2.6. Identifying Classful IP Ranges and Their Subnet Masks
When the Internet and TCP/IP were first developed, 4 billion addresses seemed like an awful lot of addresses and nobody really envisioned running out. For this reason, the founders of the Internet gave some of the larger organizations address spaces that would actually support millions of hosts. Medium-sized companies were later given address spaces that would support thousands of hosts. Small companies were given address spaces that would support hundreds of hosts. No real thought was given to the minimum that an organization could possibly use, because after all we had a "virtually unlimited" supply of addresses, or so we thought.
The addressing scheme that resulted from this is known as classful addressing. While purely classful addressing is rarely used today, it is still essential to your understanding of IP addresses. In this section, we discuss classful IP address ranges and their subnet masks.
NOTE
For more information on IP ranges and subnet masks, see Chapter 3 of the Network+ Study Guide, Fourth Edition.
2.6.1. Critical Information
You should be able to identify the address class of an IP address based on the first octet of the address. In addition, based on the address class, you should know the default subnet mask for the classful address. Finally, you should know the addresses that are not valid for IP addressing.
2.6.1.1. Classful IP Address Ranges
As we mentioned before, the subnet mask actually controls which bits of an IP address are network bits and which are host bits. Using the subnet mask, we can control the number of hosts that a given network can contain. Classful addressing aligns the first octet of an IP address with a default subnet mask in ranges called classes. IP addresses are actually divided into five classes, but we only use the first three for normal addressing. Table 2.5 illustrates the relationship of the first octet, the class, and the subnet mask.
| First Octet Address | Class | Default Subnet Mask |
|---|---|---|
| 1–126 | A | 255.0.0.0 |
| 128–191 | B | 255.255.0.0 |
| 192–223 | C | 255.255.255.0 |
You may have noticed that the number 127 is missing. This is because the 127 network is reserved for diagnostics and testing. The most notable address on this network is the loopback address 127.0.0.1, which we will discuss in later chapters. Also, note that class D addresses are reserved for multicasts and that class E addresses are reserved for experimentation and future development.
2.6.2. Exam Essentials
Know how to determine the class of an IP address based on the its first octet. The address class ranges based on the first octect of a classful address can be determined as follows:
- 1–126 Class A
- 128–191 Class B
- 192–223 Class C
- Note: 127 is reserved for diagnostic purposes, such as loopback.
Know the default subnet mask for each class of address. The default subnet masks for each class are as follows:
- A 255.0.0.0
- B 255.255.0.0
- C 255.255.255.0
Know which addresses are not valid for normal IP address assignment Only class A, B, and C addresses are valid for normal IP address assignment; the 127 network is reserved for diagnostics and testing. The 127.0.0.1 address is commonly used for a loopback address to test TCP/IP.