Comment 3 for bug 1842008

@jam it's used in the method updateForRemoteRelationIngress()

cidrs := set.NewStrings()
// If the unit is exposed, allow access from everywhere.
if unitd.applicationd.exposed {
    cidrs.Add("0.0.0.0/0")
} else {
    // Not exposed, so add any ingress rules required by remote relations.
    if err := fw.updateForRemoteRelationIngress(unitd.applicationd.application.Tag(), cidrs); err != nil {
        return nil, errors.Trace(err)
    }
}