I disagree about the database thing.
1. Create a spreadsheet with your networks. It should have headings like this: IP, mask, DNS name, use, user, phone number.
2. Separate those into subnets.
3. Print that out and put it into a binder.
4. Use something like Solarwinds to map your subnets.
5. Use a *pencil* and fill all the information into the binder.
6. Photocopy the binder and leave one copy at home, one with your boss. Carry the original with you at *all* times.
7. Update often.
I have used databases, spreadsheets, specialized tools like SolarWinds, Orion, HP OpenView, etc. Nothing beats a printed binder that is always with you. Using a pencil will ensure that future updates, additions, or deletions will not mean printing a brand new sheet.