Because the reverse domain lookup can be anything the owner of the address block wants, the second lookup is necessary to ensure that no spoofing is taking place.
因为反向域名搜索可以由地址块所有者确定，所以要防止ip欺骗的发生，必须进行第二次查找。
2
This helps to protect the corporate domain (and the company's reputation) from spoofing and identity forging.