Sorry, you cannot do that. Predict has no "by" clause, like stats, which makes it impossible to make multiple predictions.
HOWEVER
If you really need to, and don't really care for performance, you could use the map command to do something like:
... your search resulting in a list of subnets ... | map [search subnet=$subnet$ | timechart something | predict]
The map command will loop all of your subnets and execute predict for each of them. Then it will append the results of each run to the final results. Just remember this has quite the performance impact because you'll be starting a new search for each subnet.
As an alternative you might consider user the R app, which is currently only available on github: https://github.com/rfsp/r. This app will allow you to run R commands in Splunk, and R is able to make multiple predictions at a time.
