A Practical Application
| To better demonstrate how VLSM works in practical terms, Table 4-6 shows the progression from the sample network's base address (192.168.125.0) through the defined subnets. Pay particular attention to the binary and decimal translations for each subnet's base and terminal addresses. In decimal terms, you are progressing sequentially through the address space. In binary terms, you can see that each network uses a different combination of high-order bits in the last octet to identify the subnet. This might seem strange, but it is eminently logical. I distinguish between host and subnet bits in the binary address by indicating the subnet bits in bold italic and then delineating the two subfields with a dash (-). Ordinarily, you wouldn't find a dash in the middle of a bit string.
The unassigned space illustrated in Table 4-6 can be used in a number of different ways. Here are two of the more feasible scenarios for using this space:
Both of these scenarios and their implications on subnetting schemes are explored in the remainder of this section. Adding a SubnetIn the example used throughout this chapter, Subnets 1 through 3 have been used, with Subnet 0 idle. It is equal in size to a subnet defined with a mask of 255.255.255.224, but the 32 addresses it would contain are not used. I did this on purpose to show you that in many cases, old subnetting rules can be very persistent. Consequently, it is not uncommon to find Subnet 0 unused in networks today. Under today's rules for subnetting, this space doesn't have to lie fallow. To continue with the sample network, if a requirement emerges for a new subnet with 30 or fewer devices, Subnet 0 could be pressed into service. Additional unallocated addresses remain in the high end of the network address block. Addresses 64 through 255 are unused, so it is possible for additional subnets to be created in the future. Thus, you have some options for satisfying new requests for subnets. Depending on the number of hosts in a new subnet, you could assign Subnet 0 or carve Subnet 4 out of the currently unassigned address space (addresses 64 through 255). A more nettlesome question is how you accommodate growth within the existing subnets. Outgrowing a SubnetAs you were looking at Tables 4-4 and 4-5, did you notice that even VLSM isn't perfectly efficient? There are still wasted addresses. That's a direct function of the binary math that is the foundation of the address space itself, rather than a flaw in any particular approach to carving the space into subnets. You can create a subnet on any bit boundary, but each bit increments by a factor of 2. Remember: The rightmost bit in an octet has a decimal value of 1, the bit to the immediate left carries a value of 2, then 4, then 8, then 16, then 32, then 64, and ultimately 128 for the leftmost bit in the octet. Consequently, you must form your subnets in this sequence from powers of 2. You could look at this architectural feature as a negative in that it results in wasted space. Alternatively, you could take a more pragmatic perspective and appreciate the positive implications of this feature. For example, even if it were possible to create subnets of the precise size you require for any given subnet, would you really want to tailor it so concisely? Many things can happen that would require you to add endpoints to a subnet. For example, the user community on any of your subnets might hire someone new. The same thing would hold true for technological innovation. It wasn't that many years ago that printers didn't have a built-in network interface card (NIC), and you had to use a server to spool print requests to them. A more commonly encountered scenario is that the group you are supporting can simply outgrow its subnet. The point is that there are many reasons why the number of host addresses in any given subnet could change over time. Trying to add a few addresses to a tightly constructed subnetted scheme can be painful. Depending on the extent of the growth, you might find it necessary to completely renumber one or more subnets! That might not sound so bad, but it is not fun, and your users might not appreciate having to experience it either. Plus, you might discover a relatively common practice: application developers who hard-code IP addresses into their software. Renumbering a network will cause every such application to fail! To illustrate this point, let's assume that Subnet 2 of the sample network needs to grow by five endpoints. Unfortunately, that subnet has only two available IP addresses within its assigned range (192.168.125.40 through 192.168.125.47), and Subnet 3 picks up right where Subnet 2 ends, so there is no opportunity to just change the mask's size to encompass sequential but unassigned addresses. Because Subnets 2 and 3 are numerically contiguous, your only choices both involve renumbering the endpoints within Subnet 2. You have to move them to a section of the address space that offers more addresses. Your two options are as follows:
Table 4-7 shows you what the subnetting scheme would look like if you were to renumber the endpoints in Subnet 2 to use the range of addresses in Subnet 0. Doing so results in the allocation of 30 usable host addresses to a group of users that requires only 17, but you don't have any better options! The coarseness of the architecture works against you. A mask of 255.255.255.240 would yield only 14 usable hosts, which is inadequate. However, a mask of 255.255.255.224 (1 less bit in the subnet prefix) yields 30 usable hosts and is the only feasible solution. This should seem familiar to you, but if it doesn't, just refer back to Table 4-4.
Your second option for satisfying the outgrowth of Subnet 2 is to create a new subnet from the unassigned addresses. Table 4-8 demonstrates how this would be done. Pay particular attention to the binary and decimal translations for Subnet 4 to see how that would be accomplished.
If you look carefully at the progression of numbersparticularly the binary numbersyou will see a very familiar pattern. Both Subnets 3 and 4 use a 3-bit subnet mask. This makes it easy to see why one subnet ends with 00111111 and another begins with 01000000. In the other cases, mismatched subnet prefixes make it a bit tougher to follow the logic. In those cases, ignore my visual cues about subnet formation and just look at the 8-bit string as a whole. Then, things make more sense. Keep Careful Records!Looking back over the previous three tables, you must draw one simple, inescapable conclusion: It is critical to maintain an accurate and up-to-date record of address assignments! Subnetting, in its original incarnation, was deemed practical only if you used a fixed-length mask to subdivide an entire network address. Although this wasn't the most efficient way to subdivide a network, it was still far more efficient than the previous method of operation, which was to secure separate network addresses for each of your subnets. The grassroots innovation of flexible subnetting happened outside the auspices of the IETF. Thus, there was no solid base of research to draw on, no well-worn trail to follow, and no set of tools to rely on. If you chose to ignore the recommendation of using a single-sized subnet mask for your subnetting, you were on your own! This was a simplifying assumption embedded in the original subnetting RFCs. It gave ordinary network administrators a chance to improve the efficiency with which they consumed IP address space without creating a mathematics exercise that could have qualified as a Herculean task. Despite the risks of stepping outside the safe confines of an RFC, flexible subnetting became the dominant paradigm. Technical personnel realized that the price they had to pay for this was the creation and maintenance of an accurate database of address assignments. This has never been more true! |
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