This setting is optional. You can add DNS servers to your virtual network for name resolution. If you want to have name resolution ...

This setting is optional. You can add DNS servers to your virtual network for name resolution. If you want to have name resolution between this virtual network and your on-premises network, you should specify the DNS servers that are used for your on-premises name resolution. You can also specify public DNS servers. If you do not specify a DNS server, name resolution will be provided by Windows Azure.