IP summarization design

So I never quite grasped this. I skimmed some books and tried my Google fu but I couldn't quite get the answer I was looking for.

If you are trying to design a new IP space for a 100 branch office site where do you start?

Also in general when designing IP spaces (Data center, campus, Wan, branch , remote workers) where do you start? How do you lay it out?

Say you want to make a 'cookie cutter' site. 6 /24s for users, phones, servers, etc. 6 /30s for various transit networks, some /29s for slightly larger transit networks... a management network, etc.

How would you go about figuring this out....

If I try to work through this I need

• 7 /24 (one for mgmt) *6 /29 transit space *6 /30 transit space

Now I understand there may be more or less, but lets run with it for this example.

First I would need to determine the aggregate route. I'm not sure if this is right, but I think we would need to basically add up all the IP space from our composite routes (not sure if usable or count Bcast / network). (255x6)+(8x6)+(8x4) = 1610. Now looking on the binary scale that would be... 2048 so a /21.

Cool now we know each site gets a /21. And we need 100 branches (assuming no growth) meaning a /11 would summarize all the branches... right?

Okay so now we have to divy up the /21. Here is where I'm not sure how to slice it up. Do we start with the /30s first and work our way up? how do we do this part?