source
If not empty, this Forwarding Rule will only forward the traffic when the source IP address matches one of the IP addresses or CIDR ranges set here. Note that a Forwarding Rule can only have up to 64 source IP ranges, and this field can only be used with a regional Forwarding Rule whose scheme is EXTERNAL. Each source_ip_range entry should be either an IP address (for example, 1.2.3.4) or a CIDR range (for example, 1.2.3.0/24).