Update vpn-gateway-faq-nat-include.md

Author: stegag

includes/vpn-gateway-faq-nat-include.md

@@ -74,3 +74,7 @@ Yes, this is typically used when the connections are for the same on-premises ne
 ### Do I need both Ingress and Egress rules on a NAT connection?
 
 You need both Ingress and Egress rules on the same connection when the on-premise network address space overlaps with the VNet address space. If the VNet address space is unique among all connected networks, you don't need the EgressSNAT rule on those connections. You can use the Ingress rules to avoid address overlap among the on-premises networks.
+
+### What do I choose as "IP configuration ID" ?
+
+"IP configuration ID" is simply the name of the IP configuration object you want the NAT rule to use. With this setting, you are simply choosing which gateway public IP address applies to the NAT rule. If you haven't specified any custom name at gateway creation time, the gateway's primary IP address is assigned to the "default" IPconfiguration and the secondary IP is assigned to the "activeActive" IPconfiguration.