IT and Telecoms networks tend to be challenging to model as they change frequently, not just in terms of structure, capacity and topology, but also in terms of the rules that apply to them. These rules change when new technologies and policies are introduced and this often happens with little notice. Sometimes this problem is addressed by using few or no constraints, but this has consequences like poor data integrity and maintainability, as poor data can be introduced. Another challenge is multiple simultaneous network changes with long durations, for example the installation of a new cable, or the ordering of an additional item of network equipment that needs to be specially manufactured and/or shipped by sea. When these issues arise one needs to be able to interrogate and compare the current network configuration as well as the planned future configuration, and assess these against various current and future demands. The use of versioned inventory systems will address these problems when properly designed and configured. We have a great deal of experience with these problems.