Allocating VLSM Addresses

Applying the addressing scheme designed in the preceding example is very simple after the design has been worked out.

Taking the region of California as the example to address, you will now address the entire region.

Figure 3-7 shows the bit allocation that was determined.

Figure 3-7 Bit Allocation

Region:

California: 001 Campus:

San Francisco: 01 San Jose: 10 San Rafael: 11 Buildings:

Building 1: 001 Building 2: 010 Building 3: 011 Building 4: 100 Floor:

Floor 1: 001 Floor 2: 010 Floor 3: 011 Floor 4: 100 Floor 5: 101 Hosts: 1-30

NOTE Remember that you will conform to the rule of reserving the broadcast addresses in the access layer of the network.

NOTE The buildings have the same bit pattern for each campus. Remember, however, that this bit pattern is unique within the whole address space because the pattern for the campus is unique and the address must be seen in its entirety.

The third host, on the fourth floor of the second building in San Jose, California, will be given the address shown in Figure 3-8.

The address in Figure 3-8 is represented as 140.100.50.131 in dotted decimal, with a mask of 255.255.255.224.

Worked this way, from the physical topology up, it is very straightforward. Many people are given the address 140.100.50.131 and work backward, which is very confusing. To avoid confusion, it is extremely helpful to document the addressing scheme within the organization's network (to make management of the network easier and to help maintain the administrator's sanity).

Figure 3-8 Example of How to Apply VLSM

Hosts

Floor

Building Campus Region y v y y

001 10 010. 100 00011

TIP This use of VLSM shows clearly that when allocating addresses in IP, it is necessary to reduce the address to binary and to disregard the octet boundary. Reducing the address to binary and disregarding the octet boundary creates just a continuous set of bits to be applied as appropriate to address the network.

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