When a host IP packet sent to the destination is forwarded through multiple routers, does the destination IP change?

When a host IP packet sent to the destination is forwarded through multiple routers, the destination IP address remains unchanged.

When the ip packet is forwarded via routing, do the source ip and destination ip change?

This question is often asked in recent interviews with network professionals. The answer cannot be changed unless a nat conversion is performed. (Recommended learning: web front-end video tutorial)

But the mac address changes, because the sending end does not know the mac address of the destination host at first, so the mac address changes every time it passes through a router .

How to get the destination mac address?

TCP/IP uses the ARP protocol. For example, if a new intranet is created, if machine A is looking for machine B, when encapsulating FRAME (the data format used by the second layer of OSI), the other party's MAC must be encapsulated. At the beginning, A does not know B's MAC, but only knows the IP. It sends an ARP packet, the source IP is its own, the destination IP is B's, the source MAC is its own, and the destination MAC is broadcast.

Then the request packet is broadcast within the intranet. When other machines receive the packet, they compare the destination IP with their own IP. If not, they discard it. When B received it, he found that the IP was the same as his own, so he agreed to the request of the packet and sent his MAC to A. If B is a machine in another subnet, then the router will determine that B is in another subnet, and then the router will return its MAC to A. When A sends a packet to B in the future, the destination MAC will be encapsulated by the router.

Routing and forwarding process:

When the data flow sent from host A to host B is encapsulated into an IP data packet at the network layer, the header of the IP data packet contains the source address and destination address. Host A will use the 24-bit IP network mask 255.255.255.0 configured on this machine to perform an AND operation with the target address to determine whether the target network address and the network address of this machine are in the same network segment. If not forward the IP packet to the gateway.

Host A will also obtain the MAC address of the default gateway through an ARP request before sending it to the gateway. The IP data packet at the data link layer of host A is encapsulated into an Ethernet data frame and then sent to the gateway... which is a port on the router.

When the gateway router receives the Ethernet data frame and finds that the target MAC address in the data frame is the physical address of one of its own ports, the router will remove the encapsulation of the Ethernet data frame. The router thinks that this IP packet is to be forwarded through itself, and then it matches the routing table. After matching the routing entry, it sends the packet to the next address.

This is how the router forwards data packets, so it will never change the IP address. Only the MAC will be changed.

When a data packet is transmitted to the router, the router first compares its destination address with the routing table. If it is a local network, it will not be forwarded to the external network. Instead, it is forwarded directly to the destination host in the local network; but if the destination address is compared with the routing table and it is found that it is not in the local network, if there is NAT, the IP of the source address will be changed (the IP address of the original source address will be changed to the IP address of the router) ), the router forwards the data packet to the corresponding port for communication.

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