Allocating VLSM Addresses
Applying the addressing scheme designed in the preceding case study is very simple after the design has been worked out.
Taking California as the example to examine, we shall now address the entire region.
Figure 2-6 shows the bit allocation that was determined.
Figure 2-6 Bit Allocation
San Francisco: 01
San Jose: 10
San Rafael: 11
Building 1: 001
Building 2: 010
Building 3: 011
Building 4: 100
Floor 1: 001
Floor 2: 010
Floor 3: 011
Floor 4: 100
Floor 5: 101
NOTE: Remember that the case study will conform to the rule of reserving the broadcast addresses in the access layer of the network, the last level of subnetting.
Also remember that the buildings have the same bit pattern for each campus. However, 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 2-7. The address in Figure 2-7 is represented as 188.8.131.52 in dotted decimal, with a mask of 255.255.255.224.
Applying an addressing structure that uses VLSM with careful reference to the physical topology is very straightforward. When presented with a host address, it is common for people to try to determine the bit allocation working from the host address. If the addressing scheme has been well documented, network management is much easier, because as soon as the address is seen, its physical location is known. This simplifies troubleshooting, because a problem seen on a management console can be solved by member of the support staff.
NOTE: 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 a continuous set of bits to be applied as appropriate to address the network.
VLSM also enables you to allocate the required bits for addressing a particular network.
Optimizing the IP Address Space
Particularly in the use of WANs, where there is a predominance of point-to-point connections, allocating an entire subnet is very wasteful. VLSM allows refinement of the address space to exactly that which is needed and no more.
As demonstrated, dealing with VLSM to support the hierarchical design requires the consideration of the entire network topology. When using VLSM to optimize the IP address space, the network addressing can become extremely confusing if it is not clearly managed and documented.
In the preceding example, no consideration was given to the connections between the regions, campuses, and buildings—all of which could be point-to-point lines.
Now it is important to consider the last part of the network addressing, which will illustrate the use of VLSM for IP address optimization.