In case it wasn't clear in my example above:

`$ neutron router-show 5b66e186-83e3-462f-9a69-934d489fee1e | grep subnet_id`

...contained...

{"subnet_id": "57496540-3291-41ed-b58e-da879c08ec37", "ip_address": "fdd6:a1fa:cfa8:962e::1"}