For an outgoing IP packet,entering IP from an upper layer,
IP must decide whether to send the IP packet directly or indirectly,
and IP must choose a lower network interface.These choices are made by
consulting the route table.

对一个IP发送包,IP必须决定是否直接或间接发送这个包,

并且选择一个下面的网关.这些选择是由路由表来决定的.

For an incoming IP packet ,entering IP from a lower interface,IP
must decide whether to forward the IP packet or pass it to
an upper layer.If the IP packet is being forwarded,it is
treated as an outgoing IP packet.

Where an incoming IP packet arrives it is never forwarded  backout through the same network interface.

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