DNS lookup is the network process by which a DNS record is returned from a DNS server to a client. Interconnected computers, servers and smart phones need to know how to translate the email addresses and domain names people use into meaningful numerical addresses. A DNS lookup performs this function. When you type in a domain name, the request is forwarded to a DNS server, which returns the corresponding IP address. This address is then used by all the computers and routers to channel the request and responses of a user’s session.
The result is the user sees web pages as expected or has email show up in an in-box. 两种类型的DNS lookups是向前DNS lookups和反DNS lookups. 向前DNS lookup是使用互联网域名，以找到一个IP地址. 反DNS lookup是使用互联网的IP地址找到一个域名. 当你进入地址的网站浏览器 (地址是正式称为统一资源定位器, 或URL), 地址传送到附近的一个路由器，并向前DNS lookup在一个由表找到IP地址.
向前DNS (它代表域名系统) 查找的是更常见的查找由于大多数用户认为在条款的域名称而不是IP地址. 然而, 偶尔你们可以看到一个网页，网址在其域名称的一部分，是表达作为一个IP地址 (有时称为一点的地址) 并希望能够看到它的域名. 一个互联网设施，让你做任向或反DNS lookup自己被称为nslookup. 它与一些操作系统或者你可以下载的程序，并将其安装在计算机.
A reverse lookup is a query of the DNS for domain names when the IP address is known. Multiple domain names may be associated with an IP address. The DNS stores IP addresses in the form of domain names as specially formatted names in pointer (PTR) records within the infrastructure top-level domain arpa. IPv4, the domain is in-addr.arpa. For IPv6, the reverse lookup domain is ip6.arpa.
The IP address is represented as a name in reverse-ordered octet representation for IPv4, and reverse-ordered nibble representation for IPv6. When performing a reverse lookup, the DNS client converts the address into these formats before querying the name for a PTR record following the delegation chain as for any DNS query. When the DNS resolver gets a pointer (PTR) request, it begins by querying the root servers, which results in an authoritative response.
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