各种L2数据网具有不同的通信协议与帧结构，其网络节点设备可以是各种类型的数据交换机（X.25、FR、Ethernet和ATM等分组交换机）；而L3数据网（IP网或internet）具有统一的TCP/IP通信协议和分组结构，全网采用统一格式的IP地址， 节点设备统一采用面向无连接的路由器。为便于理解，我们可以将L3数据网简单地理解为构建在L2数据网平台上的一个路由器网。-L2 data network with a variety of different communication protocol and fr a me structure, the network node device can be of various types of data switches (X.25, FR, Ethernet and ATM packet switches, etc.)　and L3 data network (IP network or internet) have a unified TCP/IP communication protocol and packet structure, the whole network using a unified format for the IP address, node equipment reunification-oriented connectionless router. In order to facilitate understanding, we can L3 data network simply interpreted as a data network built on the L2 platform for a network router.