IP datagram sending and forwarding process

Hosts in the same network can communicate directly ----- Direct delivery
Routers of different networks need routers to transit ---- Indirect delivery

Sending process ：

host C To his own ip The address matches its own subnet mask , Be the host C Network address of ;
host C Then set the host F The network address of and its own subnet mask match Get the destination network address ;
Discover the host F Destination network address and C The same as your own will be delivered directly ; If it is not the same , The router is not required to forward packets for indirect delivery in the same network ;
host C Transmit the datagram to the router of the network , The router forwards the datagram to the host F

Forwarding process ：

（1） First, you need to extract the destination address from the header of the datagram , Check Head check sum Is it wrong , If there is an error, discard the datagram and notify the host .
（2） Match the subnet mask of this router , See if it can be delivered directly . That is, whether the network where the destination address is located is directly connected to the router , If connected, it can be delivered directly , There is no need to forward through other routers . otherwise , execute 3.
（3） Check whether the routing table has a corresponding Specific host routing （ Host specific routes are used to control 、 Test network ）, If yes, it will be forwarded to the next hop address specified by the specific host routing , Otherwise execution 4.
（4） Check to see if you have arrived The router of the destination network , If there is , Then forward according to the next hop address specified by the general route that meets the rules , Otherwise execution 5.
（5） If the routing table in the router is set to Default route , Then the packet is forwarded to the address indicated by the default route , Otherwise execution 6
（6） Discard the packet , And report forwarding failure

Be careful ：

1. Each router selects the next hop required by the target network according to the routing table （ The physical port number of the router or the next network address ）, and ip After the packet arrives at the target network through multiple routes , Change to target LAN Through the link layer MAC Address is addressed by broadcast ！（IP Address conversion to MAC Address , Stored in MAC In the head of the frame ） This can improve the efficiency of routing
2. although ip Datagrams have complete sources ip Address and purpose ip Address , But the router only depends on The network number of the destination address Routing ！
3. Link layer in LAN , You can only see MAC Address . And forward by route ip In groups ,ip Packets are unpacked and repackaged by routers in each network （ Framing ）, therefore MAC The address is constantly changing ！ This also determines that it cannot be passed MAC Address cross network communication ！
4. Because routers interconnect multiple networks , So not only are there multiple ip Address , There are multiple hardware addresses
5. No matter what protocol the network layer uses , When data frames are transmitted over the actual network , In the end, the hardware address must be used , So we need a way to accomplish ip Address to MAC Address mapping ----- Address resolution protocol ARP
   ARP Used to solve a problem LAN Host in / The router ip Address and MAC Address mapping problem , If the host you are looking for is not on the same network as the source host , Through ARP Found at this LAN The hardware address of a router of , Then forward the packet to the router

Broadcast storm ：

If the host A Send a broadcast datagram to each device in the network （ip All the host numbers are 1）, namely ip The destination address of the datagram is the broadcast address ,
The router receives the ip The datagram is not forwarded after the datagram ！
That is, the router isolates the broadcast domain
If the router forwards the broadcast datagram after receiving it , Will cause a huge broadcast storm , Serious waste of resources

Repeaters and hubs are at the physical layer , Neither conflict domain nor broadcast domain is isolated
Bridges and switches （ Multi port on bridge ） Working at the data link layer , You can isolate conflict domains , Can't isolate broadcast domain
Routers work on the network layer , Isolate the conflict domain , Also isolate the broadcast domain

Routers do not provide reliable transmission ！