Answer, Config VM: IP Addressing

By certskills November 24, 2014 09:05

This blog post simply answers the previous Config museum post from a few days ago. This post makes no sense without the question, so don’t look until you read the other post. No guile, no tricks, just a chance to exercise.

The topic for this post: finding the highest IP addresses in each of six subnets, and then configuring those on Cisco router interfaces. Details below the fold!

Answers: Finding and Configuring the Highest IP Addresses

To find the lowest IP addresses, first calculate the subnet ID, and then calculate the subnet broadcast address for that same subnet. From there, subtract 1 to find the highest usable IP address in the subnet, subtract one more for the next highest, and so on. I won’t go into how to find the subnet ID and broadcast address – that’s all over the books – but I’ll entertain specific questions if you have them. Then you just need to configure the IP addresses using the ip address address mask interface subcommand, as shown here.

This is a relatively simple exercise, but especially for newbies, there’s nothing wrong with asking questions. Fire away. The next Config Museum will build on this config and topology, so stay tuned!

Here’s a quick repeat of the figure, for reference:

Figure 1: Router Triangle with Sample IP Addresses for Calculations


Table 1 lists the results of choosing the highest IP addresses, by router and interface:


Location IP Mask
R1 G0/0
R1 G0/1
R1 G0/4
R2 G0/0
R2 G0/1
R2 G0/4
R3 G0/0
R3 G0/1
R3 G0/4

Table 1: IP Addresses and Locations


Example 1: R1 Config


Example 2: R2 Config


Example 3: R3 Config



Try This at Home!

The lab topology uses particular router interfaces on purpose. If you download Cisco’s All-in-One VM, the network simulator that comes with AIO (for free) implements this same topology. To do so, you just have to start the default topology, and then shutdown a few interfaces (namely, all the routers’ G0/3 interfaces). For more detail, check out this earlier post, part of a series about the AIO VM, VIRL, and Cisco’s plans.


Config VM: IPv4 Addressing, OSPF, and Fallback Static Routes
Config VM: Static Routes
By certskills November 24, 2014 09:05
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