How can I check if a network is wholly contained in another network in Python, e.g. if `10.11.12.0/24` is in `10.11.0.0/16`? 

I've tried using `ipaddress` but it doesn't work:

```
>>> import ipaddress
>>> ipaddress.ip_network('10.11.12.0/24') in ipaddress.ip_network('10.11.0.0/16')
False

```