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Then the network needs at least one subnet; and the first subnet is created in the "subnet" pane. Give it a name, and select an IP range. It is not necessary to fill in anything for Gateway IP, it has a sane default (the first usable address in your selected subnet). See this article to see which addresses to select.
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Select the desired subnet from the "Subnet" list, and submit the form. Note that the IP address is optional. That means it has a sane default (the first addressable IP in the selected subnet).
A look at the Network Topology graph should display something like so:
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