Comment 14 for bug 1821690

On the network side, the "get me a network" feature is used to create a private network ready to use (with a subnet, a subnet pool so you don't need to set a prefix/range, a router and connection to external public network).

So here the ext-subnet subnet does not matter for the test (the needed subnetpool is only used for the private subnet), and you can configure the subnetpool with pretty much any parameters - big enough to host all the addresses needed for the test of course.

Apparently you set everything correctly, the feature needs:
* a default external network to connect to
* a subnetpool that will be used for the created subnet in the tests

It will then create a net, a subnet (using the recommended pool), a router linking the net to the default external network.