The option masters is required for slave zones, and is used by our servers to determine who it should contact to transfer zone information. You must specify at least one IP address we can use to transfer zone information from for your domain.
You may specify multiple IP addresses here. If you do so, use a semi-colon to delimit the values, as shown below:
You will of course need to fill in your own appropriate IP addresses, specifically the IP addresses acting as master servers for your domain.