CCENT/CCNA ICND1 Official Exam Certification Guide - Chapter 5

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Fundamentals of IP Addressing and Routing OSI vật lý lớp (Layer 1) định nghĩa như thế nào để truyền tải các bit trên một loại mạng vật lý. OSI lớp liên kết dữ liệu (Layer 2) xác định khung, địa chỉ, phát hiện lỗi, và các quy tắc khi sử dụng các phương tiện vật lý. Mặc dù họ là quan trọng, hai lớp này không xác định làm thế nào để cung cấp dữ liệu giữa các thiết bị tồn tại xa nhau, với nhiều mạng vật lý khác nhau ngồi giữa hai máy tính. Chương này giải...

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Nội dung Text: CCENT/CCNA ICND1 Official Exam Certification Guide - Chapter 5

1828xbook.fm Page 93 Thursday, July 26, 2007 3:10 PM 5 CHAPTER Fundamentals of IP Addressing and Routing The OSI physical layer (Layer 1) deﬁnes how to transmit bits over a particular type of physical network. The OSI data link layer (Layer 2) deﬁnes the framing, addressing, error detection, and rules for when to use the physical medium. Although they are important, these two layers do not deﬁne how to deliver data between devices that exist far from each other, with many different physical networks sitting between the two computers. This chapter explains the function and purpose of the OSI network layer (Layer 3): the end-to-end delivery of data between two computers. Regardless of the type of physical network to which each endpoint computer is attached, and regardless of the types of physical networks used between the two computers, the network layer deﬁnes how to forward, or route, data between the two computers. This chapter covers the basics of how the network layer routes data packets from one computer to another. After reviewing the full story at a basic level, this chapter examines in more detail the network layer of TCP/IP, including IP addressing (which enables efﬁcient routing), IP routing (the forwarding process itself), IP routing protocols (the process by which routers learn routes), and several other small but important features of the network layer. “Do I Know This Already?” Quiz The “Do I Know This Already?” quiz allows you to assess whether you should read the entire chapter. If you miss no more than one of these 13 self-assessment questions, you might want to move ahead to the “Exam Preparation Tasks” section. Table 5-1 lists the major headings in this chapter and the “Do I Know This Already?” quiz questions covering the material in those sections. This helps you assess your knowledge of these speciﬁc areas. The answers to the “Do I Know This Already?” quiz appear in Appendix A.
1828xbook.fm Page 94 Thursday, July 26, 2007 3:10 PM 94 Chapter 5: Fundamentals of IP Addressing and Routing “Do I Know This Already?” Foundation Topics Section-to-Question Mapping Table 5-1 Foundation Topics Section Questions Overview of Network Layer Functions 1–3 IP Addressing 4–8 IP Routing 9, 10 IP Routing Protocols 11 Network Layer Utilities 12, 13 Which of the following are functions of OSI Layer 3 protocols? 1. Logical addressing a. Physical addressing b. Path selection c. Arbitration d. Error recovery e. Imagine that PC1 needs to send some data to PC2, and PC1 and PC2 are separated by 2. several routers. What are the largest entities that make it from PC1 to PC2? Frame a. Segment b. Packet c. L5 PDU d. L3 PDU e. L1 PDU f. Imagine a network with two routers that are connected with a point-to-point HDLC 3. serial link. Each router has an Ethernet, with PC1 sharing the Ethernet with Router1, and PC2 sharing the Ethernet with Router2. When PC1 sends data to PC2, which of the following is true? Router1 strips the Ethernet header and trailer off the frame received from PC1, a. never to be used again. Router1 encapsulates the Ethernet frame inside an HDLC header and sends the b. frame to Router2, which extracts the Ethernet frame for forwarding to PC2.
1828xbook.fm Page 95 Thursday, July 26, 2007 3:10 PM “Do I Know This Already?” Quiz 95 Router1 strips the Ethernet header and trailer off the frame received from PC1, c. which is exactly re-created by R2 before forwarding data to PC2. Router1 removes the Ethernet, IP, and TCP headers and rebuilds the appropriate d. headers before forwarding the packet to Router2. Which of the following are valid Class C IP addresses that can be assigned to hosts? 4. 1.1.1.1 a. 200.1.1.1 b. 128.128.128.128 c. 224.1.1.1 d. 223.223.223.255 e. What is the range of values for the ﬁrst octet for Class A IP networks? 5. 0 to 127 a. 0 to 126 b. 1 to 127 c. 1 to 126 d. 128 to 191 e. 128 to 192 f. PC1 and PC2 are on two different Ethernets that are separated by an IP router. PC1’s 6. IP address is 10.1.1.1, and no subnetting is used. Which of the following addresses could be used for PC2? 10.1.1.2 a. 10.2.2.2 b. 10.200.200.1 c. 9.1.1.1 d. 225.1.1.1 e. 1.1.1.1 f.
1828xbook.fm Page 96 Thursday, July 26, 2007 3:10 PM 96 Chapter 5: Fundamentals of IP Addressing and Routing Each Class B network contains how many IP addresses that can be assigned to hosts? 7. 16,777,214 a. 16,777,216 b. 65,536 c. 65,534 d. 65,532 e. 32,768 f. 32,766 g. Each Class C network contains how many IP addresses that can be assigned to hosts? 8. 65,534 a. 65,532 b. 32,768 c. 32,766 d. 256 e. 254 f. Which of the following does a router normally use when making a decision about 9. routing TCP/IP packets? Destination MAC address a. Source MAC address b. Destination IP address c. Source IP address d. Destination MAC and IP address e. Which of the following are true about a LAN-connected TCP/IP host and its IP routing 10. (forwarding) choices? The host always sends packets to its default gateway. a. The host sends packets to its default gateway if the destination IP address is in a b. different class of IP network than the host. The host sends packets to its default gateway if the destination IP address is in a c. different subnet than the host. The host sends packets to its default gateway if the destination IP address is in the d. same subnet as the host.
1828xbook.fm Page 97 Thursday, July 26, 2007 3:10 PM “Do I Know This Already?” Quiz 97 Which of the following are functions of a routing protocol? 11. Advertising known routes to neighboring routers. a. Learning routes for subnets directly connected to the router. b. Learning routes, and putting those routes into the routing table, for routes adver- c. tised to the router by its neighboring routers. To forward IP packets based on a packet’s destination IP address. d. Which of the following protocols allows a client PC to discover the IP address of 12. another computer based on that other computer’s name? ARP a. RARP b. DNS c. DHCP d. Which of the following protocols allows a client PC to request assignment of an IP 13. address as well as learn its default gateway? ARP a. RARP b. DNS c. DHCP d.
1828xbook.fm Page 98 Thursday, July 26, 2007 3:10 PM 98 Chapter 5: Fundamentals of IP Addressing and Routing Foundation Topics OSI Layer 3-equivalent protocols deﬁne how packets can be delivered from the computer that creates the packet all the way to the computer that needs to receive the packet. To reach that goal, an OSI network layer protocol deﬁnes the following features: Routing: The process of forwarding packets (Layer 3 PDUs). Logical addressing: Addresses that can be used regardless of the type of physical networks used, providing each device (at least) one address. Logical addressing enables the routing process to identify a packet’s source and destination. Routing protocol: A protocol that aids routers by dynamically learning about the groups of addresses in the network, which in turn allows the routing (forwarding) process to work well. Other utilities: The network layer also relies on other utilities. For TCP/IP, these utilities include Domain Name System (DNS), Dynamic Host Conﬁguration Protocol (DHCP), Address Resolution Protocol (ARP), and ping. NOTE The term path selection sometimes is used to mean the same thing as routing protocol, sometimes is used to refer to the routing (forwarding) of packets, and sometimes is used for both functions. This chapter begins with an overview of routing, logical addressing, and routing protocols. Following that, the text moves on to more details about the speciﬁcs of the TCP/IP network layer (called the internetwork layer in the TCP/IP model). In particular, the topics of IP addressing, routing, routing protocols, and network layer utilities are covered. Overview of Network Layer Functions A protocol that deﬁnes routing and logical addressing is considered to be a network layer, or Layer 3, protocol. OSI does deﬁne a unique Layer 3 protocol called Connectionless Network Services (CLNS), but, as usual with OSI protocols, you rarely see it in networks today. In the recent past, you might have seen many other network layer protocols, such as Internet Protocol (IP), Novell Internetwork Packet Exchange (IPX), or AppleTalk Datagram Delivery Protocol (DDP). Today, the only Layer 3 protocol that is used widely is the TCP/ IP network layer protocol—speciﬁcally, IP. The main job of IP is to route data (packets) from the source host to the destination host. Because a network might need to forward large numbers of packets, the IP routing process is very simple. IP does not require any overhead agreements or messages before sending a packet, making IP a connectionless protocol. IP tries to deliver each packet, but if a router or host’s IP process cannot deliver the packet, it is discarded—with no error recovery. The
1828xbook.fm Page 99 Thursday, July 26, 2007 3:10 PM Overview of Network Layer Functions 99 goal with IP is to deliver packets with as little per-packet work as possible, which allows for large packet volumes. Other protocols perform some of the other useful networking functions. For example, Transmission Control Protocol (TCP), which is described in detail in Chapter 6, “Fundamentals of TCP/IP Transport, Applications, and Security,” provides error recovery, resending lost data, but IP does not. IP routing relies on the structure and meaning of IP addresses, and IP addressing was designed with IP routing in mind. This ﬁrst major section of this chapter begins by introducing IP routing, with some IP addressing concepts introduced along the way. Then, the text examines IP addressing fundamentals. Routing (Forwarding) Routing focuses on the end-to-end logic of forwarding data. Figure 5-1 shows a simple example of how routing works. The logic illustrated by the ﬁgure is relatively simple. For PC1 to send data to PC2, it must send something to router R1, which sends it to router R2, and then to router R3, and ﬁnally to PC2. However, the logic used by each device along the path varies slightly. Routing Logic: PC1 Sending to PC2 Figure 5-1 10.1.1.1 Destination Is in Another Group; Send PC1 to Nearby Router. 10.0.0.0 My Route to that Group Is Out Serial Link. R1 168.0.0.0 My Route to that Group Is Out Frame R2 Relay. 168.11.0.0 Send Directly to PC2 R3 168.1.0.0 PC2 168.1.1.1
1828xbook.fm Page 100 Thursday, July 26, 2007 3:10 PM 100 Chapter 5: Fundamentals of IP Addressing and Routing PC1’s Logic: Sending Data to a Nearby Router In this example, illustrated in Figure 5-1, PC1 has some data to send to PC2. Because PC2 is not on the same Ethernet as PC1, PC1 needs to send the packet to a router that is attached to the same Ethernet as PC1. The sender sends a data-link frame across the medium to the nearby router; this frame includes the packet in the data portion of the frame. That frame uses data link layer (Layer 2) addressing in the data-link header to ensure that the nearby router receives the frame. The main point here is that the computer that created the data does not know much about the network—just how to get the data to some nearby router. Using a post ofﬁce analogy, it’s like knowing how to get to the local post ofﬁce, but nothing more. Likewise, PC1 needs to know only how to get the packet to R1, not the rest of the path used to send the packet to PC2. R1 and R2’s Logic: Routing Data Across the Network R1 and R2 both use the same general process to route the packet. The routing table for any particular network layer protocol contains a list of network layer address groupings. Instead of a single entry in the routing table per individual destination network layer address, there is one routing table entry per group. The router compares the destination network layer address in the packet to the entries in the routing table and makes a match. This matching entry in the routing table tells this router where to forward the packet next. The words in the bubbles in Figure 5-1 point out this basic logic. The concept of network layer address grouping is similar to the U.S. zip code system. Everyone living in the same vicinity is in the same zip code, and the postal sorters just look for the zip codes, ignoring the rest of the address. Likewise, in Figure 5-1, everyone in this network whose IP address starts with 168.1 is on the Ethernet on which PC2 resides, so the routers can have just one routing table entry that means “all addresses that start with 168.1.” Any intervening routers repeat the same process: the router compares the packet’s destination network layer (Layer 3) address to the groups listed in its routing table, and the matched routing table entry tells this router where to forward the packet next. Eventually, the packet is delivered to the router connected to the network or subnet of the destination host (R3), as shown in Figure 5-1. R3’s Logic: Delivering Data to the End Destination The ﬁnal router in the path, R3, uses almost the exact same logic as R1 and R2, but with one minor difference. R3 needs to forward the packet directly to PC2, not to some other router. On the surface, that difference seems insigniﬁcant. In the next section, when you read about how the network layer uses the data link layer, the signiﬁcance of the difference will become obvious.
1828xbook.fm Page 101 Thursday, July 26, 2007 3:10 PM Overview of Network Layer Functions 101 Network Layer Interaction with the Data Link Layer When the network layer protocol is processing the packet, it decides to send the packet out the appropriate network interface. Before the actual bits can be placed onto that physical interface, the network layer must hand off the packet to the data link layer protocols, which, in turn, ask the physical layer to actually send the data. And as was described in Chapter 3, “Fundamentals of LANs,” the data link layer adds the appropriate header and trailer to the packet, creating a frame, before sending the frames over each physical network. The routing process forwards the packet, and only the packet, end-to-end through the network, discarding data-link headers and trailers along the way. The network layer processes deliver the packet end-to-end, using successive data-link headers and trailers just to get the packet to the next router or host in the path. Each successive data link layer just gets the packet from one device to the next. Figure 5-2 points out the key encapsulation logic on each device, using the same examples as in Figure 5-1. Network Layer and Data Link Layer Encapsulation Figure 5-2 10.1.1.1 Encapsulate IP Packet in PC1 Ethernet Eth. IP Packet 10.0.0.0 Extract IP Packet and R1 Encapsulate in HDLC 168.10.0.0 HDLC IP Packet Extract IP Packet, and Encapsulate in R2 Frame Relay FR IP Packet 168.11.0.0 Extract IP Packet, and Encapsulate in R3 Ethernet Eth IP Packet 168.1.0.0 PC2 168.1.1.1
1828xbook.fm Page 102 Thursday, July 26, 2007 3:10 PM 102 Chapter 5: Fundamentals of IP Addressing and Routing Because the routers build new data-link headers and trailers (trailers not shown in the ﬁgure), and because the new headers contain data-link addresses, the PCs and routers must have some way to decide what data-link addresses to use. An example of how the router determines which data-link address to use is the IP Address Resolution Protocol (ARP). ARP is used to dynamically learn the data-link address of an IP host connected to a LAN. You will read more about ARP later in this chapter. Routing as covered so far has two main concepts: The process of routing forwards Layer 3 packets, also called Layer 3 protocol data ■ units (L3 PDU), based on the destination Layer 3 address in the packet. The routing process uses the data link layer to encapsulate the Layer 3 packets into ■ Layer 2 frames for transmission across each successive data link. IP Packets and the IP Header The IP packets encapsulated in the data-link frames shown in Figure 5-2 have an IP header, followed by additional headers and data. For reference, Figure 5-3 shows the ﬁelds inside the standard 20-byte IPv4 header, with no optional IP header ﬁelds, as is typically seen in most networks today. IPv4 Header Figure 5-3 0 8 16 24 31 Header Version DS Field Packet Length Length Identification Flags (3) Fragment Offset (13) Protocol Time to Live Header Checksum Source IP Address Destination IP Address Of the different ﬁelds inside the IPv4 header, this book, and the companion ICND2 Ofﬁcial Exam Certiﬁcation Guide, ignore all the ﬁelds except the Time-To-Live (TTL) (covered in Chapter 15 in this book), protocol (Chapter 6 of the ICND2 book), and the source and destination IP address ﬁelds (scattered throughout most chapters). However, for reference, Table 5-2 brieﬂy describes each ﬁeld.
1828xbook.fm Page 103 Thursday, July 26, 2007 3:10 PM Overview of Network Layer Functions 103 IPv4 Header Fields Table 5-2 Field Meaning Version Version of the IP protocol. Most networks use version 4 today. IHL IP Header Length. Deﬁnes the length of the IP header, including optional ﬁelds. DS Field Differentiated Services Field. It is used for marking packets for the purpose of applying different quality-of-service (QoS) levels to different packets. Packet length Identiﬁes the entire length of the IP packet, including the data. Identiﬁcation Used by the IP packet fragmentation process; all fragments of the original packet contain the same identiﬁer. Flags 3 bits used by the IP packet fragmentation process. Fragment offset A number used to help hosts reassemble fragmented packets into the original larger packet. TTL Time to live. A value used to prevent routing loops. Protocol A ﬁeld that identiﬁes the contents of the data portion of the IP packet. For example, protocol 6 implies that a TCP header is the ﬁrst thing in the IP packet data ﬁeld. Header Checksum A value used to store an FCS value, whose purpose is to determine if any bit errors occurred in the IP header. Source IP address The 32-bit IP address of the sender of the packet. Destination IP The 32-bit IP address of the intended recipient of the packet. address This section next examines the concept of network layer addressing and how it aids the routing process. Network Layer (Layer 3) Addressing Network layer protocols deﬁne the format and meaning of logical addresses. (The term logical address does not really refer to whether the addresses make sense, but rather to contrast these addresses with physical addresses.) Each computer that needs to communicate will have (at least) one network layer address so that other computers can send data packets to that address, expecting the network to deliver the data packet to the correct computer. One key feature of network layer addresses is that they were designed to allow logical grouping of addresses. In other words, something about the numeric value of an address implies a group or set of addresses, all of which are considered to be in the same grouping. With IP addresses, this group is called a network or a subnet. These groupings work just like USPS zip (postal) codes, allowing the routers (mail sorters) to speedily route (sort) lots of packets (letters).
1828xbook.fm Page 104 Thursday, July 26, 2007 3:10 PM 104 Chapter 5: Fundamentals of IP Addressing and Routing Just like postal street addresses, network layer addresses are grouped based on physical location in a network. The rules differ for some network layer protocols, but with IP addressing, the ﬁrst part of the IP address is the same for all the addresses in one grouping. For example, in Figures 5-1 and 5-2, the following IP addressing conventions deﬁne the groups of IP addresses (IP networks) for all hosts on that internetwork: Hosts on the top Ethernet: Addresses start with 10 ■ Hosts on the R1-R2 serial link: Addresses start with 168.10 ■ Hosts on the R2-R3 Frame Relay network: Addresses start with 168.11 ■ Hosts on the bottom Ethernet: Addresses start with 168.1 ■ NOTE To avoid confusion when writing about IP networks, many resources (including this one) use the term internetwork to refer more generally to a network made up of routers, switches, cables, and other equipment, and the word network to refer to the more speciﬁc concept of an IP network. Routing relies on the fact that Layer 3 addresses are grouped. The routing tables for each network layer protocol can have one entry for the group, not one entry for each individual address. Imagine an Ethernet with 100 TCP/IP hosts. A router that needs to forward packets to any of those hosts needs only one entry in its IP routing table, with that one routing table entry representing the entire group of hosts on the Ethernet. This basic fact is one of the key reasons that routers can scale to allow hundreds of thousands of devices. It’s very similar to the USPS zip code system. It would be ridiculous to have people in the same zip code live far from each other, or to have next-door neighbors be in different zip codes. The poor postman would spend all his time driving and ﬂying around the country! Similarly, to make routing more efﬁcient, network layer protocols group addresses. Routing Protocols Conveniently, the routers in Figures 5-1 and 5-2 somehow know the correct steps to take to forward the packet from PC1 to PC2. To make the correct choices, each router needs a routing table, with a route that matches the packet sent to PC2. The routes tell the router where to send the packet next. In most cases, routers build their routing table entries dynamically using a routing protocol. Routing protocols learn about all the locations of the network layer “groups” in a network and advertise the groups’ locations. As a result, each router can build a good routing table dynamically. Routing protocols deﬁne message formats and procedures, just like any other protocol. The end goal of each routing protocol is to ﬁll the routing table with all known destination groups and with the best route to reach each group.
1828xbook.fm Page 105 Thursday, July 26, 2007 3:10 PM IP Addressing 105 The terminology relating to routing protocols sometimes can get in the way. A routing protocol learns routes and puts those routes in a routing table. A routed protocol deﬁnes the type of packet forwarded, or routed, through a network. In Figures 5-1 and 5-2, the ﬁgures represent how IP packets are routed, so IP would be the routed protocol. If the routers used Routing Information Protocol (RIP) to learn the routes, RIP would be the routing protocol. Later in this chapter, the section “IP Routing Protocols” shows a detailed example of how routing protocols learn routes. Now that you have seen the basic function of the OSI network layer at work, the rest of this chapter examines the key components of the end-to-end routing process for TCP/IP. IP Addressing IP addressing is absolutely the most important topic for the CCNA exams. By the time you have completed your study, you should be comfortable and conﬁdent in your understanding of IP addresses, their formats, the grouping concepts, how to subdivide groups into subnets, how to interpret the documentation for existing networks’ IP addressing, and so on. Simply put, you had better know addressing and subnetting! This section introduces IP addressing and subnetting and also covers the concepts behind the structure of an IP address, including how it relates to IP routing. In Chapter 12, “IP Addressing and Subnetting,” you will read about the math behind IP addressing and subnetting. IP Addressing Definitions If a device wants to communicate using TCP/IP, it needs an IP address. When the device has an IP address and the appropriate software and hardware, it can send and receive IP packets. Any device that can send and receive IP packets is called an IP host. NOTE IP Version 4 (IPv4) is the most widely used version of IP. The ICND2 Ofﬁcial Exam Certiﬁcation Guide covers the newer version of IP, IPv6. This book only brieﬂy mentions IPv6 in Chapter 12 and otherwise ignores it. So, all references to IP addresses in this book should be taken to mean “IP version 4” addresses. IP addresses consist of a 32-bit number, usually written in dotted-decimal notation. The “decimal” part of the term comes from the fact that each byte (8 bits) of the 32-bit IP address is shown as its decimal equivalent. The four resulting decimal numbers are written in sequence, with “dots,” or decimal points, separating the numbers—hence the name dotted decimal. For instance, 168.1.1.1 is an IP address written in dotted-decimal form; the actual binary version is 10101000 00000001 00000001 00000001. (You almost never need to write down the binary version, but you will see how to convert between the two formats in Chapter 12.)
1828xbook.fm Page 106 Thursday, July 26, 2007 3:10 PM 106 Chapter 5: Fundamentals of IP Addressing and Routing Each decimal number in an IP address is called an octet. The term octet is just a vendor- neutral term for byte. So, for an IP address of 168.1.1.1, the ﬁrst octet is 168, the second octet is 1, and so on. The range of decimal numbers in each octet is between 0 and 255, inclusive. Finally, note that each network interface uses a unique IP address. Most people tend to think that their computer has an IP address, but actually their computer’s network card has an IP address. If you put two Ethernet cards in a PC to forward IP packets through both cards, they both would need unique IP addresses. Also, if your laptop has both an Ethernet NIC and a wireless NIC working at the same time, your laptop will have an IP address for each NIC. Similarly, routers, which typically have many network interfaces that forward IP packets, have an IP address for each interface. Now that you have some idea of the basic terminology, the next section relates IP addressing to the routing concepts of OSI Layer 3. How IP Addresses Are Grouped The original speciﬁcations for TCP/IP grouped IP addresses into sets of consecutive addresses called IP networks. The addresses in a single network have the same numeric value in the ﬁrst part of all addresses in the network. Figure 5-4 shows a simple internetwork that has three separate IP networks. Sample Network Using Class A, B, and C Network Numbers Figure 5-4 Network 199.1.1.0 All IP addresses that begin with 199.1.1 Network Network 130.4.0.0 8.0.0.0 All IP addresses All IP addresses that that begin with 8 begin with 130.4 The conventions of IP addressing and IP address grouping make routing easy. For example, all IP addresses that begin with 8 are in the IP network that contains all the hosts on the Ethernet on the left. Likewise, all IP addresses that begin with 130.4 are in another IP network that consists of all the hosts on the Ethernet on the right. Along the same lines, 199.1.1 is the preﬁx for all IP addresses on the network that includes the
1828xbook.fm Page 107 Thursday, July 26, 2007 3:10 PM IP Addressing 107 addresses on the serial link. (The only two IP addresses in this last grouping will be the IP addresses on each of the two routers.) By following this convention, the routers build a routing table with three entries—one for each preﬁx, or network number. For example, the router on the left can have one route that refers to all addresses that begin with 130.4, with that route directing the router to forward packets to the router on the right. The example indirectly points out a couple of key points about how IP addresses are organized. To be a little more explicit, the following two rules summarize the facts about which IP addresses need to be in the same grouping: All IP addresses in the same group must not be separated by a router. ■ IP addresses separated by a router must be in different groups. ■ As mentioned earlier in this chapter, IP addressing behaves similarly to zip codes. Everyone in my zip code lives in a little town in Ohio. If some members of my zip code were in California, some of my mail might be sent to California by mistake. Likewise, IP routing relies on the fact that all IP addresses in the same group (called either a network or a subnet) are in the same general location. If some of the IP addresses in my network or subnet were allowed to be on the other side of the internetwork compared to my computer, the routers in the network might incorrectly send some of the packets sent to my computer to the other side of the network. Classes of Networks Figure 5-4 and the surrounding text claim that the IP addresses of devices attached to the Ethernet on the left all start with 8 and that the IP addresses of devices attached to the Ethernet on the right all start with 130.4. Why only one number (8) for the “preﬁx” on the Ethernet on the left and two numbers (130 and 4) on the Ethernet on the right? Well, it all has to do with IP address classes. RFC 791 deﬁnes the IP protocol, including several different classes of networks. IP deﬁnes three different network classes for addresses used by individual hosts—addresses called unicast IP addresses. These three network classes are called A, B, and C. TCP/IP deﬁnes Class D (multicast) addresses and Class E (experimental) addresses as well. By deﬁnition, all addresses in the same Class A, B, or C network have the same numeric value network portion of the addresses. The rest of the address is called the host portion of the address. Using the post ofﬁce example, the network part of an IP address acts like the zip (postal) code, and the host part acts like the street address. Just as a letter-sorting machine three states away from you cares only about the zip code on a letter addressed to you,
1828xbook.fm Page 108 Thursday, July 26, 2007 3:10 PM 108 Chapter 5: Fundamentals of IP Addressing and Routing a router three hops away from you cares only about the network number that your address resides in. Class A, B, and C networks each have a different length for the part that identiﬁes the network: Class A networks have a 1-byte-long network part. That leaves 3 bytes for the rest of ■ the address, called the host part. Class B networks have a 2-byte-long network part, leaving 2 bytes for the host portion ■ of the address. Class C networks have a 3-byte-long network part, leaving only 1 byte for the host part. ■ For example, Figure 5-4 lists network 8.0.0.0 next to the Ethernet on the left. Network 8.0.0.0 is a Class A network, which means that only 1 octet (byte) is used for the network part of the address. So, all hosts in network 8.0.0.0 begin with 8. Similarly, Class B network 130.4.0.0 is listed next to the Ethernet on the right. Because it is a Class B network, 2 octets deﬁne the network part, and all addresses begin with 130.4 as the ﬁrst 2 octets. When listing network numbers, the convention is to write down the network part of the number, with all decimal 0s in the host part of the number. So, Class A network “8,” which consists of all IP addresses that begin with 8, is written as 8.0.0.0. Similarly, Class B network “130.4,” which consists of all IP addresses that begin with 130.4, is written as 130.4.0.0, and so on. Now consider the size of each class of network. Class A networks need 1 byte for the network part, leaving 3 bytes, or 24 bits, for the host part. There are 224 different possible values in the host part of a Class A IP address. So, each Class A network can have 224 IP addresses—except for two reserved host addresses in each network, as shown in the last column of Table 5-3. The table summarizes the characteristics of Class A, B, and C networks. Sizes of Network and Host Parts of IP Addresses with No Subnetting Table 5-3 Any Network of This Number of Network Number of Host Number of Addresses Per Network* Class Bytes (Bits) Bytes (Bits) 224 – 2 A 1 (8) 3 (24) 216 – 2 B 2 (16) 2 (16) 28 – 2 C 3 (24) 1 (8) *There are two reserved host addresses per network. Based on the three examples from Figure 5-4, Table 5-4 provides a closer look at the numeric version of the three network numbers: 8.0.0.0, 130.4.0.0, and 199.1.1.0.
1828xbook.fm Page 109 Thursday, July 26, 2007 3:10 PM IP Addressing 109 Sample Network Numbers, Decimal and Binary Table 5-4 Network Number Binary Representation, with the Host Part in Bold 8.0.0.0 00001000 00000000 00000000 00000000 130.4.0.0 10000010 00000100 00000000 00000000 199.1.1.0 11000111 00000001 00000001 00000000 Even though the network numbers look like addresses because of their dotted-decimal format, network numbers cannot be assigned to an interface to be used as an IP address. Conceptually, network numbers represent the group of all IP addresses in the network, much like a zip code represents the group of all addresses in a community. It would be confusing to have a single number represent a whole group of addresses and then also use that same number as an IP address for a single device. So, the network numbers themselves are reserved and cannot be used as an IP address for a device. Besides the network number, a second dotted-decimal value in each network is reserved. Note that the ﬁrst reserved value, the network number, has all binary 0s in the host part of the number (see Table 5-4). The other reserved value is the one with all binary 1s in the host part of the number. This number is called the network broadcast or directed broadcast address. This reserved number cannot be assigned to a host for use as an IP address. However, packets sent to a network broadcast address are forwarded to all devices in the network. Also, because the network number is the lowest numeric value inside that network and the broadcast address is the highest numeric value, all the numbers between the network number and the broadcast address are the valid, useful IP addresses that can be used to address interfaces in the network. The Actual Class A, B, and C Network Numbers The Internet is a collection of almost every IP-based network and almost every TCP/IP host computer in the world. The original design of the Internet required several cooperating features that made it technically possible as well as administratively manageable: Each computer connected to the Internet needs a unique, nonduplicated IP address. ■ Administratively, a central authority assigned Class A, B, or C networks to companies, ■ governments, school systems, and ISPs based on the size of their IP network (Class A for large networks, Class B for medium networks, and Class C for small networks). The central authority assigned each network number to only one organization, helping ■ ensure unique address assignment worldwide. Each organization with an assigned Class A, B, or C network then assigned individual ■ IP addresses inside its own network.
1828xbook.fm Page 110 Thursday, July 26, 2007 3:10 PM 110 Chapter 5: Fundamentals of IP Addressing and Routing By following these guidelines, as long as each organization assigns each IP address to only one computer, every computer in the Internet has a globally unique IP address. NOTE The details of address assignment have changed over time, but the general idea described here is enough detail to help you understand the concept of different Class A, B, and C networks. The organization in charge of universal IP address assignment is the Internet Corporation for Assigned Network Numbers (ICANN, www.icann.org). (The Internet Assigned Numbers Authority (IANA) formerly owned the IP address assignment process.) ICANN, in turn, assigns regional authority to other cooperating organizations. For example, the American Registry for Internet Numbers (ARIN, www.arin.org) owns the address assignment process for North America. Table 5-5 summarizes the possible network numbers that ICANN and other agencies could have assigned over time. Note the total number for each network class and the number of hosts in each Class A, B, and C network. All Possible Valid Network Numbers* Table 5-5 First Octet Valid Network Total Number for This Number of Hosts Numbers* Class Range Class of Network Per Network 27 – 2 (126) 224 – 2 (16,777,214) A 1 to 126 1.0.0.0 to 126.0.0.0 214 (16,384) 216 – 2 (65,534) B 128 to 191 128.0.0.0 to 191.255.0.0 221 (2,097,152) 28 – 2 (254) C 192 to 223 192.0.0.0 to 223.255.255.0 *The Valid Network Numbers column shows actual network numbers. Networks 0.0.0.0 (originally defined for use as a broadcast address) and 127.0.0.0 (still available for use as the loopback address) are reserved. Memorizing the contents of Table 5-5 should be one of the ﬁrst things you do in preparation for the CCNA exam(s). Engineers should be able to categorize a network as Class A, B, or C with ease. Also, memorize the number of octets in the network part of Class A, B, and C addresses, as shown in Table 5-4. IP Subnetting Subnetting is one of the most important topics on the ICND1, ICND2, and CCNA exams. You need to know how it works and how to “do the math” to ﬁgure out issues when subnetting is in use, both in real life and on the exam. Chapter 12 covers the details of subnetting concepts, motivation, and math, but you should have a basic understanding of the concepts before
1828xbook.fm Page 111 Thursday, July 26, 2007 3:10 PM IP Addressing 111 covering the topics between here and Chapter 12. IP subnetting takes a single Class A, B, or C network and subdivides it into a number of smaller groups of IP addresses. The Class A, B, and C rules still exist, but now, a single Class A, B, or C network can be subdivided into many smaller groups. Subnetting treats a subdivision of a single Class A, B, or C network as if it were a network itself. In fact, the name “subnet” is just shorthand for “subdivided network.” You can easily discern the concepts behind subnetting by comparing one network topology that does not use subnetting with the same topology but with subnetting implemented. Figure 5-5 shows such a network, without subnetting. Backdrop for Discussing Numbers of Different Networks/Subnetworks Figure 5-5 150.1.0.0 150.2.0.0 Ray Hannah A B Fay Jessie Frame Relay 150.5.0.0 150.6.0.0 C D 150.4.0.0 150.3.0.0 Kris Wendell Vinnie The design in Figure 5-5 requires six groups of IP addresses, each of which is a Class B network in this example. The four LANs each use a single Class B network. In other words, each of the LANs attached to routers A, B, C, and D is in a separate IP network. Additionally, the two serial interfaces composing the point-to-point serial link between routers C and D use one IP network because these two interfaces are not separated by a router. Finally, the three router interfaces composing the Frame Relay network with routers A, B, and C are not separated by an IP router and would use a sixth IP network.
1828xbook.fm Page 112 Thursday, July 26, 2007 3:10 PM 112 Chapter 5: Fundamentals of IP Addressing and Routing Each Class B network has 216 – 2 host addresses—far more than you will ever need for each LAN and WAN link. For example, the upper-left Ethernet should contain all addresses that begin with 150.1. Therefore, addresses that begin with 150.1 cannot be assigned anywhere else in the network, except on the upper-left Ethernet. So, if you ran out of IP addresses somewhere else, you could not use the large number of unused addresses that begin with 150.1. As a result, the addressing design shown in Figure 5-5 wastes a lot of addresses. In fact, this design would not be allowed if it were connected to the Internet. The ICANN member organization would not assign six separate registered Class B network numbers. In fact, you probably would not get even one Class B network, because most of the Class B addresses are already assigned. You more likely would get a couple of Class C networks with the expectation that you would use subnetting. Figure 5-6 illustrates a more realistic example that uses basic subnetting. Using Subnets Figure 5-6 150.150.1.0 150.150.2.0 Ray Hannah 150.150.1.1 150.150.2.1 A B Fay Jessie 150.150.1.2 150.150.2.2 Frame Relay 150.150.5.0 150.150.6.0 C D 150.150.4.0 150.150.3.0 Kris Wendell Vinnie 150.150.4.2 150.150.4.1 150.150.3.1 As in Figure 5-5, the design in Figure 5-6 requires six groups. Unlike Figure 5-5, this ﬁgure uses six subnets, each of which is a subnet of a single Class B network. This design subdivides the Class B network 150.150.0.0 into six subnets. To perform subnetting, the third octet (in this example) is used to identify unique subnets of network 150.150.0.0.