Module 3: IP Addressing

Transcription

1 Module 3 IP Addressing Module 3: IP Addressing 3-1

2 AN233 Implementing WAN Solutions Student Manual 3-2

3 Module 3 IP Addressing Overview This module provides an overview of IP addressing. Lessons Module 3 includes the following lessons: Lesson 3-1: Classful Addressing Lesson 3-2: Classless Addressing Lesson 3-3: Automatic Assignment of IP Addresses Objectives After completing this module, you will be able to: Explain the IP addressing scheme Identify methods for the automatic assignment of IP addresses 3-3

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5 Module 3 IP Addressing Lesson 3-1: Classful Addressing Lesson Overview Lesson 3-1 provides an overview of classful addressing. Lesson Objective After completing this lesson, you will be able to: Describe the characteristics, addresses, and benefits of the IP addressing scheme Introduction Classful addressing was the first IP addressing scheme used for the Internet. In a classful addressing, an 8-, 16-, or 24-bit prefix determines whether the IP header uses 8, 16, or 24 bits to represent the host portion of the IP address. The remaining bits can represent individual devices on the network. 3-5

6 AN233 Implementing WAN Solutions Student Manual IP Address Format An IP address consists of two parts: the network address and the host address. The network portion of the address identifies all of the systems that are located on the same physical segment. For networks that have a direct connection to the Internet, the network portion of the address must be unique. The host portion of the address identifies each workstation, server, router, or other TCP/IP device on the network segment. A host address must be unique within a network. An address class sets a boundary between the network portion and the host portion of an IP address. The boundary establishes the number of bits that are available for both the network and the host portions of the address. This boundary determines the number of networks that are possible for a given class, as well as the number of hosts that can exist on a network of a given class. Types of IP Addresses Traditionally, IP addresses have been divided into three address classes based on the number of bits in the network prefix. Figure 3-1 shows these address classes. Class A: 0 7-Bit Prefix 24-Bit Host ID Class B: Bit Prefix 16-Bit Host ID Class C: Bit Prefix 8-Bit Host ID Figure 3-1: IP Address Classes 3-6

7 Module 3 IP Addressing Class A IP Addresses This class of IP address uses seven bits for the network prefix and 24 bits for the host portion of the address. The first bit is always zero in a Class A address. Seven bits for the network prefix allows for only 2 7 (128) different Class A networks. On the other hand, each of these networks can be huge, with 2 24 (more than 16 million) computers on each. Class A addresses are in the range (The address is reserved for the loopback interface.) Class B IP Addresses This class of IP address uses 14 bits for the network prefix and 16 bits for the host portion. This allows for just over Class B networks with more than IP network stations on each. Class B addresses are in the range Class C IP Addresses This last class of IP address uses 21 bits for the network prefix and just eight bits for the host portion. There are more than two million Class C networks, but each of these networks can have only 254 network stations. 1 Class C addresses are in the range In addition to these three address classes, Class D and E addresses are sometimes mentioned. Class D IP Addresses This class of IP address is used for IP multicasting. Class D addresses have the binary digits 1110 as the first four bits and are in the range Class E IP Addresses This class of IP address is reserved for future use. Class E addresses have the binary digits as the first five bits of the address field = 256. In real IP networks, however, the first (all zeros in the host portion) and the last (all ones) are reserved for special purposes, leaving just 254 usable IP addresses. 3-7

8 AN233 Implementing WAN Solutions Student Manual Limitations of the IP Addressing Structure Theoretically, an IP address size of 32 bits should allow more than 4 billion computers to have unique IP addresses, which should be sufficient for even the Internet in the foreseeable future. However, because IP addresses have been assigned to organizations in chunks of complete Class A, B, or C addresses, large portions of the address space are wasted. For instance, half the available address space (more than two billion addresses) is assigned to just 128 Class A networks. Another problem with this allocation scheme is the small number of reasonably sized IP networks available. For most organizations, Class A or Class B addresses would be the most attractive, but the total number of these addresses is very small compared to the number of organizations worldwide wanting unique IP addresses. Consequently, most organizations are assigned Class C addresses. But since one Class C address can accommodate at most 254 hosts, many organizations have to build networks from several Class C networks. Using Several Class C Addresses Using several Class C networks has the following consequences: Routers are usually necessary. Routers are needed inside larger organizations because different IP networks must always be connected using a router. Network administrators must manage large address tables. Routing tables in Internet backbone routers grow extremely large, and they take a great deal of time to maintain. Someone must track every IP network address. For these reasons, the classful address assignment scheme (that is, an assignment scheme that only checks the first three bits of an IP address when making routing decisions and ignores the subnet mask) described above is gradually being replaced with the classless network prefix/host portion interpretation of IP addresses. A classless address assignment is a more hierarchical structure, where each domain s IP address is related to the level above it. The classful assignment scheme uses fewer IP addresses and conserves backbone router resources. However, large portions of the IP address space have already been assigned as Class A and B addresses to organizations that are not yet using them. 3-8