On the page showing firewall details, associate desired subnets with a firewall. After a firewall is associated with a subnet, the firewall denies all traffic to and from the subnet until you add rules to allow traffic.
The selected subnets are associated with the firewall.
Subnets that have already been associated with firewalls will not be displayed on the page for you to select. One-click subnet association and disassociation are not currently supported. Furthermore, a subnet can only be associated with one firewall. If you want to reassociate a subnet that has already been associated with another firewalls, you must first disassociate the subnet from the original firewalls.