Update concepts.md

Author: anavinahar

articles/load-balancer/concepts.md

@@ -61,6 +61,7 @@ Availability Zones | Standard load balancer supports additional abilities in reg
 
 ## <a name = "limitations"></a>Limitations
 
+- A load balancer rule can't span two virtual networks.  Front-ends and their related backend instances must be located in the same virtual network.  
 - Web Worker Roles without a virtual network and other Microsoft platform services can be accessible from instances behind only an internal standard load balancer. Don't rely on this accessibility, as the respective service itself or the underlying platform can change without notice. If outbound connectivity is required when using a standard internal load balancer, [outbound connectivity](load-balancer-outbound-connections.md) must be configured.
 - Load balancer provides load balancing and port forwarding for specific TCP or UDP protocols. Load-balancing rules and inbound NAT rules support TCP and UDP, but not other IP protocols including ICMP.