Every network should have a customized manual of sorts with maps showing the network layout and written documentation on all the basic aspects of the network.
Although you or some other IT superhero might have a mental map of the network stored in your brain, having up-to-date written documentation is still vital. What happens if that IT superhero leaves all of a sudden? If you have adequate documentation, the learning curve for a new IT staff member will be much easier. The same applies to any IT contractors who might be called in to help with the network in the future.
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Network documentation can aid in other situations as well. A properly documented network should be able to help with disaster recovery. Any other IT professional should be able to come in and install and configure the same basic network from scratch using the documentation, for instance if the hardware becomes damaged or broken. The documentation can also help with security because during the auditing and documenting process, some security risks may become apparent.