network address translator (nat)
a network address translator not only examines but also modifies the header information in packets flowing across the boundary, allowing many hosts behind the nat to share the use of a smaller number of public ip addresses (often one). network address translators in turn have two main varieties:
网络地址转换（nat）
当有数据包通过时，网络地址转换器不仅检查包的信息，还要将包头中的ip地址和端口信息进行修改。以使得处于nat之后的机器共享几个仅有的公网ip地址（通常是一个）。网络地址转换器主要有两种类型：
basic nat
a basic nat maps an internal host's private ip address to a public ip address without changing the tcp/udp port numbers in packets crossing the boundary. basic nat is generally only useful when the nat has a pool of public ip addresses from which to make address bindings on behalf of internal hosts.
基础nat
