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I am reading this article https://www.practicalnetworking.net/series/packet-traveling/host-to-host-through-a-router/ , trying to learn how a packet is delivered between different networks.

It says:

**From the perspective of each Router, the Routing Table is the map of all networks in existence. The Routing Table starts empty, and is populated as the Router learns of new routes to each network.

and

Again, the Routing Table is a map of every network that exists (from the perspective of each router). If a router receives a packet destined to a network it does not have a route for, then as far as that router is concerned, that network must not exist. Therefore, a router will discard a packet if its destination is in a network not in the Routing Table.

What does "the Routing Table is the map of all networks in existence" mean?

Does it mean that every router in public network knows all the other public interconnected networks in the world? (Of course for routers in local/private networks, I think they don't know other public networks, but only know the default gateway (the outermost router).)

Take these 2 images as basic model, if they are correct.

enter image description here

enter image description here

Let's say for example I am in Japan, in network 11.11.11.x and my destination is 222.222.222.222 in Canada, which is in network 222.222.222.x (What I want to say is that there are now having so many more networks between 11.11.11.x and 222.222.222.x). Will the router R1 has a route like:

 Method      network     Interface/Next-Hop
  ...        ...            ...
   ?     222.222.222.x   22.22.22.2

R1 has to know the existence of 222.222.222.x(all the other public networks in the world), or R1 won't be able to deliver the packet even if it knows network 22.22.22.x, according to the article.

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 15 '19 at 18:44
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the Routing Table is a map of every network that exists

Yes, this is more or less true. However, the every network that exists does not mean every network in the whole world.

From your first picture, every networks means 11.11.11.0/24, 22.22.22.0/0, 33.33.33.0/0. As their is nothing else, then that's the whole list. Keep in mind that routers can learn default route or summarized routes as well. As an example, R1 could work without knowing 33.33.33.0/0, but with a default route via R2. That mean whenever their is a destination that R1 does not know (so everything except 11.11.11.0/24 and 22.22.22.0/24), it will send the packet to R2, which then know where 33.33.33.0/24 is and can forward the packet to the final destination.

Let's say for example I am in Japan, in network 11.11.11.x and my destination is 222.222.222.222 in Canada, which is in network 222.222.222.x (What I want to say is that there are now having so many more networks between 11.11.11.x and 222.222.222.x). Will the router R1 has a route like:

Yes. When you're using the Internet, BGP routers know every routes in the public IPv4 range. As of 2018, this is equivalent to 760,000 routes.

  • Thanks! The last paragraph is very helpful. I just wanted to check the number of routers but didn't know where to. But one question with your answer: according to the quoted text from the article, it's said that in order to deliver a packet, R1 must know the existence of 222.222.222.x beforehand, though not directly connected to. (Your answer is contradictory to this point of view.) – Rick Sep 10 '19 at 6:28
  • I've re-edtied my question and clarified something. I think I get your point. You said "the every network that exists does not mean every network in the whole world.". Yes, but I mean every router in public network knows all the other public networks. :) – Rick Sep 10 '19 at 6:45
  • Yes, R1 must know where 222.222.222.0/24 is. But the only needed information is what is the next hop to reach this subnet, so the IP address of the next router in the path. – Nakrule Sep 10 '19 at 6:52
  • Ah yes, that's my point. At the very least the "next hop" information must be known beforehand. I would have come up my own logical guessing if had known there are only so few BGP routers. – Rick Sep 10 '19 at 6:54

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