Activity: Calculating VLSMs

Objective

Subnet an address space to satisfy the requirements of a 4 building campus.

Scenario

All American University has acquired network 130.16.32.0/24 from its ISP for use at its 4 building campus. The host requirements vary from building to building. The network administrator will use VLSM to divide the address space to accommodate the addressing needs of each building. (For this exercise, you will assume that all subnets are useable.)

Step 1

You should begin by looking at the largest pool of addresses needed in this scenario. Based on the graphic, which building has the greatest number of hosts?

How many hosts are required?

In order to address these hosts, how many bits will you borrow in the first round of VLSM?

How many subnets will that yield?

How many host addresses will each subnet provide?

VLSM subnetting is most easily performed when the address space is presented in binary. Since the first 3 octets of this address cannot be changed, only the last octet will be presented in binary.

Step 2

Although the first round of VLSM subnetting in this example is identical to classful subnetting, use the step by step process that is used in later steps. To perform VLSM subnetting in this and subsequent rounds of subnetting, follow these steps:

1.       Write the address in binary. You may choose to write only those octets where subnetting will occur in binary, as in this example.

2.       Draw a vertical line at the current network or subnet boundary. This line will identify those bits that have already been assigned (network bits) and those bits that are “borrowable” (host bits). Only bits to the right of this line can be borrowed.

3.       Draw a second vertical line x bits to the right of the boundary, where x = the number of bits borrowed in this round.

4.       Using the bits between the lines, count from lowest to highest in binary. These are the VLSM subnets for this round.

In our example, we will first divide our network into two subnets by borrowing 1 bit. This will leave 7 host bits to meet the requirement of our largest network. Following the steps we have:

1.       Binary representation of available bits:

2.       Draw a line at the current subnet boundary:

3.       Draw second line x=1 bits from boundary:

4.       Count up in binary using bits between the lines:

Since we borrowed 1 bit in this round, we have 2¹ = 2 subnets. Each subnet has 2⁷ – 2 = 126 hosts each. We can use one of these subnets for the Administration building.

If the first subnet is assigned to the Administration building, what is the network and mask?

What are the valid host addresses that can be used?

From:                                                                To:                                                                   

Step 3

After the first round of VLSM, the address space has been divided into two subnets. The first subnet has been assigned to the Administration building.

Since there are still 3 buildings to consider, we must perform further subnetting on the 2^nd VLSM subnet to provide address space to these buildings. Since the first subnet has been assigned, we must be careful not to overlap any of those addresses.

Notice that /25 in the 2 VLSM subnets indicate that 25 bits are used for network identification and 7 bits remain in the host portion. Using this information to draw a vertical line at this boundary during our next round of VLSM subnetting will ensure that no addresses overlap.

To prepare for the next round of subnetting, VLSM subnet 2 should be represented in binary. Write this address in the table below.

Step 4

The second VLSM subnet, 130.16.32.128/25 can be divided in several ways. We must consider how to divide it properly in order to meet the host requirements of the remaining 3 buildings.

If we borrow 2 of the remaining bits, we have:

2² = 4 subnets

2⁵ – 2 = 30 hosts each

While this would provide enough subnets, the South Building needs at least 45 host addresses.

Consider the results of borrowing 1 bit:

2¹ = 2 subnets

2⁶ – 2 = 62 hosts each

This will provide a subnet with enough hosts for the South Building. The other subnet can then be further subnetted for the remaining 2 buildings.

Complete the next round of VLSM subnetting by borrowing 1 bit to create 2 subnets. The first subnet will be assigned to the South Building.

1.       Binary representation of last octet of 130.16.32.128/25:

2.       Draw a line at the current subnet boundary. (Remember that the VLSM subnet, 130.16.32.128/25 has a different boundary than we used in round one.)

3.       Draw second line x=1 bits from boundary. (This round borrows 1 bit.)

4.       Count up in binary using bits between the lines:

Assign the first of these two subnets to the South Building. Identify the network, mask and host address ranges for this network in decimal:

Network and Mask:                                                                                                                   

Host Range From:                                                           To:                                                      

Step 5

So far we have divided the original address space to meet the needs of the Administration Building and the South Building. The last round of VLSM subnetting will divide the last unassigned subnet between the remaining 2 networks. Each subnet will provide 30 useable host addresses, which is sufficient for the host addressing needs of the North and East Buildings.

Complete the last round of VLSM subnetting. Use the table below to complete each step in the process.

Assign VLSM subnet 2.2.1 to the North Building and 2.2.2 to the East Building. Use your calculations from this and previous steps to complete the table with information about each subnet at All American University.

Viewed graphically, the VLSM scheme at AAU shows how the original address; 130.16.32.0/24 has been subnetted to accommodate the specific needs of this network.