IP addresses route a packet across the whole global Internet, while MAC addresses help the packet make the small, local hop between hardware devices.
IP地址在整个全球因特网中路由数据包，而MAC地址则帮助数据包在两个硬件设备间做狭小的局部的传递。
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The NAT router translates the inside global address of the packet to the inside local address and then checks the routing table before it sends it to the destination computer.
When a computer on the stub domain that has an inside local address wants to communicate outside the network, the packet goes to one of the NAT routers by way of normal routing to the default-gateway.