5.如果是 .tw 之類的在 root dns server 沒記錄，會再往下找，可以輸入以下指令試看看
dig +trace https://www.webnode.tw/
DNS Server 種類
Root Name server
TLDs Server
Authoritative name server
過程
local ISP's DNS server will send a query to the root server. The root servers won’t have information on a specific IP address for www.securitytrails.com, but it will know where the name servers that serve that TLD(.com) are.
Root servers will return the list of TLD servers so the provider or configured server can again send a query, this time to a TLD server.
The TLD server will then return the authoritative name server where the desired domain is stored.
Once the request has reached the authoritative server, it will respond to the requesting server with the IP address
NS record 通常會有多個 DNS server 用來表示有哪些 DNS Server 可供查詢，SOA record 通常只有一個，指向 Master DNS Server，可讓其他 Slave DNS Server Sync 資料，並且 SOA record 後面會跟著一些數字代表 Master 與 Slave 的資料同步策略。
NS records are used to redirect DNS resolver to the next DNS server that is hosting the next level zone. And, SOA record is used by cluster of DNS servers to sync latest changes from the master to secondary servers
It’s because of the limitations of the original DNS infrastructure, which used only IPv4¹ containing 32 bytes. The IP addresses needed to fit into a single packet, which was limited to 512 bytes at that time. So, each of the IPv4 addresses is 32 bits, and 13 of them come to 416 bytes, leaving the remaining 96 bytes for protocol information.
