Do you have a network layout diagram?
Here's a not entirely hypothetical question: suppose that your machines are coming up after a building-wide power outage, except that a scattering of them (on various different networks) are either not up or not reachable. Could you look at what machines are failing to appear and identify whether there's likely to be a switch that's failed, and if so, where that switch would be?
(This happened to us yesterday. There was some confusion, because guess what we don't have.)
That sort of question is why you want to have a network layout diagram, something that tells you how your logical networks flow through your physical infrastructure and reach your various physical locations. Without a network layout diagram, you're relying on a combination of human memory and tracing connections around; even in the best of times, this is going to be slower and more error-prone than looking it up.
Another advantage of having things in an actual diagram is that it's generally easier to reason about things when you have a physical diagram in front of you. Without an actual diagram, everyone involved has to more or less reconstruct one in their mind in order to see the relationships; with a diagram, well, you just look at it, and you can just point at various bits to explain things.
For some people having an actual diagram for this will sound silly. To those people I say that you haven't gotten big enough yet.
(If you're relatively big yet you're still keeping all of this in your head and consider it good enough, ask yourself how you're going to bring a new person up to speed on the network structure.)
For some people, this will sound like a motherhood and apple pie issue; of course you have a network layout diagram. The problem with this view is that old issue: documentation is not free, and network layout diagrams are a form of documentation. If you have a non-trivial network that is under constant evolution (some would say 'churn'), keeping your network layout diagram up to date is going to take a commitment of time, and everyone concerned (management included) has to accept that this is going to slow down work.
(How much time it takes depends on what format you keep your network layout diagram in and how easy the format is to update. My personal choice would be graphviz format, because it's plain text, but I haven't actually tried to do this; it's possible that real network layouts are too complex for graphviz's automatic routing and layout.)