# Simple routing table question

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?

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

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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

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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