how to monitor connections of Oracle and SQL databases

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Hi we get messages from our users that there are connections loss between their client and our Oracle and/or SQL databases

Is the someone who has a suggestion how we can monitor the connections to find our problem

Our databases are in out x.x.1.x sigment and the clients are in the x.x.1.x and x.x.2.x sigment

We don't have Oracle experts in the house but we know a lot.