The answer, like so many others, is "it depends."
If you are trying to calculate a single availability number for the entire network of 10 subnets, then uptime is simply the percentage of time that the entire network is fully functional.
The other way to present the availability statistics is to consider each network separately. The advantage is that you'll have higher individual numbers for most, or all, of the subnets than you will for the combined set. The disadvantage is that you'll be presenting 10 numbers (or 11 with the inclusion of a combined figure) to management.
You should, by the way, also find that the 11th number is the product of the first 10 since that is how availability statistics work.
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This was first published in December 2002