Chapter 3, “Fixed-Length Subnet Masks,” showed you how FLSM lets you conserve the IP address space by creating locally significant subnetwork addresses. The benefit of this is that you can use a single network address to service multiple local networks. But in the real world, those local networks are seldom the same size. Thus, implementing FLSM actually wastes IP addresses. To better illustrate this point, consider the network shown in Figure 4-1. We will use this basic network diagram as the basis for exploring VLSM throughout this chapter. (more…)

Ordinarily, I would point out an IETF source document that is the basis for an Internet technology and then expound on that technology. In the case of Variable-Length Subnet Masking (VLSM), there is no clear-cut genesis document. Searching the Internet or the RFC Editor’s database turns up a variety of references, mostly in documents dedicated to other topics. The more salient of these tangential reference documents are RFC 1009 and RFC 1878. They provide you with the context for the development of variable-length subnets and supporting mathematics and helps you appreciate a more thorough examination of VLSM. The following sections discuss each of these RFCs. (more…)

Fixed-length subnet masking was a tremendous step in the evolution of the IP address architecture. It offered the capability to develop locally significant subnetworks without affecting global routability and gave network administrators the flexibility to create whichever sized subnet suited their needs best. Despite this radical leap forward, the potential for waste remained high. Thus was born the great irony of subnetting. Subnetting, in general, was designed to enable more efficient use of address space by permitting class-based network address blocks to be subdivided into smaller address blocks. Yet the way subnetting was originally implemented was far from efficient. (more…)

Having explored the basics of subnetting, you are ready for a more in-depth examination of how they are formed and the mathematics on which they are founded. Starting with a 24-bit network address (equivalent to a Class C network), the formation of subnets is limited to between 2- and 6-bit masks. The limited size of masks in a 24-bit network makes for an ideal case study to demonstrate the potential for confusion as to where a subnet begins and ends and why some addresses look valid but aren’t.

Let’s start by examining the basic trade-off proposition that exists between the number of subnets that can be formed with any given mask size and the subsequent number of usable hosts per subnet. This information is presented Table 3-3. (more…)

Subnetting, as this process is more commonly called, is a remarkably logical and mathematical process. Understanding the mathematics of subnetting helps you develop and implement efficient subnetting schemes that make better use of available address spaces. That is the explicit goal of subnetting: to use an address space more efficiently. Unfortunately, subnetting is the most confusing and least-understood aspect of IPv4 . This is largely due to the fact that it makes sense only when viewed in binary numbers, yet most people think in terms of decimal numbers. For that reason alone I rely extensively on the use of binary-to-decimal translations to demonstrate the concept and applications of subnetting throughout this chapter. (more…)

Having examined the mathematics upon which the IP address space is founded, it is time to explore its hierarchical organization. The hierarchy is best described as being compound, because there are two aspects:

• Two levels of addressing within each IP address.

• Classes of addresses based on differing bit allocations to the two levels of addresses. Having segmented the address’s bit string into four 8-bit components makes it very easy to create address classes because you have logical groupings to work with.

Each of these hierarchical aspects is explored in the following sections. (more…)

IP and its addressing scheme evolved slowly, sometimes even erratically, over time. They were not, contrary to any current appearances, carefully designed prior to implementation! For example, RFC 1, published in April 1969, tells us that the original IP address space was specified at just 5 bits! As you will see in this chapter, that’s enough for just 32 addresses! Strange as that might sound, in 1969 that was more than enough for the embedded base of computers that were being internetworked. Over time, the number of bits allocated to host addressing was increased to 6, and then 8, and finally up to the familiar 32-bit format that is in use today. (more…)

Making publicly available standards documents that stipulate every nuance of a technology is known as open standards. Open standards offer tremendous benefits that have been proven time and again since the introduction of this concept. In the days before the Internet, the previous paradigm was tightly integrated proprietary platforms. In other words, every aspect of a networked computing architecture (including endpoint devices, cable interfaces, computing platforms, operating systems, applications, and printers) was tightly linked by the manufacturers. You couldn’t mix and match components from different manufacturers; you had to select one vendor for all your needs. (more…)

The way that technical standards are developed for the Internet might seem arcane from the outside looking in, but this process is eminently logical, and it has served the Internet well for years. This process is documented in the IETF’s RFC 2026, which is also currently the Internet’s BCP #9. This document can be accessed at www.ietf.org/rfc/rfc2026.txt.

If the terms RFC and BCP are alien to you, read on. The remainder of this chapter is devoted to helping you understand the inner workings of this vital function. The roles of Internet drafts, RFCs, STDs (standards), and BCPs are all explored and explained. (more…)

Numerous organizations, standards bodies, and even corporations function in different capacities. All of them contribute in some way to the Internet. Some allocate domain names (such as cisco.com) or assign IP addresses to the Internet’s end users. Others create the technologies that make the Internet work or that let you use the Internet. All these entities are integral to the Internet’s operation. We’ll look at each one in this chapter, but only one can truly be considered the Internet’s caretaker. That organization is the Internet Engineering Task Force (IETF). (more…)