Undocumented networks are extremely common.
Many times this situation is related more to the difficulty of keeping the documentation up to date than to the complexity of the documentation process itself. Many LAN administrators start with big dreams of keeping elaborate drawings, detailing every aspect of the network. However, networks tend to change too frequently for such drawings to stay current. The fact is, having a well-documented network can help you solve problems quickly when they arise, and documentation is vital to the overall security of your network. Perhaps the most important part of a good network documentation plan is to label everything.