Layer 3 forwarding is the forwarding of routers at different times. It generally works on the third layer of the TCP/IP network model. Layer 3 forwarding can be complex or simple. Today we will talk about the definition of the network layer and the basic principles of three-layer forwarding.
1. Definition of the network layerNetwork layer: It is located in the third layer of the OSI model, between the transport layer and the data link layer, providing information transmission services between the sender and the destination. The data of the network layer is transmitted in the form of an IP datagram.
IP provides an unreliable, connectionless datagram delivery service. It defines the specific format for transmitting data over the Internet. IP implements two basic functions: addressing and fragmentation.
The IP address consists of a network number and a host number. The network number identifies a specific network in the Internet. The host number identifies a specific host in the network. The most commonly used address representation method is dotted decimal notation. For example, 192.168.10.1 for the specific explanation of the IP protocol, see "TCP/IP Basic Learning"
3.ARPARP, an address resolution protocol, implements mapping from an IP address to a physical address.
In the Ethernet protocol, a host in the same local area network must communicate with another host directly, and must know the MAC address of the target host. In the TCP/IP protocol stack, the network layer and the transport layer only care about the target host. IP address.
This results in the Ethernet protocol of the data link layer being connected to the data provided by the upper layer IP protocol when the IP protocol is used in the Ethernet, including only the IP address of the destination host. Therefore, a method is needed to obtain its MAC address according to the IP address of the destination host. This is what the ARP protocol is going to do. (For specific knowledge of ARP, see "TCP/IP Fundamental Learning")
4. The concept of basic forwardingIn a data communication network, communication between devices is basically based on the TCP/IP protocol. A device with a specific function running in an IP network can be called an end system. A PC is one of the most common end systems, and a network. The routers and switches are used to interconnect different end systems for transparent communication.
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