This greatly depends on the complexity of your network. For smaller networks, a notebook with device configuration an electronic backups and Visio drawings is fine. There are even some freeware/shareware applications that can help with the visual part of this.
For security and operations requirements, documentation is very important. IP ranges, protocols used by each application, routing methods, VLANs, etc, should all be documented. If this is a complex network with manning corporate applications, there are some very powerful applications for maping networks, VLANs, protocols, etc. They are also very expensive.
Would need to know more about the scope before a good recommendation could be made. But I recommend maintaining some level of documentation even for small networks. I use a freeware product (be careful which one you download so as not to introduce a security vulnerability) for the small network I maintain for a family business. For my main job, I use several powerful tools from Computer Associates and a few smaller vendors to map and maintain the network.