If you have a set of computers that can talk to each other directly, you have a network. For computers on your network to talk to computers on another network, you need a default gateway. The default gateway is a computer to which other local computers send data that is destined for a non-local computer.

一般说来，数据链路层发出的数据包称为frame，地址是链路层的地址，如mac地址；网络层发出的数据包称为packet，地址是网络层地址，如ip地址；传输层发出的数据包称为segment，地址是传输层地址，如TCP的端口号。

http://www.inetdaemon.com/tutorials/internet/ip/routing/default_gateway.shtml

Default gateway ARP