def allocate_fixed_ip(self, context, tenant_id, quantum_net_id, vif_rec):
"""Allocates a single fixed IPv4 address for a virtual interface."""
admin_context = context.elevated()
network = db.network_get_by_uuid(admin_context, quantum_net_id)
address = None
if network['cidr']:
address = db.fixed_ip_associate_pool(admin_context,
network['id'],
vif_rec['instance_id'])
values = {'allocated': True,
'virtual_interface_id': vif_rec['id']}
db.fixed_ip_update(admin_context, address, values)
return address
