1. Navigate to Interfaces | LAN.
2. Check the Enable Interface checkbox:

3. Choose an IPv4 Configuration Type (usually Static IPv4).
4. Choose an IPv6 Configuration Type (or leave it set to None).
5. Enter an IPv4 Address in the appropriate field, and the correct CIDR in the adjacent drop-down box. Leave IPv4 Upstream gateway set to None.
6. If you enabled IPv6 by setting the IPv6 Configuration Type, enter an IPv4 Address in the appropriate field and the correct CIDR in the adjacent drop-down box.
7. Leave Block private networks and Block bogon networks unchecked (they should be unchecked by default).
8. When you are done making changes, click on the Save button. When the page reloads, click on the Apply Changes button.