Scenarios-3 | Kickoff

Scenarios-3

8 Mar

Scenarios

The following scenario and questions are designed to draw together the content of the chapter and exercise your understanding of the concepts. There is not necessarily a right answer. The thought process and practice in manipulating the concepts is the goal of this section. The answers to the scenario questions are found at the end of this chapter.

Scenario 3-1
This scenario concentrates on correcting an addressing scheme in the company discussed in Chapter 2, CyberKit. A network administrator devised the addressing scheme before there was any intention of connecting the company to the Internet and before the company had regional offices. Addresses were subsequently allocated without any policy or administrative control. This has led to problems in the current organization, which now needs to summarize its addresses. Using the addressing scheme in Figure 3-9, answer the following questions.

Figure 3-9 An Addressing Scheme for Scenario 3-1

Addressing Scheme for Scenario

1. There are serious problems with the addressing scheme in Figure 3-9. If the network had this addressing scheme, would summarization be possible?

2. Design an alternative addressing scheme using VLSM that would summarize to the regional level.

3. Write out the addressing scheme in both binary and dotted decimal notation.

4. Could these addressing requirements be achieved with a Class C address?
5. If the answer to the preceding question is yes, write out the dotted decimal and binary notation to support it. If the answer is no, how many Class C addresses would be required? (Again, write out the dotted decimal and binary notation to support your argument.)

Scenario Answers

The answers provided in this section are not necessarily the only possible answers to the questions. The questions are designed to test your knowledge and to give practical exercise in certain key areas. This section is intended to test and exercise skills and concepts detailed in the body of this chapter.

If your answer is different, ask yourself whether it follows the tenets explained in the answers provided. Your answer is correct not if it matches the solution provided in the book, but rather if it has included the principles of design laid out in the chapter.

In this way, the testing provided in these scenarios is deeper: It examines not only your knowledge, but also your understanding and capability to apply that knowledge to problems.

If you do not get the correct answer, refer back to the text and review the subject tested. Be certain to also review your notes on the question to ensure that you understand the principles of the subject.

Scenario 3-1 Answers
1. There are serious problems with the addressing scheme in Figure 3-9. If the network had this addressing scheme, would summarization be possible?

Summarization is not possible, for the following reasons:
— The buildings do not share the same high-order bits as the campus.
— The campuses do not share the same high-order bits as the region.
— Depending on the physical design, the California campus and Building 1 could be seen as duplicate addresses.

2. Design an alternative addressing scheme using VLSM that would summarize to the regional level.
See Table 3-6.
3. Write out the addressing scheme in both binary and dotted decimal notation.
See Table 3-6.

Table 3-6 shows an alternative solution using the same address as before and the same bit allocation. If you have changed the bit allocation, ensure that there are enough bits for each level of the network. The requirements have not been stated, so you will have to state them for yourself or use the limited information that is provided. The question identifies three states, and you can assume three campuses in each state. Each campus has four buildings.

Table 3-6 Alternative Addressing Scheme

Alternative Addressing Scheme

4. Could these addressing requirements be achieved with a Class C address?

It would not be possible to address this network, using a hierarchical design, with one Class C address. Given the minimum requirements shown in the question of three states, three campuses, and four buildings at each campus, 6 bits would be required. A Class C address allows only 8 bits in total, leaving 2 bits for host allocation. The rule of not using all 0s or all 1s applies to the host portion of the address, so 2 bits would not enable you to address only two hosts.

5. If the answer to the preceding question is yes, write out the dotted decimal and binary notation to support it. If the answer is no, how many Class C addresses would be required (again write out the dotted decimal and binary notation to support your argument)?

Remember that although a Class C address has a prefix of /24, not all addresses with a /24 prefix are Class C addresses. A Class C address is a classful address that has been obtained from the IANA. It is interesting that although the first guess is that two are better than one, two Class C addresses do not really improve the situation. The need to address 12 buildings requires 4 bits, which would allow only 14 hosts in each building. The network could be addressed with two Class C addresses if 14 hosts in each building are all that is required. There is very little growth allowance in this scheme, making it inadvisable.

The most efficient addressing scheme with Class C addresses would be to use 40 Class C addresses. Consider, for example, the addressing scheme using Class C addresses.

A Class C address would be allocated to each building. This would allow 254 hosts in each building and subnetting to the floor, if necessary. The other three Class C addresses would be used with VLSM to identify the regions and campuses. Table 3-7 shows the addressing scheme for the one Class C address to address one region, three campuses, and four buildings.

Each region or state will now advertise five networks—the four Class C addresses for the buildings and the shared network for the state. One Class C network can be used for the state if the connections are point to point. Because there are 15 connections—four buildings per region, and three regions—this means 12 connections to the buildings, plus three connections to the state. A Class C address would easily accommodate this, even with redundant connections built into the design.

The reason that 40 Class C networks are needed is that the analysis of the state must be extrapolated to the entire organization. The organization covers three states, each with three regions, and each region has four buildings. Although the addressing described previously is correct, it would need to be extended to the other regions. This is calculated as follows:

The number of buildings requiring Class C networks in three states, each with three regions, and each region in turn with four buildings is 3 * 3 * 4 = 36. Add to the three states requiring Class C networks the additional network required for the core cloud that connects the states, and you have 36 + 3 + 1 = 40. In total, therefore, 40 Class C networks will be required.

Other than for academic interest in torturous addressing, this scenario would be an excellent candidate for a private Class B address.

Table 3-7 The Class C Used to Identify the Campuses for Each Region

Campuses for Each Region

Table 3-8 shows how to address the departments or floors within each building. For this discussion, use 210.10.64.0 as the example Class C address. Four bits taken in the fourth octet allows 14 networks, either distributed between the floors or between departments, with 14 hosts on each subnet.

Table 3-8 How to Address a Building Using a Class C Network Address

Building Using a Class

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