficient processing, to enable larger or richer models. What changes in the dependencies or effects result from a given change in the network can be determined (830).