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When the router builds its routing table , it uses the routing updates with its neighbors , but the routers on the internet don't have entry for every other destination network on the internet right ? and the routing table will be limited to a number of entries.

My question , Does the router receive updates from routers with limited distance ? like for example it will keep receive updates from all the routers 15 hops away ,but not more than that (or using any similar distance algorithm) or the router will keep adding any valid path (or Best paths with regard to Routing metrics and Administrative Distance) to its routing table until the size of routing table is full ?

PS: I'm new to routing so Please Forgive my poor knowledge

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    No. In fact, the Internet IPv4 full routing table is about 1,000,000 entries.
    – Ron Maupin
    May 10, 2023 at 23:19
  • @RonMaupin thanks for the reply , so is it possible that a router can hold all these entries inside its memory ? and if it adds until the size is full what happens when new update for new route arrive after that ? or the router just use a default route afterwards ?
    – sam
    May 10, 2023 at 23:45
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    Many Internet routers will hold the full table, but, with aggregation, not all need to. If you think about it, the ultimate aggregation is 0.0.0.0/0, but not all aggregation needs to be so complete. Different companies do what works for them, but questions about networks not under your direct control are off-topic here.
    – Ron Maupin
    May 10, 2023 at 23:55
  • @RonMaupin thanks a lot for the clarification
    – sam
    May 11, 2023 at 0:26

2 Answers 2

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but the routers on the internet don't have entry for every other destination network on the internet right ?

In fact, many routers on the Internet DO have all entries. The size of the Internet IPv4 routing table is just under 1,000,000 routes (May, 2023). These routers comprise the "Default-Free Zone." It's called the Default-Free Zone because these routers do not have a default route -- if they don't have a route to the prefix, it is dropped.

Routers outside this zone do have a default route for all unknown prefixes. Your home router is a simple example. A router in a large organization may have hundreds of internal routes, but still has a default route for all unknown routes.

Traffic from your router to unknown destinations is forwarded (via the default route) to your ISP. Your ISP's routers will have many more routes, but they may also have a default route for prefixes unknown to them. Those get forwarded to their ISP, who will likely have routers with the full Internet routing table, and traffic will be forwarded to its destination.

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  • thanks a lot all clear now
    – sam
    May 13, 2023 at 12:24
  • just one last question , my home router is only built with one WAN Port , i see 3 entries in the routing table , one to forward for the internal private LAN and the 2 others one for the network connected between the home router and ISP router and the default route , so based on one port for the WAN in home routers does this the case always ? or not always the rule ,but most of the time yes ?
    – sam
    May 13, 2023 at 12:29
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When the router builds its routing table , it uses the routing updates with its neighbors

That's one way. Directly connected networks are added automatically, and the admin can also add static routes manually.

but the routers on the internet don't have entry for every other destination network on the internet right ?

Not all routers on the Internet require a full table. Simple routers with just a single uplink use a default route to 0.0.0.0/0.

However, with two or more uplinks or gateways, the router needs to make a decision which one to use for each packet. When that decision needs to be made on the basis of distance (and possibly other factors) then you start to require a full(er) routing table. Those routers use BGP to exchange routes with their peers to learn of their connections.

and the routing table will be limited to a number of entries.

Generally, yes. But the routing table needs to be large enough for the purpose of the router.

Does the router receive updates from routers with limited distance ?

Unless you set a default route ('gateway of last resort'), the router needs to know all routes. If there's no route for a packet to be forwarded then the packet can only be dropped. Note that with very few exceptions routing information is only exchanged with adjacent/neighboring routers, which includes routes that those routers have learned from ones farther away.

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  • thanks a lot for the simple explanation and the valuable information
    – sam
    May 13, 2023 at 12:23

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