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I am trying to understand how a router's forward table works. As such, I am working on the following problem.

Problem:
A router’s forwarding table has the following entries. \begin{array} {|r|r|}\hline Network \, Number & Next \, Hop \\ \hline 0.0.0.0/0 & 15.15.1.2 \\ \hline 192.140.20.0/24 & 15.15.1.8 \\ \hline 192.140.20.0/20 & 15.15.1.9 \\ \hline \end{array}

Now, if the router is given an IP address like $192.140.20.3$ which row of the table would be used?

Here is how I see it. The first row of the table is not a match. It is going to match either the second or third row. The addresses given in the two rows are the same expect for the qualification. That is, the first address has a subnet mask of $24$ bits. The second one has a subnet mask of 20 bits. We do not know how big the subnet mask of the IP Address given is. So, I do not know what the router will do.

In the forwarding table given there is no metric field. If there was, in this case, it would be used to select either row 2 or row 3. Am I right about this?

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    $\begingroup$ 1. 0.0.0.0/0 matches anything. It's the default route. 2. Addresses don't have subnet masks. An address is an address. Thus the address you're given matches all three lines. But one of them is the most specific. $\endgroup$
    – rici
    Commented Sep 27, 2021 at 2:14
  • $\begingroup$ @rici What you are telling me is that the router will select either row 1 or row 2. Is that right? $\endgroup$
    – Bob
    Commented Sep 27, 2021 at 11:54
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    $\begingroup$ Line 2 is more specific, so I'd expect it to apply. If it didn't, the line would be useless, and routers don't like to store useless data. $\endgroup$
    – rici
    Commented Sep 27, 2021 at 13:45
  • $\begingroup$ some routers also have source specific entries which could even override longest match preference. but that is out of question scope $\endgroup$
    – FirstName LastName
    Commented Jul 14, 2022 at 2:37

1 Answer 1

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the router should choose the longest prefix match, or the most specific prefix that matches. so, here it will choose line 2, because of longest prefix (24 bit).

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