How the router determines to which subnet to send an ARP request when forwarding packets?

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There is a network with such a routing table, everything works fine, but I can't explain to the teacher the question in the title. In advance thank you!)
July 12th 19 at 17:12
5 answers
July 12th 19 at 17:14
Solution
For the destination IP address and the routing table determines which interface to send the packet.
If the destination address is directly available in the network interface that sent the ARP request in this interface.
If the destination address is not directly available, the packet is sent to the next router according to the routing table.
This is a common scheme of work IP-routing, it is the same that the Cisco gateway, in the Windows of the house.
"According to the destination IP address and the routing table determines which interface is" here it is possible more in detail? What the data in the routing table it looks? - delbert_Schuppe commented on July 12th 19 at 17:17
: He looks at LPM. Finds the Longest Prefix Match, there is - or nexthop address, if a route via a different router interface or nexthop, if the network directly to the router connected. If there interface makes a broadcast ARP request to a specific node via the specified interface, if the address - it recursively looks for how to get to this address in the same procedure.
When prescribing the ip address on whichever interface the router automatically adds for that subnet nexthop interface entry in its routing table with the interface on which that address. - nina commented on July 12th 19 at 17:20
: thank you - delbert_Schuppe commented on July 12th 19 at 17:23
July 12th 19 at 17:16
The router sends nothing between subnets. The package has destination IP. For this IP address within the network segment sends a broadcast ARP request.
Anything with a large network will not happen. For this and there are the cached MAC addresses, switches instead of hubs, routers. Show the teacher at least an article in Wiki https://ru.wikipedia.org/wiki/ARP If the teacher is very high, you can take a book of Tanenbaum or Cisco ICND.
The teacher is a normal question asked. It is evident that the student cannot explain. - delbert_Schuppe commented on July 12th 19 at 17:19
: I don't blame him (the teacher), but he is very picky) - nina commented on July 12th 19 at 17:22
: Yes, right. Such things must be understood at the level of physics. - delbert_Schuppe commented on July 12th 19 at 17:25
Just redid the network now consists of 3 subnets with a mask of 255.255.255.192, duck here when some kind of network he receives the ARP request, it does not send it to anyone who is connected and sends only the desired network, how he does it I want to understand - Madge commented on July 12th 19 at 17:28
Yeah, the only specialty is not a technical - delbert_Schuppe commented on July 12th 19 at 17:31
Although met admins who do not understand or work for a decent RFP. - Madge commented on July 12th 19 at 17:34
: So the topic is not difficult. - delbert_Schuppe commented on July 12th 19 at 17:37
: How the router determines to which subnet to send an ARP request when forwarding packets? I would appreciate an answer) I understand the case in the gateway? - delbert_Schuppe commented on July 12th 19 at 17:40
The gateway keeps a routing table. - Madge commented on July 12th 19 at 17:43
July 12th 19 at 17:18
He did not broadcast the request?
When I said it sends an ARP request, he said, "And? Sends all chtoli? Imagine what would happen with a large network?" So probably no) - delbert_Schuppe commented on July 12th 19 at 17:21
Nothing will happen to a large network. - nina commented on July 12th 19 at 17:24
July 12th 19 at 17:20
View a lecture, maybe it will be clearer. 40 min. about your topic.
July 12th 19 at 17:22
Well, firstly, it sends a broadcast only when he wants to find the next hop, and sends it to find the Mac address of this next hop.
Second, theoretically it is possible to get around without a ARP'as it was earlier when you used the hub.

I'll try to explain what is happening in the diagram when the upper left computer address e.g. 192.168.0.64/25 sends the packet to the right down the computers address for example 192.168.0.192/25

1. Examines the routing table to understand (a physical interface), then ->
2. Realizing that it is necessary to not send to another zone, it is necessary to choose the address of the next hop or gateway
3. Checked the ARP table->IP(at the moment, it looks up the Mac address of our router and not the receiver on the link layer them on top of each other do not care, maybe they are on different networks which are served by the router) and if no match then optasense package in which instead of the mac addresses of the recipient are FF:FF:FF:FF:FF:FF, and the package will receive the router and sends an answer which will replace all FF on your Mac address. Notice that this procedure is for switches, it is useless if the network no switch, but will still be run(there are plenty of examples where there is no poppy of address)
4. After we learned the Mac address, the system allows us to make the next higher level request and the request is an IP and there are already TCP\UDP packet.
5. when the package daydet up to the router then the router looks that this package is locally connected to its other interface, and then he performs the same procedure zapraszamy Mac address, and sends the package to the recipient
6. If the recipient should send the packet back the same again, and the former recipient again looks to its ARP table if no match shouts again and asks at all, the router is receiving ARP package seeing that it will respond accordingly and after that "former" recipient sends a reply, and the router taking again see what is on the neighboring interface, and again otpravlyaet ARP request.

and if we had 10 routers between the clients, each router before sending the package to the neighbor every time I asked him a MAC by means of ARP.
Yes, and your table is not full, all the tables are always specified and even interface - delbert_Schuppe commented on July 12th 19 at 17:25
: 4th column, not the interface? - nina commented on July 12th 19 at 17:28
in this case, the alias, and then this emulator.. well, as you can understand the interface is both something physical between channel<->physical, and in this case, the "interface" level "operating system(or program)"<->"service which performs a network communication" - delbert_Schuppe commented on July 12th 19 at 17:31

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