When documenting changes to the network infrastructure I feel that a diagram is nearly always necessary to allow support staff to fully understand the system. My preferred application is MS Visio which has a wealth of options for creating diagrams including UML, floor plans, network diagrams and more. You can quickly and easily create very professional looking documentation. I am lucky that my organisation provides Visio as it is expensive to purchase as an individual, so I was interested to come across a free alternative called Dia.
Dia describes itself as roughly inspired by Visio and not only is it free but it is available on Windows, Linux, and Mac OS. Using Dia you quickly realise that it is not as polished as Visio but it is perfectly capable of being used to create good quality diagrams. This icons in particualr are not a pretty as Visio but they are clear and look fine when printed. This is ideal for smaller companies or people that only infrequently create network diagrams. You can check out Dia and Visio on the links below.