Ccna Course By K.nagi

INDEX •

Networks

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Topology

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Ethernet

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Cable and modes of transmissions

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Devices used in networking

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Cable installation

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OSI model

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Protocols and TCP/IP model

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Addressing and types of logical Address

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IP address

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Subneting

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Router back panel

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Router accessing Modes

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Password setting in Router

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Telnet

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Switching

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Commands

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Networks One way to categorize the different types of computer network designs is by their scope or scale. For historical reasons, the networking industry refers to nearly every type of design as some kind of area network. Common examples of area network types are: •

LAN - Local Area Network

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WLAN - Wireless Local Area Network

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WAN - Wide Area Network

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MAN - Metropolitan Area Network

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SAN - Storage Area Network, System Area Network, Server Area Network, or sometimes Small Area Network

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CAN - Campus Area Network, Controller Area Network, or sometimes Cluster Area Network

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PAN - Personal Area Network

•

DAN - Desk Area Network

LAN and WAN were the original categories of area networks, while the others have gradually emerged over many years of technology evolution. Note that these network types are a separate concept from network topologies such as bus, ring and star.

LAN - Local Area Network

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A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs (perhaps one per room), and occasionally a LAN will span a group of nearby buildings. In TCP/IP networking, a LAN is often but not always implemented as a single IP subnet. In addition to operating in a limited space, LANs are also typically owned, controlled, and managed by a single person or organization. They also tend to use certain connectivity technologies, primarily Ethernet and Token Ring.

WAN - Wide Area Network

As the term implies, a WAN spans a large physical distance. The Internet is the largest WAN, spanning the Earth.

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A WAN is a geographically-dispersed collection of LANs. A network device called a router connects LANs to a WAN. In IP networking, the router maintains both a LAN address and a WAN address. A WAN differs from a LAN in several important ways. Most WANs (like the Internet) are not owned by any one organization but rather exist under collective or distributed ownership and management. WANs tend to use technology like ATM, Frame Relay and X.25 for connectivity over the longer distances.

Metropolitan area network A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of buildings to entire cities. MANs can also depend on communications channels of moderate-to-high data rates. A MAN might be owned and operated by a single organization, but it usually will be used by many individuals and organizations. MANs might also be owned and operated as public utilities. They will often provide means for internetworking of local networks. Metropolitan area networks can span up to 50km, devices used are modem and wire/cable

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Devices used in networking LAN Card A LAN card, more commonly referred to as a NIC, is a device that allows computers to be joined together in a LAN, or local area network. Networked computers communicate with each other using a given protocol or agreed-upon language for transmitting data packets between the different machines, known as nodes. The network interface card acts as the liaison for the machine to both send and receive data on the LAN. The most common language or protocol for LANs is Ethernet, sometimes referred to as IEEE 802.3. A lesser-used protocol is Token Ring. When building a LAN, a network interface card must be installed in each computer on the network and all NICs in the network must be of the same architecture. For example, all must either be Ethernet cards, Token Ring cards, or an alternate technology. An Ethernet network interface card is installed in an available slot inside the computer. The NIC assigns a unique address called a MAC (media access control) to the machine. The MACs on the network are used to direct traffic between the computers. The back plate of the network interface card features a port that looks similar to a phone jack, but is slightly larger. This port accommodates an Ethernet cable, which resembles a thicker version of a standard telephone line. Ethernet cable must run from each network interface card to a central hub or switch. The hub or switch acts like a relay, passing information between computers using the MAC addresses and allowing resources like printers and scanners to be shared along with data. A network interface card does not have to be hard wired with physical cable. Wireless Ethernet cards are installed like their wired counterparts, but rather than a port for an Ethernet cable, the card features a small antenna. The card communicates with the central wireless switch or hub via radio waves. Wireless LANs may have some restrictions 5

depending on the material the building is made from. For example, lead in walls can block signals between the network interface card and hub or switch. When buying components for a LAN, make sure the NICs and hub or switch have the same capabilities. The entire network must be either wired or wireless, so a wireless network interface card cannot talk to a wired switch or hub. In addition, newer versions of hardware will likely support more features and/or greater speeds than older versions. Make sure your central switch or hub can utilize the highest capabilities of the network interface card under consideration. For those who wish to connect LANs located in different areas of the city, state or country, ATM (asynchronous transfer mode) can create wide area networks or WANs by connecting LANs together. LANs are still built with a network interface card in each networked computer, but ATM uses broadband Internet access to link the LANs to online ATM switches. This type of ATM WAN is referred to as an Internetwork.

Switch (Network Switch) A network switch is a small hardware device that joins multiple computers together within one local area network (LAN). Technically, network switches operate at layer two (Data Link Layer) of the OSI model. Network switches appear nearly identical to network hubs, but a switch generally contains more "intelligence" (and a slightly higher price tag) than a hub. Unlike hubs, network switches are capable of inspecting data packets as they are received, determining the source and destination device of that packet, and forwarding it appropriately. By delivering each message only to the connected device it was intended for, a network switch conserves network bandwidth and offers generally better performance than a hub.

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As with hubs, Ethernet implementations of network switches are the most common. Mainstream Ethernet network switches support 10 Mbps, 100 Mbps, or 10/100 Mbps Ethernet standards. Different models of network switches support differing numbers of connected devices. Most consumer-grade network switches provide either four or eight connections for Ethernet devices. Switches can be connected to each other. Such "daisy chaining" allows progressively larger number of devices to join the same LAN.

Routers Routers are physical devices that join multiple wired or wireless networks together. Technically, a wired or wireless router is a Layer 3 gateway, meaning that the wired/wireless router connects networks (as gateways do), and that the router operates at the network layer of the OSI model. Home networkers often use an Internet Protocol (IP) wired or wireless router, IP being the most common OSI network layer protocol. An IP router such as a DSL or cable modem broadband router joins the home's local area network (LAN) to the wide-area network (WAN) of the Internet. By maintaining configuration information in a piece of storage called the "routing table," wired or wireless routers also have the ability to filter traffic, either incoming or outgoing, based on the IP addresses of senders and receivers. Some routers allow the home networker to update the routing table from a Web browser interface. Broadband routers combine the functions of a router with those of a network switch and a firewall in a single unit.

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Ethernet hub A special type of network device called the hub can be found in many home and small business networks. Though they've existed for many years, the popularity of hubs has exploded recently, especially among people relatively new to networking. Do you own a hub, or are you considering purchasing one? This article explains the purpose of hubs and some of the technology behind them... (see below)

General Characteristics of Hubs A hub is a small rectangular box, often made of plastic that receives its power from an ordinary wall outlet. A hub joins multiple computers (or other network devices) together to form a single network segment. On this network segment, all computers can communicate directly with each other. Ethernet hubs are by far the most common type, but hubs for other types of networks such as USB also exist. A hub includes a series of ports that each accept a network cable. Small hubs network four computers. They contain four or sometimes five ports, the fifth port being reserved for "uplink" connections to another hub or similar device. Larger hubs contain eight, 12, 16, and even 24 ports.

Key Features of Hubs Hubs classify as Layer 1 devices in the OSI model. At the physical layer, hubs can support little in the way of sophisticated networking. Hubs do not read any of the data passing through them and are not aware of their source or destination. Essentially, a hub simply receives incoming packets, possibly amplifies the 8

electrical signal, and broadcasts these packets out to all devices on the network - including the one that originally sent the packet! Technically speaking, three different types of hubs exist: •

passive

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active

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intelligent

Passive hubs do not amplify the electrical signal of incoming packets before broadcasting them out to the network. Active hubs, on the other hand, do perform this amplification, as does a different type of dedicated network device called a repeater. Some people use the terms concentrator when referring to a passive hub and multiport repeater when referring to an active hub. Intelligent hubs add extra features to an active hub that are of particular importance to businesses. An intelligent hub typically is stackable (built in such a way that multiple units can be placed one on top of the other to conserve space). It also typically includes remote management capabilities via SNMP and virtual LAN (VLAN) support.

Bridge In computer networking, a bridge divides a LAN into two segments, selectively forwarding traffic across the network boundary it defines. A bridge is not quite the same as a switch.

Repeaters Network repeaters regenerate incoming electrical, wireless or optical signals. With physical media like Ethernet or Wi-Fi, data transmissions can only span a limited distance before the quality of the signal degrades. Repeaters attempt to preserve signal integrity and extend the distance over which data can safely travel. 9

Actual network devices that serve as repeaters usually have some other name. Active hubs, for example, are repeaters. Active hubs are sometimes also called "multiport repeaters," but more commonly they are just "hubs." Other types of "passive hubs" are not repeaters. In Wi-Fi, access points function as repeaters only when operating in socalled "repeater mode." Higher-level devices in the OSI model like switches and routers generally do not incorporate the functions of a repeater. All repeaters are technically OSI physical layer devices.

Modem Traditional modems used in dial-up networking convert data between the analog form used on telephone lines and the digital form used on computers. Standard dial-up network modems transmit data at a maximum rate of 56,000 bits per second (56 Kbps). However, inherent limitations of the public telephone network limit modem speeds to 33.6 Kbps or lower in practice. Broadband modems that are part of cable and DSL Internet service use more advanced signaling techniques to achieve dramatically higher network speeds than traditional modems. Broadband modems are sometimes called "digital modems" and those used for traditional dial-up networking, "analog modems." Cellular modems that establish Internet connectivity through a digital cell phone also exist.

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Topologies In computer networking, topology refers to the layout of connected devices. This article introduces the standard topologies of networking.

Topology in Network Design Think of a topology as a network's virtual shape or structure. This shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle in a family room, but it would be highly unlikely to find a ring topology there. Network topologies are categorized into the following basic types: •

bus

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ring

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star

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tree

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mesh

More complex networks can be built as hybrids of two or more of the above basic topologies.

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Bus Topology

Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message. Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling options many years ago for bus topologies. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.

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Ring Topology

In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network. To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology. Ring topologies are found in some office buildings or school campuses.

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Star Topology

Many home networks use the star topology. A star network features a central connection point called a "hub" that may be a hub, switch or router. Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet. Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entire LAN. (If the hub fails, however, the entire network also fails.)

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Tree Topology

Tree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the "root" of a tree of devices. This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.

Mesh Topology 15

Topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that even in a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, most notably the Internet, employ mesh routing. A mesh network in which every device connects to every other is called a full mesh. As shown in the illustration below, partial mesh networks also exist in which some devices connect only indirectly to others.

Ethernet Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the physical layer, through means of network access at the Media Access Control (MAC)/Data Link Layer, and a common addressing format. Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting end systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN technology. It has been in use from around 1980[1] to the present, largely replacing competing LAN standards such as token ring, FDDI, and ARCNET. 16

Ethernet Cabling Ethernet cabling is an important discussion, especially if you are planning on taking the Cisco exams. Three types of Ethernet cables are available: _ Straight-through cable _ Crossover cable _ Rolled cable We will look at each in the following sections.

Straight-Through Cable The straight-through cable is used to connect _ Host to switch or hub _ Router to switch or hub Four wires are used in straight-through cable to connect Ethernet devices. It is relatively simple to create this type; Figure 1.22 shows the four wires used in a straight-through Ethernet cable. Notice that only pins 1, 2, 3, and 6 are used. Just connect 1 to 1, 2 to 2, 3 to 3, and 6 to 6 and you’ll be up and networking in no time. However, remember that this would be an Ethernet-only cable and wouldn’t work with voice, Token Ring, ISDN, and

so on. Crossover Cable The crossover cable can be used to connect 17

_ Switch to switch _ Hub to hub _ Host to host _ Hub to switch _ Router direct to host The same four wires are used in this cable as in the straight-through cable; we just connect different pins together. Figure 1.23 shows how the four wires are used in a crossover Ethernet cable. Notice that instead of connecting 1 to 1, 2 to 2, and so on, here we connect pins 1 to 3 and 2 to 6 on each side of the cable.

Rolled Cable Although rolled cable isn’t used to connect any Ethernet connections together, you can use a rolled Ethernet cable to connect a host to a router console serial communication (com) port. If you have a Cisco router or switch, you would use this cable to connect your PC running HyperTerminal to the Cisco hardware. Eight wires are used in this cable to connect serial devices, although not all eight are used to send information, just as in Ethernet networking. Figure 1.24 shows the eight wires used in a rolled cable.

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The OSI Reference Model One of the greatest functions of the OSI specifications is to assist in data transfer between disparate Hosts—meaning, for example, that they enable us to transfer data between a UNIX host and a PC or a Mac. The OSI isn’t a physical model, though. Rather, it’s a set of guidelines that application Developers can use to create and implement applications that run on a network. It also provides a framework for creating and implementing networking standards, devices, and internetworking schemes. The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted end to end.

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The upper layers • Provides a user interface • Presents data • Handles processing such as encryption • Keeps different applications’ data separate Application Presentation Session Transport Network Data Link Physical We understand that the user interfaces with the computer at the Application layer and also that the upper layers are responsible for applications communicating between hosts. Remember that none of the upper layers knows anything about networking or network addresses. That’s the responsibility of the four bottom layers. Lower Layer you can see that it’s the four bottom layers that define how data is transferred through a physical wire or through switches and routers. These bottom layers also determine how to rebuild a data stream from a transmitting host to a destination host’s application. The lower layers The following network devices operate at all seven layers of the OSI model: _ Network management stations (NMSs) _ Web and application servers _ Gateways (not default gateways) _ Network hosts Basically, the ISO is pretty much the Emily Post of the network protocol world. Just as Ms. Post wrote the book setting the standards—or protocols—for human social interaction, the ISO developed the OSI reference model as the precedent and guide for an 20

open network protocol set. Defining the etiquette of communication models, it remains today the most popular means of comparison for protocol suites. The OSI reference model has seven layers: _ Application layer (layer 7) _ Presentation layer (layer 6) _ Session layer (layer 5) _ Transport layer (layer 4) _ Network layer (layer 3) _ Data Link layer (layer 2) _ Physical layer (layer 1) The Application Layer The Application layer of the OSI model marks the spot where users actually communicate to the computer. This layer only comes into play when it’s apparent that access to the networks going to be needed soon. Take the case of Internet Explorer (IE). You could uninstall every trace of networking components from a system, such as TCP/IP, NIC card, and so on, and you could still use IE to view a local HTML document —no problem. But things would definitely get messy if you tried to do something like view an HTML document that must be retrieved using HTTP or nab a file with FTP or TFTP. That’s because IE will respond to requests such as those by attempting to access the Application layer. And what’s happening is that the Application layer is acting as an interface between the actual application program—which isn’t at all a part of the layered structure—and the next layer down by providing ways for the application to send information down through the protocol stack. In other words, IE doesn’t truly reside within the Application layer—it interfaces with Application layer protocols when it needs to deal with remote resources. The Application layer is also possible for identifying and establishing the availability of the intended communication partner and determining whether sufficient resources for the intended communication exist. These tasks are important because computer applications sometimes require more than only desktop resources. Often, they’ll unite communicating components from more than one network application. Prime examples are file transfers and email, as well as enabling remote 21

access, network management activities, client/server processes, and information location. Many network applications provide services for communication over enterprise networks, but for present and future internetworking, the need is fast developing to reach beyond the limits of current physical networking. The Presentation Layer The Presentation layer gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting. This layer is essentially a translator and provides coding and conversion functions. A successful data-transfer technique is to adapt the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then convert the data back into its native format for actual reading (for example, EBCDIC to ASCII). By providing Translation services, the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another one. The OSI has protocol standards that define how standard data should be formatted. Tasks like data compression, decompression, encryption, and decryption are associated with this layer. Some Presentation layer standards are involved in multimedia operations too. The Session Layer The Session layer is responsible for setting up, managing, and then tearing down sessions Between Presentations layer entities. This layer also provides dialog control between devices, or nodes. It coordinates communication between systems and serves to organize Their communication by offering three different modes: simplex, half duplex, and full Duplex. To sum up, the Session layer basically keeps different applications’ data separate From other applications’ data. The Transport Layer The Transport layer segments and reassembles data into a data stream. Services located in the Transport layer segment and reassemble data from upper-layer applications and unite it into the same data. They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork.

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Some of you are probably familiar with TCP and UDP already. (But if you’re not, no worries—I’ll tell you all bout them in Chapter 2.) If so, you know that both work at the Transport layer and that TCP is a reliable service and UDP is not. This means that application developers have more options because they have a choice between the two protocols when working with TCP/IP protocols.

IP Addressing One of the most important topics in any discussion of TCP/IP is IP addressing. An IP address is a numeric identifier assigned to each machine on an IP network. It designates the specific location of a device on the network. An IP address is a software address, not a hardware address—the latter is hard-coded on a network interface card (NIC) and used for finding hosts on a local network. IP addressing was designed to allow hosts on one network to communicate with a host on a different network regardless of the type of LANs the hosts are participating in. IP Terminology Throughout this chapter you’ll learn several important terms vital to your understanding of the Internet Protocol. Here are a few to get you started: Bit A bit is one digit, either a 1 or a 0. Byte A byte is 7 or 8 bits, depending on whether parity is used. For the rest of this chapter, always assume a byte is 8 bits. Octet An octet, made up of 8 bits, is just an ordinary 8-bit binary number. In this chapter, the terms byte and octet are completely interchangeable. Network address This is the designation used in routing to send packets to a remote network— for example, 10.0.0.0, 172.16.0.0, and 192.168.10.0. Broadcast address The address used by applications and hosts to send information to all Nodes on a network are called the broadcast address. Examples include 255.255.255.255 23

Which is all networks, all nodes; 172.16.255.255, which is all subnets and hosts on network 172.16.0.0; and 10.255.255.255, which broadcasts to all subnets and hosts on Network 10.0.0.0. The Hierarchical IP Addressing Scheme An IP address consists of 32 bits of information. These bits are divided into four sections, Referred to as octets or bytes, each containing 1 byte (8 bits). You can depict an IP address using one of three methods: _ Dotted-decimal, as in 172.16.30.56 _ Binary, as in 10101100.00010000.00011110.00111000 _ Hexadecimal, as in AC.10.1E.38 All these examples truly represent the same IP address. Hexadecimal isn’t used as often as Dotted-decimal or binary when IP addressing is discussed, but you still might find an IP address stored in hexadecimal in some programs. The Windows Registry is a good example of a program that stores a machine’s IP address in hex. The 32-bit IP address is a structured or hierarchical address, as opposed to a flat or nonhierarchical address. lthough either type of addressing scheme could have been used, hierarchical addressing was chosen for a good reason. The advantage of this scheme is that it can handle a large number of addresses, namely 4.3 billion (a 32-bit address space with two possible values for each position—either 0 or 1—gives you 232, or 4,294,967,296). The disadvantage of the flat addressing scheme, and the reason it’s not used for IP addressing, relates to Routing. If every address were unique, all routers on the Internet would need to store the Address of each and every machine on the Internet. This would make efficient routing impossible, even if only a fraction of the possible addresses were used. The solution to this problem is to use a two- or three-level hierarchical addressing scheme that is structured by network and host or by network, subnet, and host. This two- or three-level scheme is comparable to a telephone number. The first section, the area code, designates a very large area. The second section, the prefix, narrows the scope to a local calling area. The final segment, the customer number, zooms in on the specific connection. IP addresses use the same type of layered structure. Rather than all 32 bits being treated 24

as a unique identifier, as in flat addressing, a part of the address is designated as the network address and the other part is designated as either the subnet and host or just the node address. In the following sections, I’m going to discuss IP network addressing and the different classes of address we can use to address our networks. Network Addressing The network address (which can also be called the network number) uniquely identifies each network. Every machine on the same network shares that network address as part of its IP address. In the IP address 172.16.30.56, for example, 172.16 is the network address. The node address is assigned to, and uniquely identifies, each machine on a network. This part of the address must be unique because it identifies a particular machine—an individual—as opposed to a network, which is a group. This number can also be referred to as a host address. In the sample IP address 172.16.30.56, the 30.56 is the node address. The designers of the Internet decided to create classes of networks based on network size. For the small number of networks possessing a very large number of nodes, they created the rank Class A network. At the other extreme is the Class C network, which is reserved for the numerous networks with a small number of nodes. The class distinction for networks between very large and very small is predictably called the Class B network. Subdividing an IP address into a network and node address is determined by the class designation of one’s network. Figure 2.12 summarizes the three classes of networks—

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To ensure efficient routing, Internet designers defined a mandate for the leading-bits section of the address for each different network class. For example, since a router knows that a Class A network address always starts with a 0, the router might be able to speed a packet on its way after reading only the first bit of its address. This is where the address schemes define the difference between a Class A, a Class B, and a Class C address. In the next sections, I’ll discuss the differences between these three classes, followed by a discussion of the Class D and Class E addresses (Classes A, B, and C are the only ranges that are used to address hosts in our networks). Network Address Range: Class A The designers of the IP address scheme said that the first bit of the first byte in a Class A network address must always be off, or 0. This means a Class A address must be between 0 and 127 in the first byte, inclusive. Consider the following network address: 0xxxxxxx If we turn the other 7 bits all off and then turn them all on, we’ll find the Class A range of network addresses: 00000000 = 0 01111111 = 127 So, a Class A network is defined in the first octet between 0 and 127, and it can’t be less 26

or more. (Yes, I know 0 and 127 are not valid in a Class A network. I’ll talk about reserved addresses in a minute.) Network Address Range: Class B In a Class B network, the RFCs state that the first bit of the first byte must always be turned on but the second bit must always be turned off. If you turn the other 6 bits all off and then all on, you will find the range for a Class B network: 10000000 = 128 10111111 = 191 As you can see, a Class B network is defined when the first byte is configured from 128 to 191. Network Address Range: Class C For Class C networks, the RFCs define the first 2 bits of the first octet as always turned on, but the third bit can never be on. Following the same process as the previous classes, convert from binary to decimal to find the range. Here’s the range for a Class C network: 11000000 = 192 11011111 = 223 So, if you see an IP address that starts at 192 and goes to 223, you’ll know it is a Class C IP address.

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Variable Length Subnet Masks (VLSMs) Classless routing protocols, however, do support the advertisement of subnet information. Therefore, you can use VLSM with routing protocols such as RIPv2, EIGRP, and OSPF. (EIGRP and OSPF will be discussed in Chapter 7.) The benefit of this type of network is that you save a bunch of IP address space with it. As the name suggests, with VLSMs we can have different subnet masks for different router interfaces. In a typical classful network design (RIP or IGRP routing protocols), you could subnet a network like this: 192.168.10.0 = Network 255.255.255.240 (/28) = Mask

Our subnets would be (you know this part, right?) 0, 16, 32, 48, 64, 80, etc. This allows us to assign 16 subnets to our internetwork. But how many hosts would be available on each network? Well, as you probably know by now, each subnet provides only 14 hosts. 28

This means that each LAN has 14 valid hosts available—one LAN doesn’t even have enough addresses needed for all the hosts! But the point-to-point WAN link also has 14 valid hosts. It’s too bad we can’t just nick some valid hosts from that WAN link and give them to our LANs! All hosts and router interfaces have the same subnet mask—again, this is called classful routing. And if we want this network to be more efficient, we definitely need to add different masks to each router interface. But there’s still another problem—the link between the two routers will never use more than two valid hosts! This wastes valuable IP address space, and it’s the big reason I’m going to talk to you about VLSM network design.

VLSM Design Let’s take Figure above and use a classless design…which will become the new network shown in Figure below In the previous example, we wasted address space—one LAN didn’t have enough addresses because every router interface and host used the same subnet mask. Not so good. What would be good is to provide only the needed number of hosts on each router interface. To do this, we use what are referred to as Variable Length Subnet Masks (VLSMs). Now remember that we can use different size masks on each router interface. And if we use /30 on our WAN links and a /27, /28, and /29 on our LANs, we’ll get 2 hosts per WAN interface, and 30, 14, and 8 hosts per LAN interface

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ice! This makes a huge difference—not only can we get just the right amount of hosts

Different Protocols A network protocol defines rules and conventions for communication between network devices. Protocols for computer networking all generally use packet switching techniques to send and receive messages in the form of packets. Network protocols include mechanisms for devices to identify and make connections with each other, as well as formatting rules that specify how data is packaged into messages sent and received. Some protocols also support message acknowledgement and data compression designed for reliable and/or high-performance network communication. Hundreds of different computer network protocols have been developed each designed for specific purposes and environments. 30

File Transfer Protocol (FTP) File Transfer Protocol (FTP) is the protocol that actually lets us transfer files, and it can accomplish this between any two machines using it. But FTP isn’t just a protocol; it’s also a program Operating as a protocol, FTP is used by applications. As a program, it’s employed by users to perform file tasks by hand. FTP also allows for access to both directories and files and can accomplish certain types of directory operations, such as relocating into different ones. FTP teams up with Telnet to transparently log you into the FTP server and then provides for the transfer of files. Simple Mail Transfer Protocol (SMTP) Simple Mail Transfer Protocol (SMTP), answering our ubiquitous call to e-mail, uses a spooled, or queued, method of mail delivery. Once a message has been sent to a destination, the message is spooled to a device—usually a disk. The server software at the destination posts a vigil, regularly checking this queue for messages. When it detects them, it proceeds to deliver them to their destination. SMTP is used to send mail; POP3 is used to receive mail Internet Protocol (IP) Internet Protocol (IP) essentially is the Internet layer. The other protocols found here merely exist to support it. IP holds the big picture and could be said to “see all,” in that it’s aware of all the interconnected networks. It can do this because all the machines on the network have software, or logical, address called an IP address. IP looks at each packet’s address. Then, using a routing table, it decides where a packet is to be sent next, choosing the best path. The protocols of the Network Access layer at the bottom of the DoD model don’t possess IP’s enlightened scope of the entire network; they deal only with physical links (local networks). HTTP 31

HTTP (Hypertext Transfer Protocol) is the set of rules for transferring files (text, graphic images, sound, video, and other multimedia files) on the World Wide Web. As soon as a Web user opens their Web browser, the user is indirectly making use of HTTP. HTTP is an application protocol that runs on top of the TCP/IP suite of protocols (the foundation protocols for the Internet). HTTP concepts include (as the Hypertext part of the name implies) the idea that files can contain references to other files whose selection will elicit additional transfer requests. Any Web server machine contains, in addition to the Web page files it can serve, an HTTP daemon, a program that is designed to wait for HTTP requests and handle them when they arrive. Your Web browser is an HTTP client, sending requests to server machines. When the browser user enters file requests by either "opening" a Web file (typing in a Uniform Resource Locator or URL) or clicking on a hypertext link, the browser builds an HTTP request and sends it to the Internet Protocol address (IP address) indicated by the URL. The HTTP daemon in the destination server machine receives the request and sends back the requested file or files associated with the request. (A Web page often consists of more than one file.)

Telnet Telnet is the chameleon of protocols—its specialty is terminal emulation. It allows a user on a remote client machine, called the Telnet client, to access the resources of another machine, the Telnet server. Telnet achieves this by pulling a fast one on the Telnet server and making the client machine appears as though it were a terminal directly attached to the local network. This projection is actually a software image—a virtual terminal that can interact with the chosen remote host. These emulated terminals are of the text-mode type and can execute refined procedures like displaying menus that give users the opportunity to choose options from them and access the applications on the duped server.

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Users begin a Telnet session by running the Telnet client software and then logging into the Telnet server.

Switching Circuit Switching Switched circuits allow data connections that can be initiated when needed and terminated when communication is complete. This works much like a normal telephone line works for voice communication. Integrated Services Digital Network (ISDN) is a good example of circuit switching. When a router has data for a remote site, the switched circuit is initiated with the circuit number of the remote network. In the case of ISDN circuits, the device actually places a call to the telephone number of the remote ISDN circuit. When the two networks are connected and authenticated, they can transfer data. When the data transmission is complete, the call can be terminated. Figure 3-3 illustrates an example of this type of circuit.

Packet Switching Packet switching is a WAN technology in which users share common carrier resources. Because this allows the carrier to make more efficient use of its infrastructure, the cost to the customer is generally much better than with point-to-point lines. In a packet switching setup, networks have connections into the carrier’s network, and many customers share the carrier’s network. The carrier can then create virtual circuits between customers’ sites by which packets of data are delivered from one to the other through the network. The section of the carrier’s network that is shared is often referred to as a cloud. Some examples of packet-switching networks include Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Services (SMDS), and X.25. Figure 3-4 shows an example packet-switched circuit.

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