B: find network iface by CIDR containment
B: find network iface by CIDR containment

rather than a string prefix, in case the ip in the addr of the network
interface isn't precisely the external ip address

Signed-off-by: Aidan Oldershaw <30ef7a6e0d016bc846f26d89dacbd23aceded206@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse