OSDOCS-3801: Add OVN-Kubernetes internal subnet configuration

- https://issues.redhat.com/browse/OSDOCS-3801

Author: jab-rh

modules/nw-operator-cr.adoc

@@ -289,6 +289,22 @@ endif::ibm-cloud[]
 ====
  While migrating egress traffic, you can expect some disruption to workloads and service traffic until the Cluster Network Operator (CNO) successfully rolls out the changes.
 ====
+
+|`v4InternalSubnet`
+|
+If your existing network infrastructure overlaps with the `100.64.0.0/16` IPv4 subnet, you can specify a different IP address range for internal use by OVN-Kubernetes. You must ensure that the IP address range does not overlap with any other subnet used by your {product-title} installation. The IP address range must be larger than the maximum number of nodes that can be added to the cluster.
+
+For example, if the `clusterNetwork.cidr` is `10.128.0.0/14` and the `clusterNetwork.hostPrefix` is `/23`, then the maximum number of nodes is `2^(23-14)=128`. An IP address is also required for the gateway, network, and broadcast addresses. Therefore the internal IP address range must be at least a `/24`.
+
+This field cannot be changed after installation.
+|The default value is `100.64.0.0/16`.
+
+|`v6InternalSubnet`
+|
+If your existing network infrastructure overlaps with the `fd98::/48` IPv6 subnet, you can specify a different IP address range for internal use by OVN-Kubernetes. You must ensure that the IP address range does not overlap with any other subnet used by your {product-title} installation. The IP address range must be larger than the maximum number of nodes that can be added to the cluster.
+
+This field cannot be changed after installation.
+| The default value is `fd98::/48`.
 |====
 
 ifdef::ibm-cloud[]