In the shared network, each instance is assigned a private IP address from the common pool of Oracle provided IP addresses. In an IP network, you can define an IP subnet in your account. The address range of the IP network is defined by the IP address prefix defined when creating the IP network. The following diagram from Oracle Documentation  explains the two network offerings:
The main differences between the two network options are:
- The cloud instance IP can change In a shared network after you stop or delete the instance. Therefore, you need to keep your router or any applications connecting to the instance updating the IP after these operations.
- IP network allows you to isolate the private network from the public Internet.