Can someone who uses(or knows about) managed switches answer simple ?

my goal is to monitor traffic by IP. my network uses the 10.214.6.xxx ip range and there are roughly 100 computers connected to it. All computers are connected to a switch which then connects to another switch (unmanaged) which then connects to the internet modem. my question is:

If I get a cheap/used Managed switch off ebay etc, can I just replace the Unmanaged switch connected to our internet modem... would i be able to monitor the traffic of each ip? or does it just monitor traffic to each port so I could only see how much traffic is coming from each other switch?

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Or would you guys suggest any other method of monitoring traffic which would work for our current setup? maybe getting some other device (router with dd-wrt?) to replace that main switch?

thanks