code: def _make_ip_subnet_lookup(vpc_info):
      
      # We create a reverse lookup from the instances private IP addresses to the
      # subnets they are associated with. This is used later on in order to
      # determine whether routes should be set in an RT: Is the RT's subnet
      # associated with ANY of the IP addresses in the route spec? To make this
      # easy, we collect the IPs and subnets of all EC2 instances in the VPC.
      # Once we get a route spec, we create a list of subnets for only the
      # cluster nodes. The assumption is that not all EC2 instances in the VPC
      # necessarily belong to the cluster. We really want to narrow it down to
      # the cluster nodes only. See make_cluster_node_subnet_list().
      vpc_info['ip_subnet_lookup'] = {}
      for instance in vpc_info['instances']:
          for interface in instance.interfaces:
              subnet_id = interface.subnet_id
              for priv_addr in interface.private_ip_addresses:
                  vpc_info['ip_subnet_lookup'][priv_addr.private_ip_address] = \
                                                                      subnet_id
docs: Updates the vpc-info object with a lookup for IP -> subnet.