What is a DNS Zone?
All pool members must be managed in the same DNS zone when creating pools that span multiple subnets.
A DNS zone is where you store name information about one or more domains you manage. You can divide your network into multiple subordinate DNS zones to aid in management, organization, or speed. The DNS zone you use for HA pools can be your organization-wide DNS zone but is more likely to be a subspace.
For example, solarwinds.com is our organization-wide DNS zone, but we could decide to manage all North American subnetworks on a separate DNS zone and use the na.solarwinds.com subdomain to access them. While the DNS server for solarwinds.com will have some records for na.solarwinds.com, the DNS server in the na.solarwinds.com DNS zone is the authoritative name server and all management activities for that subdomain occur in that DNS zone.