If you have a network in your office and you don't have a diagram that looks like this, or your IT person has not drawn one up, then you need to do a top to bottom assessment - here's why:
Your network administrator is only human, and in the land of the blind, the one eyed man is king. Many network admins don't write it up because they keep it in their heads. If, heaven forbid, something happened to your network admin like he went on vacation in Tahiti and decided he liked it there so much that he was not going to come back, who would know about your Network design?
How about if someone needed to build out your system? Networks are like buildings, and you should have the blueprints to yours.