In reply to:

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Can any of the network guys here give a good reason as to why a big company might choose not to implement DHCP on each of its subnets? Why fixed addressing would outweigh the benefits of DHCP?

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The only reason we use a mix of fixed and dynamic is for hosting departmental web servers. A recent project of mine was remote control of ISDN's in a radio studio. Hanging a home-made webserver off the back mean't that I could keep track of which ISDN's were dialled up and to what numbers etc. I had to use a static address for obvious reasons. Our IT department has a nifty way of keeping track of it's allocated static IP's, it's called a spreadsheet.
Combined with the fact that mortal users within the Beeb are not capable of altering their network settings.