I don't know of any 'standard' diagrams, and you are correct, that everyone has their own way of doing this.
For my money a Layer 1 diagram will show the cabling layout. Where the cat5 (or 5e or 6) cables run, and where the RJ45 outlets are located. It should also show any backbone cabling, either copper or fibre. This should be overlayed on a set of building or site plans. It helps you find the physical location of any network connection point.
A layer 2 diagram is more conceptual diagram, and indicates how the various elements in the network are connected together in relation to each other. This does not necessarily represent the physical location, but how they interconnect. So if two switches are connected, then they are shown on the diagram with a line between them, and the colour of the line should indicate the media type, and possibly the speed, and at each end a label to show the port it is connected to on that device. That is the sort of thing I thinkl should be on these.
A layer 3 diagram shows the protocol connections. How different devices are collectively grouped into subnets, or VLANs, completely regardless of location. Include the IP addresses and VLAN numbers on this diagram.
The actual layout of any of these diagrams will depend on the size and scope of the network. Try to use standard icons for the elements. I tend to use the Cisco conceptual ones, as these are widely understood, even when drawing diagrams of other vendor's equipment. If you use Visio (good for these), or Powerpoint (OK, but not so good), then you can download these icons for free from Cisco. Search for Visio on their website.
I have seen many sites with only one of these, usually the layer 3 diagram. I think that every site should have all of these, as each one has different information, that is extremely useful in diagnosing problems, and also when making changes or additions to the network. This is particularly true for anyone who was not involved in the original design and installation, and when combined with the network devices configurations, it should then give them all the information they need to understand the network.
It goes without saying (but I will anyway) that these should always be updated as soon as possible following any changes, otherwise their usefulness is greatly diminished.
I think if you follow the above guidelines, then you won't go too far wrong.
Well... you can find some interesting network diagram examples at <a href="http://www.ratemynetworkdiagram.com/index2.php">RateMyNetworkDiagram</a>. The best diagram is the one that raises the fewest questions in the intended audience.
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