You cannot reserve elastic IP addresses for an elastic network created after you enable the multiple external egresses feature in the controller plug-in.
On the top navigation bar, click Cloud Services, and then select Elastic IPs from the Network menu.
From the left navigation pane, select Advanced Settings > Elastic Networks.
Click an elastic network name.
Click the Reserved IPs tab, and then click Create.
Select a subnet, and then enter an IP address or IP address range.