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How to do this:
1) Log into the web management tool via port 5000 or 5001.
2) Go to Control Panel -> "Security" -> Select the "Firewall" Tab
3) Click Create. The "Create Firewall Rules" window will come up
4) Ports -> All
5) Source IP -> Subnet
6) IP Address: 18.0.0.0
7) Subnet Mask: 255.0.0.0
8) Action -> Allow
9) Click OK
10) Click on the "Save" button right under the tabs. If you forget to save like I often do, and try to leave the page or close the window, it will tell you "Changes are not saved. Are you sure you want to leave?" Click on "No" and make sure you save the setting.
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Again, remember to save. Fortunately if you attempt to navigate away or close the window without saving, it will let you know again that changes are not saved.
If both of your ethernet ports are being used, you will need to apply these settings to both ethernet cards. On the tab level on the far right side you'll see a pull down menu.