Category: CCNA Study Guide Chapter 05

Since there are literally millions of IP addresses available, the IETF originally designated what are known as classes of IP addresses. The purpose of these classes was to break up the IP address space into ranges that accounted for networks of different sizes. The term “classful” is used to describe addresses that are looked at according to their class. In reality, the world of IP addressing has changed such that classes of addresses are much less important than they used to be – later in the chapter, we’ll take a look at classless addressing, including how and why it came about.

You’ll definitely need to be familiar with classful addressing, since it forms the basis upon which IP addresses were originally defined, and is still a factor with routing protocols such as RIP version 1 and IGRP. Five different classes of addresses exist, and are distinguished according to the values found in their first octet. The table below outlines each of the five ranges.

Class	First Octet Decimal Value	Network and Host Portions	Hosts Supported Per Network	Details
A	0-126	N.H.H.H	16,777,214	Intended for the largest networks only
B	128-191	N.N.H.H	65,534	Intended for medium sized organizations
C	192-223	N.N.N.H	254	Intended for small organizations
D	224-239	N/A	N/A	Reserved range used for multicasting
E	240+	N/A	N/A	Experimental range

The value of the first octet of an IP address holds the immediate answer to the class an address falls into. Notice that Class A addresses always begin with a value between 0 and 126. As such, the address 64.12.203.1 can safely be identified as Class A. From the table above, you should also note that in a Class A address, the first octet uniquely identifies the network (designated by the “N”), while the last three octets uniquely identify a host (designated by the “H”) on that network. Only Class A, B, and C addresses are valid to assign to hosts. Class D addresses are used to support multicasting, while Class E addresses are reserved for experimental use.

You may have noticed that the first octet value of 127 is missing from the table above. What is the reason for this? The 127 range is actually reserved for diagnostic functions – for example, the address 127.0.0.1 is the loopback address. Ping that address, and you’re actually testing the TCP/IP connectivity of the source machine.

Back in Chapter 4, we took a very basic and introductory look at the concept of an IP address. Recall that IP addresses are logical addresses made up of two parts – the first part represents a network, and the other, a unique host on that network. Before we get into the details of IP addressing, you’ll first need to know a little more about how binary and decimal numbers relate to each other.

Binary-Decimal Conversions

Remember that IP addresses are 32-bits in length, but are usually represented in dotted-decimal notation. As such, the address 192.168.2.200 can also be represented as:

11000000 10101000 00000010 11001000

But why should you care about the binary version? The answer is absolutely critical – in order to truly understand how IP addressing works, you must always take a look at elements such as addresses and subnet masks in binary form. After you’ve had lots of practice in binary, you’ll get very good at “understanding” the numbers when you see them in decimal. Just remember that when starting off, it is important that you convert addresses to binary – doing so will ultimately unlock the secrets of subnetting, determining if hosts are on the same or different networks, and whether IP addresses are valid.

Having said that, our first logical step is learning how to do decimal-to-binary conversions and vice versa. Recall that a 32-bit address is actually grouped into four octets in its decimal form – each octet represents 8 bits in binary. The figure below outlines the decimal and binary values for each of the 8 bits.

DECIMAL	128	64	32	16	8	4	2	1
BINARY	1	1	1	1	1	1	1	1

Notice that each of the eight binary digits has a single decimal value associated with it. An important pattern should be clear in the decimal row – moving from right to left, the value doubles in each successive column.

This is part of what make binary numbering so useful – for any given decimal number, there is one (and only one), way to represent it in binary. After a few examples, understanding how binary numbering works will be clear.

We’ll start with converting binary to decimal, since this is the easiest way to illustrate the conversion process. Imagine you wanted to convert the binary number 01010101 to decimal. Using Table 5-1, you’ll notice that each binary “1” value corresponds to the decimal value above it. If we add together the decimal values that correspond to the 1s in the binary number, we’ll have completed the conversion process. For example, the decimal value of the binary number 01010101 would be:

0 + 64 + 0 + 16 + 0 + 4 + 1 = 85

All I did was take the decimal value of each of the 1s in the binary number and replace them with their corresponding decimal value. In cases where the binary digit was 0, I did not add that value. Consider another example, the binary number 11100011. In this case, the corresponding decimal value would be calculated as:

128 + 64 + 32 + 0 + 0 + 0 + 2 + 1 = 227

I’ve left the 0 decimal values in my examples to act as placeholders – you should consider doing this until you feel comfortable that you remember the decimal values associated with each binary digit.