diff --git a/modules/net-vpc/README.md b/modules/net-vpc/README.md
index 5271814a..5e982798 100644
--- a/modules/net-vpc/README.md
+++ b/modules/net-vpc/README.md
@@ -163,3 +163,4 @@ module "vpc-host" {
 | subnets | Subnet resources. |  |
 <!-- END TFDOC -->
 
+The key format is `subnet_region/subnet_name`. For example `europe-west1/my_subnet`.