How does a router determine which subnet to send an ARP request to when forwarding packets?

Question

Computer networks

There is such a network with such a routing table, everything works fine, but I can’t explain to the teacher the question that is in the title. Thanks in advance!)

Answer 1

The destination IP address and the routing table determine which interface to send the packet to.
If the destination address is directly available on the network on an interface, then an ARP request is sent to that interface.
If the destination address is not directly accessible, then the packet is sent to the next router according to the routing table.
This is the usual scheme of IP routing, it is the same on the Cisco gateway, in Windows at home.

Answer 2

The router does not send anything between subnets. The packet has a destination IP. For this IP address, an ARP broadcast request is sent within the network segment.
Nothing will happen to a large network. For this, there are cached MAC address tables, switches instead of hubs, routers. Show the teacher at least an article on Wiki https://ru.wikipedia.org/wiki/ARP If the teacher is very stubborn, then you can take Tanenbaum's book or Cisco ICND.

Answer 3

Watch the lecture , maybe it will be clearer. For 40 min. about your topic.

Answer 4

Doesn't he send a broadcast request?

Answer 5

Well, firstly, it broadcasts only when it wants to find the next hop, and then it sends to find the poppy address of this very next hop.
Secondly, purely theoretically, you can get around without ARP'a, as it was before when hub was used.
I will try to explain what happens in the diagram when the upper left computer with an address, for example 192.168.0.64/25, sends a packet to the lower right computer with an address, for example 192.168.0.192/25
1. The routing table is analyzed to understand (which physical interface), then ->
2. Understanding what needs to be sent to another zone, then you need to select the next hop or gateway address
3. The ARP->IP table is checked (at the moment, the poppy address of our router is being searched and not the recipient at the link level, they don’t care about each other, because they are served by a router or several on different networks), if there are no matches, then a packet is opted in which instead of mac addresses of the destination are FF:FF:FF:FF:FF:FF, and the packet will receive the router and send a response where it will replace all FFs with its mac address. I note that this procedure is done only for switches, it is useless if there is no switch on the network, but it will still be executed (there are a lot of examples where there are no MAC addresses)
4. After we know the MAC address, the system allows us to make the next higher level request and this request is an IP and there is already a TCP\UDP packet.
5. when the packet reaches the router, the router sees that this packet is locally connected to its other interface, and then it performs the same procedure, requests the poppy address, and sends the packet to the recipient
6. If the recipient needs to send the packet back, then again the same thing , and the former recipient again looks at his ARP table, if there are no matches, he screams again and asks everyone, the router receiving the ARP packet seeing that it will answer him accordingly and after that the "former" recipient sends a response, and the router accepting again see what it is on the neighboring interface and again sends an ARP request.
and if there were 10 routers between these clients, then each router, before sending a packet to its neighbor, would each time ask its MAC using ARP.