sperera:
This has been covered in various ways in the posts above, but just in case it needs further clarification...
You need to differentiate between the original concentrate (or powder), the stock solution (if any) and the working solution.
The instructions for commercially prepared developers take one of three forms:
1) for some developers, the instructions recommend that you take the developer in the form that it comes from the store (either powder or liquid) and make a stock solution from it. Then, when you want to actually use the developer, you take that stock solution and dilute it to make a working solution. D-76 is one example of this type of developer (although you can use the stock undiluted). If you follow the instructions on the bottle of HC-110, it too is said to work this way, although many here prefer instead to take small amounts of the concentrate (~6-10 ml) and dilute it directly to working dilution just before use;
2) for some developers (almost invariably liquid developers), the instructions recommend that you take the developer in the form that it comes from the store and, when you want to actually use the developer, you take it and dilute it to make a working solution. T-Max developer works this way;
3) for some developers, the instructions require that you prepare two baths, and each different bath is used (and re-used) at full strength. Diafine works this way.
FWIW, I use HC110 and I do frequently make up a stock solution, but not with the entire bottle of concentrate. I only do this because I have more confidence in my ability to measure 120ml (as compared to 6ml) accurately.
Matt