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If you miss assigning a Private IP to your VM during the creation you can still do so after the fact.
1. Right click on the VM and select Properties.
2. Click on the Hardware tab, scroll to the NICs section and click on the Add button.
3. Choose Organization VDC network and click Next.
4. Select your network and click Finish.
5. Your Network will now be attached. You can define the IP address you want to assign with Static - Manual in IP Mode or leave it as IP Pool.
That's it. Don't forget to configure that IP inside your Virtual Machine (Guest OS level).