0:23 switch 0:54 1:10 packet 1:55 *switch only cares about the MAC address* , does not care about IP address at all 2:08 router 2:10 router的两种接口 - LAN + WAN 2:14 LAN 2:25 the only one WAN 2:38 gateway *4:01* 子网的意义 - 4:04 according to TCP/IP protocol, 2 subnets cannot communicate directly with each other 4:08 need a gateway to forward data 4:12 there are 2 network cards in a gateway