IP Addresses 399 Figure 7-27 Broadcast Address Figure 7-28 Network Address In a Class B network address, the first two octets, written as dotted-decimal numbers, are assigned by default. The last two octets contain 0s because those 16 bits are for host numbers and identify devices that are attached to the network. This is called a unicast address (uni means one). A unicast address points to just one host on the net- work. The IP address in the example (176.10.0.0) is reserved for the network address and is never used as an address for any device that is attached to it. An example of an IP address for a device on the 176.10.0.0 network is 176.10.16.1. In this example, 176.10 is the network address portion, and 16.1 is the host address portion. To send data to all the devices on a network, a broadcast address is needed. A broadcast occurs when a source sends data to all devices on a network, as shown in Figure 7-29. This Class B address is the broadcast address for this network. When packets are received with this destination address, the data is processed by every computer. To ensure that 1102.book Page 399 Tuesday, May 20, 2003 2:53 PM 400 Chapter 7: TCP/IP Protocol Suite and IP Addressing all the other devices on the network process the broadcast, the sender must use a desti- nation IP address that they can recognize and process. Broadcast IP addresses end with binary 1s in the entire host part of the address (the Host field). Figure 7-29 Broadcast Address For the network 176.10.0.0, where the last 16 bits make up the Host field (or the host part of the address), the broadcast that is sent to all devices on that network includes a destination address of 176.10.255.255 (because 255 is the decimal value of an octet containing 11111111). Public and Private Addresses Internet stability depends directly on the uniqueness of publicly used network addresses. As shown in Figure 7-30, there is an issue with the networking addressing scheme. Both networks have a network address of 198.150.11.0. When data transmissions reach the router, which network would it forward to? A scheme such as this one would greatly increase the amount of network traffic and would defeat a router’s basic function. Therefore, some mechanism was needed to ensure that addresses were, in fact, unique. This responsibility originally rested with the InterNIC (Internet Network Information Center). This organization is now defunct and has been succeeded by the Internet Assigned Numbers Authority (IANA). IANA carefully manages the remaining supply of IP addresses to ensure that duplication of publicly used addresses does not occur. Such duplication would cause instability in the Internet and compromise its capability to deliver datagrams to networks using the duplicated addresses. 1102.book Page 400 Tuesday, May 20, 2003 2:53 PM IP Addresses 401 Figure 7-30 Required Unique Addresses Public IP addresses are unique. No two machines that connect to a public network can have the same IP address, because public IP addresses are global and standardized. All machines connected to the Internet agree to adhere to the system. Public IP addresses must be obtained from an Internet service provider (ISP) or a registry at some expense. With the rapid growth of the Internet, public IP addresses were beginning to run out, so new addressing schemes such as classless interdomain routing (CIDR) and IPv6 were developed to help solve the problem. CIDR and IPv6 are discussed later. Another solution that was developed is the use of private IP addresses, as shown in Table 7-8. As stated previously, Internet hosts require a globally unique IP address. However, private networks that are not connected to the Internet can use any valid address, as long as it is unique within the private network. Many private networks exist alongside public networks. Grabbing “just any address” is strongly discouraged because that network might eventually be connected to the Internet. RFC 1918 sets aside three blocks of IP addresses (a single Class A address, a range of Class B addresses, and a range of Class C addresses) for private, internal use. Addresses in this range are not routed on the Internet backbone; Internet routers immediately discard private addresses. 1102.book Page 401 Tuesday, May 20, 2003 2:53 PM 402 Chapter 7: TCP/IP Protocol Suite and IP Addressing If you are addressing a nonpublic intranet, a test lab, or a home network, these private addresses can be used instead of globally unique addresses. Private IP addresses can be intermixed with public IP addresses, as shown in Figure 7-31, to conserve the number of addresses used for internal connections. Figure 7-31 Using Private IP Addresses Within the WAN Connecting a network to the Internet using private addresses requires translating the private addresses to public addresses. This translation process is called Network Address Translation (NAT). A router usually is the device that performs NAT. Introduction to Subnetting Another way to conserve IP addresses, like CIDR, IPv6, and private addresses, is the use of subnetting. This method of dividing full network address classes into smaller pieces has helped prevent complete IP address exhaustion. Figure 7-32 shows a Class B network (131.108.0.0) divided into three subnetworks. It is impossible to cover TCP/IP without mentioning subnetting. As a system administrator, you must understand sub- netting as a means of dividing and identifying separate networks throughout the LAN. It is not always necessary to subnet a small network, but for large or extremely large networks, subnetting is required. Simply stated, subnetting a network means using the subnet mask to divide the network and break a large network into smaller, more effi- cient, more manageable segments, or subnets, as shown in Figure 7-33. This is like the Table 7-8 Private IP Addresses IP Address Class RFC 1918 Internal Address Range Class A 10.0.0.0 to 10.255.255.255 Class B 172.16.0.0 to 172.31.255.255 Class C 192.168.0.0 to 192.168.255.255 Site A Site B Site C Site D 207.21.24.0/27 207.21.24.32/27 207.21.24.64/27 207.21.24.96/27 10.0.0.4/30 10.0.0.8/30 10.0.0.12/30 Internet 1102.book Page 402 Tuesday, May 20, 2003 2:53 PM IP Addresses 403 American telephone system, which breaks the system into area codes, and then exchange codes, and finally local numbers. These elements of the phone system are comparable to network numbers, subnets, and individual host addresses, respectively, in an IP internetwork. Figure 7-32 Addressing with Subnets Figure 7-33 Subnet Addresses The system administrator must resolve these issues when adding and expanding the network. It is important to know how many subnet/networks are needed and how many hosts are allowed to be on each network. With subnetting, the network is not limited to the standard Class A, B, or C network masks and there is more flexibility in the network design. 1102.book Page 403 Tuesday, May 20, 2003 2:53 PM 404 Chapter 7: TCP/IP Protocol Suite and IP Addressing Subnet addresses include the Class A, Class B, or Class C network portion, plus a Subnet field and a Host field. These fields are created from the original host portion for the entire network. The ability to decide how to divide the original host portion into the new Subnet and Host fields provides addressing flexibility for the network administrator. To create a subnet address, a network administrator borrows bits from the Host field and designates them as the Subnet field, as shown in Table 7-9. The minimum number of bits that can be borrowed is 2. If you were to borrow only 1 bit, to create a subnet, you would have only a network number (the .0 network) and a broadcast number (the .255 network). The maximum number of bits that can be borrowed can be any number that leaves at least 2 bits for the host number. In Table 7-9’s example of a Class C IP address, bits from the Host field have been borrowed for the Subnet field. IPv4 Versus IPv6 When TCP/IPa was adopted in the 1980s, it relied on a two-level addressing scheme, which at the time offered adequate scalability. Unfortunately, the architects of TCP/IP could not have predicted that their protocol would eventually sustain a global network of information, commerce, and entertainment. More than 20 years ago, IPv4 offered an addressing strategy that, although scalable for a time, resulted in an inefficient allo- cation of addresses. Table 7-9 Subnet Addresses Decimal Notation for First Host Octet Number of Subnets Number of Class A Hosts Per Subnet Number of Class B Hosts Per Subnet Number of Class C Hosts Per Subnet .192 2 4,194,302 16,382 62 .224 6 2,097,150 8,190 30 .240 14 1,048,574 4,094 14 .248 30 524,286 2,046 6 .252 62 262,142 1,022 2 .254 126 131,070 510 — .255 254 65,534 254 — Lab Activity IP Addressing Basics This exercise helps you develop an understanding of IP addresses and how TCP/IP networks operate. 1102.book Page 404 Tuesday, May 20, 2003 2:53 PM IP Addresses 405 Class A and B addresses make up 75 percent of the IPv4 address space, as shown in Figure 7-34, but a relative handful of organizations (fewer than 17,000) can be assigned a Class A or B network number. Class C network addresses are far more numerous than Class A and Class B addresses, but they account for only 12.5 percent of the possible 4 billion IP addresses. Figure 7-34 IPv4 Address Allocation Unfortunately, Class C addresses are limited to 254 hosts, not meeting the needs of larger organizations that cannot acquire a Class A or B address. Even if there were more Class A, B, and C addresses, too many network addresses would cause Internet routers to grind to a halt under the weight of the enormous routing tables required to store the routes to reach the networks. As early as 1992, the IETF identified two specific concerns: ■ Exhaustion of the remaining, unassigned IPv4 network addresses—At the time, the Class B space was on the verge of depletion. ■ The rapid and substantial increase in the size of Internet routing tables because of the Internet’s growth—As more Class C networks came online, the resulting flood of new network information threatened the capability of Internet routers to cope effectively. Over the past two decades, numerous extensions to IPv4 have been developed that are specifically designed to improve the efficiency with which the 32-bit address space can be used. Two of the more important are subnet masks and CIDR. Meanwhile, an even more extensible and scalable version of IP, IPv6, has been defined and developed. IPv6 uses 128 bits rather than the 32 bits currently used in IPv4, as 1102.book Page 405 Tuesday, May 20, 2003 2:53 PM 406 Chapter 7: TCP/IP Protocol Suite and IP Addressing shown in Figure 7-35. IPv6 uses hexadecimal numbers to represent the 128 bits. It pro- vides 16 billion IP addresses (3.4 × 10 38 addresses). This version of IP should provide sufficient addresses for future communication needs. Figure 7-35 IPv4 and IPv6 The IPv6 shorthand representation of the 128 bits uses eight 16-bit numbers, shown as four hexadecimal digits, as shown in Figure 7-36. The groups of four hex digits are separated by colons. If there are leading 0s in the hex digits, they may be omitted. Figure 7-36 IPv4 and IPv6 After years of planning and development, IPv6 is slowly being implemented in select networks. Eventually, IPv6 might replace IPv4 as the dominant Internet protocol. 1102.book Page 406 Tuesday, May 20, 2003 2:53 PM IP Address Assignment, Acquisition, and Hierarchy 407 IP Address Assignment, Acquisition, and Hierarchy This section discusses how network devices obtain IP addresses. For a network to keep functioning, the IP addresses must be assigned according to a specific hierarchy. How and why this is done are discussed in the following section. IP addresses can be assigned either statically or dynamically. Both methods are covered here. Obtaining an Internet Address For a host on a network to function on the Internet, it needs to obtain a globally unique address. A host’s physical or MAC address is only locally significant. Being locally sig- nificant means that the address can only identify the host in its own LAN. It has no meaning to any device that is not in that LAN. IP is the most widely used global addressing scheme. It is a hierarchical addressing scheme that allows individual addresses to be associated and treated as groups, as shown in Figure 7-37. These groups of addresses allow efficient transfer of data across the Internet. Figure 7-37 Internet Address Hierarchy There are essentially two methods for assigning IP addresses—static addressing and dynamic addressing. The next few sections cover static and dynamic addressing. Regard- less of which addressing scheme is chosen, no two interfaces can have the same IP address. This would cause a conflict that might cause both the hosts involved not to operate properly. 1.0.0.0 2.0.0.0 3.0.0.0 10.0.0.0 11.0.0.0 255.0.0.0 10.1.0.0 10.2.0.0 10.3.0.0 10.255.0.0 10.2.1.0 10.2.2.0 10.2.3.0 10.2.255.0 10.255.1.0 10.255.2.0 10.255.3.0 10.255.255.0 10.2.2.1 10.2.2.2 10.2.2.3 10.2.255.0 10.255.2.1 10.255.2.2 10.255.2.3 10.255.2.255 Internet Address 1102.book Page 407 Tuesday, May 20, 2003 2:53 PM 408 Chapter 7: TCP/IP Protocol Suite and IP Addressing Static Assignment of an IP Address When IP addresses are assigned statically, each device must be configured with an IP address. Each operating system has its own way of configuring TCP/IP. This method requires records of the address assignments to be kept, because problems can occur in a network if duplicate IP addresses are used. Some operating systems, such as Win- dows 95 and Windows NT, send an ARP request to check for a duplicate IP address when they attempt to initialize TCP/IP. If a duplicate is discovered, the operating system does not initialize TCP/IP and generates an error message. Not all operating systems identify duplicate IP addresses. This again emphasizes the need for good record-keeping. The main reason that a device would be assigned a static IP address is if the device needs to be referenced by other devices. A good example is a web server. If a web server got a new IP address each time it started up, it would be difficult to find the web server. As an example of this address changing, if a city were to constantly change street names and building addresses, maps would no longer help you locate a particular building. If an address changes, it is no longer easy to return to the location. If a building is diffi- cult to get to, people will stop trying to locate it. Certain types of devices need to maintain a static IP address. Web servers, network printers, application servers, and routers are good examples of devices that require permanent IP addresses. Address Resolution Protocol For devices to communicate, the sending device needs the destination device’s IP address and MAC address. When a device tries to communicate with a device whose IP addresses it knows, it must determine the MAC addresses. The TCP/IP suite has a protocol called Address Resolution Protocol (ARP) that can automatically obtain the MAC address. ARP lets a computer find the MAC address of the computer that is associated with an IP address, as shown in Figure 7-38. Some devices keep ARP tables, which contain the MAC addresses and IP addresses of other devices that are connected to the same LAN. ARP tables map IP addresses to the corresponding MAC addresses. ARP tables are sections of RAM memory that are maintained automatically on each device, as shown in Tables 7-10 and 7-11. It is rare that you must manually make an ARP table entry. Each computer on a network main- tains its own ARP table. 1102.book Page 408 Tuesday, May 20, 2003 2:53 PM . Per Subnet .19 2 2 4 ,19 4,3 02 16 ,3 82 62 .22 4 6 2, 097 ,15 0 8 ,19 0 30 .24 0 14 1, 048,574 4,094 14 .24 8 30 524 ,28 6 2, 046 6 .25 2 62 2 62 ,14 2 1, 022 2 .25 4 12 6 13 1,070 510 — .25 5 25 4 65,534 25 4 — Lab Activity. properly. 1. 0.0.0 2. 0.0.0 3.0.0.0 10 .0.0.0 11 .0.0.0 25 5.0.0.0 10 .1. 0.0 10 .2. 0.0 10 .3.0.0 10 .25 5.0.0 10 .2 .1. 0 10 .2. 2.0 10 .2. 3.0 10 .2. 255.0 10 .25 5 .1. 0 10 .25 5 .2. 0 10 .25 5.3.0 10 .25 5 .25 5.0 10 .2. 2 .1. 10 .2. 2 .1 10 .2. 2 .2 10 .2. 2.3 10 .2. 255.0 10 .25 5 .2 .1 10 .25 5 .2. 2 10 .25 5 .2. 3 10 .25 5 .2. 255 Internet Address 11 02. book Page 407 Tuesday, May 20 , 20 03 2: 53 PM 408 Chapter 7: TCP/IP Protocol Suite and IP