This article puts together an IP addressing scheme based on the requirements of an imaginary organization. The math that is required to come up with each of the networks will be covered and visually displayed to ensure that the concepts are clearer to understand.

In an effort to take the concepts that were covered in “IP Subnetting Basics” and “Advanced IPv4 Subnet Concepts and Examples” and put them to use in a more real world example, this article will put together an IP addressing scheme based on the requirements of an imaginary organization. The math that is required to come up with each of the networks will be covered and visually displayed to ensure that the concepts are clearer to understand.

Requirements

The Acme University is planning to deploy an IP network in their main campus location and it wants to ensure that it is developed in a way to ensure that easy growth is possible. As the university operates strictly online, its main campus location includes only a single physical building with 4 floors and 8 total departments: Administration, Admissions, Financial Aid, Business school, Liberal Arts school, Internet Technology school, Science school, and History school. Each of the different departments needs to be separate and have their own IP address space. The address space that has been allocated to the University by their Internet Service Provider (ISP) is 172.16.0.0/23. Each of the different departments requires at least 40 different usable addresses and at least 10 extra addresses allocated for future growth.

Calculating the Correct Subnet Mask

The first thing to Figure out is whether the amount of addresses allocated for the organization is enough to meet the requirements with it. Figure 1 below shows the allocated space given by the ISP and how it is displayed in binary.

Creating a Basic IP Addressing Scheme - Figure 1

Figure 1 First and Last Range Addresses

As shown in the Figure, this gives a total range from 172.16.0.0 through 172.16.1.255 for a total of 512 addresses. The requirements stated that each of the 8 departments needed at a minimum 40 addresses with an additional 10 allocated for future growth for a total of 50 required addresses assigned per department. Given this requirement, what is the smallest subnet that would be required? Figure 2 below shows that the smallest subnet available would be allocating each network 64 total addresses.

Creating a Basic IP Addressing Scheme - Figure 2

Figure 2 Subnet Mask Calculation

In this case, the organization has the ability to extend the number of bits that are allocated for the subnetwork. This enables the allocation of 3 additional bits (borrowed from the assigned ISP host space) for separate subnetworks and 6 bits for host devices for a total of 64 total addresses per subnetwork.

The next thing to Figure out is whether the number of the addresses allocated by the ISP is enough to meet the requirements of the organization. The ISP allocated the range from 172.16.0.0 through 172.16.1.255 which equals 512 total addresses; 512 ÷ 8 (departments) = 64 addresses.

Network Range Calculation

Now that it has been calculated that the ISPs allocation of addresses is enough to meet the requirements of the organization, the next task is to come up with the different ranges that will be used to allocate to each department.

Figure 3 shows the calculation of the first range using the 255.255.255.192 (/26) subnet mask calculated above.

Creating a Basic IP Addressing Scheme - Figure 3

Figure 3 First Subnetwork Range

The first department (Administration) will be allocated addresses from 172.16.0.0 through 172.16.0.63. The next thing to calculate is which of these addresses are usable and which are reserved for the subnet and broadcast addresses. As the subnet address is the lowest number in the range, it is 172.16.0.0; as the broadcast address is the highest address in the range, it is 172.16.0.63, leaving the addresses from 172.16.0.1 through 172.16.0.62 as usable. This is shown in Figure 4:

Creating a Basic IP Addressing Scheme - Figure 4

Figure 4 First Usable Subnetwork Range

The second subnetwork will begin where the first left off at 172.16.0.64 and go up to 172.16.0.127; this range is shown in Figure 5:

Creating a Basic IP Addressing Scheme - Figure 5

Figure 5 Second Subnetwork Range’

The second department (Admissions) will be allocated addresses from 172.16.0.64 through 172.16.0.127. Again, the subnet and broadcast address must be found and reserved. As the subnet address is the lowest number in the range it is 172.16.0.64 and the broadcast address is the highest address in the range it is 172.16.0.127, leaving the addresses from 172.16.0.65 through 172.16.0.126 as usable; this is shown in Figure 6:

Creating a Basic IP Addressing Scheme - Figure 6

Figure 6 Second Usable Subnetwork Range

The third subnetwork will begin where the second left off at 172.16.0.128 and go up to 172.16.0.191; this range is shown in Figure 7:

Creating a Basic IP Addressing Scheme - Figure 7

Figure 7 Third Subnetwork Range

The third department (Financial Aid) will be allocated addresses from 172.16.0.128 through 172.16.0.191. The subnet and broadcast address must be found and reserved. As the subnet address is the lowest number in the range, it is 172.16.0.128; as the broadcast address is the highest address in the range, it is 172.16.0.191, leaving the addresses from 172.16.0.129 through 172.16.0.190 as usable. This is shown in Figure 8:

Creating a Basic IP Addressing Scheme - Figure 8

Figure 8 Third Usable Subnetwork Range

The fourth subnetwork will begin where the third left off at 172.16.0.192 and go up to 172.16.0.255; this range is shown in Figure 9:

Creating a Basic IP Addressing Scheme - Figure 9

Figure 9 Fourth Subnetwork Range’

The fourth department (Business school) will be allocated addresses from 172.16.0.192 through 172.16.0.255. Again, the subnet and broadcast address must be found and reserved. As the subnet address is the lowest number in the range, it is 172.16.0.192; as the broadcast address is the highest address in the range, it is 172.16.0.255, leaving the addresses from 172.16.0.193 through 172.16.0.254 as usable. This is shown in Figure 10:

Creating a Basic IP Addressing Scheme - Figure 10

Figure 10 Fourth Usable Subnetwork Range

The fifth subnetwork will begin where the fourth left off at 172.16.1.0 and go up to 172.16.1.63; this range is shown in Figure 11:

Creating a Basic IP Addressing Scheme - Figure 11

Figure 11 Fifth Subnetwork Range

The fifth department (Liberal Arts school) will be allocated addresses from 172.16.1.0 through 172.16.1.63. The subnet and broadcast address must be found and reserved. As the subnet address is the lowest number in the range, it is 172.16.1.0; as the broadcast address is the highest address in the range, it is 172.16.1.63, leaving the addresses from 172.16.1.1 through 172.16.1.62 as usable. This is shown in Figure 12:

Creating a Basic IP Addressing Scheme - Figure 12

Figure 12 Fifth Usable Subnetwork Range

The Sixth subnetwork will begin where the fifth left off at 172.16.1.64 and go up to 172.16.1.127; this range is shown in Figure 13:

Creating a Basic IP Addressing Scheme - Figure 13

Figure 13 Sixth Subnetwork Range

The sixth department (Internet Technology school) will be allocated addresses from 172.16.1.64 through 172.16.1.127. The subnet and broadcast address must be found and reserved. As the subnet address is the lowest number in the range, it is 172.16.1.64; as the broadcast address is the highest address in the range, it is 172.16.1.127, leaving the addresses from 172.16.1.65 through 172.16.1.126 as usable. This is shown in Figure 14:

Creating a Basic IP Addressing Scheme - Figure 14

Figure 14 Sixth Usable Subnetwork Range

The seventh subnetwork will begin where the sixth left off at 172.16.128 and go up to 172.16.1.191; this range is shown in Figure 15:

Creating a Basic IP Addressing Scheme - Figure 15

Figure 15 Seventh Subnetwork Range

The seventh department (Science school) will be allocated addresses from 172.16.1.128 through 172.16.1.191. The subnet and broadcast address must be found and reserved. As the subnet address is the lowest number in the range, it is 172.16.1.128; as the broadcast address is the highest address in the range, it is 172.16.1.191, leaving the addresses from 172.16.1.129 through 172.16.1.190 as usable. This is shown in Figure 16:

Creating a Basic IP Addressing Scheme - Figure 16

Figure 16 Seventh Usable Subnetwork Range

The eighth and final subnetwork will begin where the seventh left off at 172.16.1.192 and go up to 172.16.1.255; this range is shown in Figure 17:

Creating a Basic IP Addressing Scheme - Figure 17

Figure 17 Eighth Subnetwork Range

The Eighth department (History school) will be allocated addresses from 172.16.1.192 through 172.16.1.255. The subnet and broadcast address must be found and reserved. As the subnet address is the lowest number in the range, it is 172.16.1.192; as the broadcast address is the highest address in the range, it is 172.16.1.255, leaving the addresses from 172.16.1.193 through 172.16.1.254 as usable. This is shown in Figure 18:

Creating a Basic IP Addressing Scheme - Figure 18

Figure 18 Eighth Usable Subnetwork Range

Summary

It is obvious to anyone who has already learned subnetting or for those attempting to learn it that it can be quite challenging. Taking the time to learn the basics and through practice anyone can learn and become proficient at subnetting. Hopefully the content of this article, along with its earlier companion articles, enable a better understanding of how subnetting can be calculated. The last article in this series, “Configuring an IP Addressing Scheme,” will show the subnets calculated in this article configured on Cisco routing equipment, assigned to the specific routing devices allowing department to department connectivity.

Requirements

Calculating the Correct Subnet Mask

Network Range Calculation

Summary

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Sean Wilkins