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I'm having difficulties understanding the solution to the following question:

Consider the following description of a LAN:

The main Internet connection is connected to our gateway router. This in turn is part of the backbone network on the 65.21.1.0/24 subnet. The backbone also supports two further routers SALES and RESEARCH. There is one Sales subnetwork 65.21.6.0/23, and two research subnetworks 65.21.100.0/24 and 65.21.200.0/26.

If the Interface addresses are:   

• i1–65.21.1.1
• i2–65.21.1.2
• i3–65.21.6.1
• i4–65.21.1.3
• i5–65.21.100.1
• i6–65.21.200.1

What are the routing tables for the SALES and RESEARCH routers?

enter image description here

How is the via and interface column derived in the solution?

enter image description here

Thanks in advance.

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Is this a homework exercise? If so, it has probably been derived with one of the methods presented in lecture or in the companion book. –  Raphael Jan 17 '13 at 13:35
2  
The answer is already in the figure you drew. Just follow the links .. –  AJed Jan 18 '13 at 2:37

1 Answer 1

up vote 4 down vote accepted

This is a simple question on the definition of routing tables. I will add a brief explanation.

Let's take the SALES router. It has two interfaces.

The interface top (65.21.1.2) is connected to the network 65.21.1.0/24.

The interface bottom (65.21.6.1) is connected to the network 65.21.6.0/23.

This can be expressed as the following routing table:

network          via        interface
---------------+-----------+---------
65.21.1.0/24     direct      top
65.21.6.0/24     direct      bottom

Now we have two research networks accessible via the router RESEARCH. This router has an ip address 65.21.1.3 on the network 65.21.1.0/24. This network is already in the routing table, so the traffic to the research networks can be routed with the following rules.

network          via        interface
---------------+-----------+---------
65.21.100.0/24   65.21.1.3   top
65.21.200.0/24   65.21.1.3   top

Finally the traffic to all other networks should be sent to the GATEWAY for further processing. Here is the final routing table.

network          via        interface
---------------+-----------+---------
65.21.1.0/24     direct      top
65.21.6.0/23     direct      bottom
65.21.100.0/24   65.21.1.3   top
65.21.200.0/24   65.21.1.3   top
default          65.21.1.1   top

The routing table for the RESEARCH router can be assembled following the same logic.

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