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Chapter 1

Chapter 1. Network Routing and Review. Learning Objectives. Describe the function of the seven layers of the OSI model Identify network segmentation using repeaters, bridges and routers Define IP address classes and create subnet masks

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Chapter 1

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  1. Chapter 1 Network Routing and Review

  2. Learning Objectives • Describe the function of the seven layers of the OSI model • Identify network segmentation using repeaters, bridges and routers • Define IP address classes and create subnet masks • Understand basic router components and configurations on a Cisco router • Troubleshoot router connectivity problems using ping, trace and debug commands

  3. Chapter Overview • This text is a continuation and expansion of the concepts contained in Course Technology’s CCNA Guide to Networking Fundamentals. • This chapter is a review of several key networking and routing concepts. Those concepts include: • OSI reference model • Network segmentation • IP addressing • Basic router components and configuration • Router connectivity troubleshooting

  4. The OSI Reference Model • The OSI reference model was created in 1984 by the ISO and provides a seven-layer, conceptual model for how devices communicate on a network. • The OSI model solves many problems associated with conceptualizing how devices communicate. • Benefits of using the model include: • Compatibility and standardization between vendors • Interoperability between vendors • Simplified networking model that eases understanding of the communication process • Modular architecture which allows one layer to change without affecting other layers

  5. The OSI Reference Model Continued • The seven layers of the OSI model from the top down are the Application layer, Presentation layer, Session layer, Transport layer, Network layer, Data Link layer and Physical layer. • You can remember the layers from the top down using the following mnemonic: All People Seem To Need Data Processing. • You can remember the layers from the bottom up using the following mnemonic: Please Do Not Throw Sausage Pizza Away.

  6. Seven Layers of the OSI Model

  7. Peer-to-Peer OSI Communication • The seven layers of the OSI reference model communicate with one another via peer-to-peer communication. • Each layer will only talk to its peer on the receiving end. • As a result, each layer is shielded from the activities of all other layers of the model. This is why error checking can occur in two separate layers simultaneously. • Each layer does provide some services to the layer above and receives services from the layer below, but the layers do not acknowledge these services. • Each layer concentrates just on its function in the overall communication process.

  8. Peer-to-Peer Communication

  9. The Seven Layers of the OSI Reference Model • Physical layer - layer 1: The Physical layer is responsible for putting the data in the form of bits represented as voltages onto the physical media. The process of representing bits as voltages is sometimes referred to as encoding. Cables, connectors, and repeaters are common networking devices that function at the Physical layer. • Data Link layer - layer 2: This layer is concerned with MAC addressing, media access, network topology, packaging data into frames, and flow control. The CRC is added as a trailer at this layer. The Data Link layer is further divided into the Logical Link Control and Media Access Control sublayers. Bridges and switches are common networking devices that function at the Data Link layer.

  10. The Seven Layers of the OSI Reference Model Continued • Network layer - layer 3: This layer is responsible for routing packets along the best path between multiple networks. Routing decisions are based upon logical addresses such as TCP/IP addresses and IPX/SPX addresses assigned by the network administrator. Routers function at the Network layer. The IP and IPX protocols function at this layer. • Transport layer - layer 4: This layer ensures that packets arrive intact and in sequence, without duplication. The Transport layer can provide connection-oriented services via the TCP or other connection-oriented protocols. Connectionless services may also be used via UDP or other connectionless protocols. The TCP, UDP, and SPX protocols function at this layer.

  11. The Seven Layers of the OSI Reference Model Continued • Session layer - layer 5: This layer creates, synchronizes, maintains and terminates sessions between applications. Sessions are dialogs between two applications. A good example is an SQL request to retrieve information from a database. The SQL and RPC protocols operate at this layer. • Presentation layer - layer 6: This layer translates data into an intermediary form that can be passed down the OSI stack. If encryption and compression occur, they occur at the Presentation layer. The JPEG, GIF, BMP, WAV, MPEG, MidI, EBCDIC and ASCII protocols and standards operate at this layer.

  12. The Seven Layers of the OSI Reference Model Continued • Application layer - layer 7: This layer interacts directly with network applications. The SMTP protocol operates at this layer. FTP and Telnet are applications that function at this layer. • Although the OSI model specifies seven layers, many other protocol stacks have their own model which may have a different number of layers defined in a somewhat different manner. Still the OSI reference model is the standard and most models can be mapped to the OSI model.

  13. Quick Quiz • At which layer does encryption occur? • At which layer does interhost communication setup and synchronization occur? • At which layer do communications originate? • At which layer does best path selection occur? • With which layer is connection-oriented service associated?

  14. Network Segmentation • Network segmentation is a popular topic on the CCNA exam. You must understand network segmentation using repeaters, bridges, switches, and routers. • Ethernet networks based on the IEEE 802.3 standards use the CSMA/CD access method. • CSMA/CD involves each computer listening to the media to determine if it is free of packets. • If no other computer has packets on the media, the computer can transmit.

  15. Network Segmentation Continued • It is possible for two computers to listen, find the wire empty, and send packets at the same exact time. The end result is a collision in which both packets are destroyed. • The CSMA/CD access method specifies that computers that determine a collision has taken place must perform the Backoff Algorithm. The computers wait a random amount of time before making an attempt to retransmit data. • Problems occur when there are too many computers on the same network segment. • As the number of computers increase, the amount of traffic on a segment increases, as does the chance that two computers will transmit data at exactly the same time.

  16. Network Segmentation Continued • Too many computers on a segment cause the number of collisions to increase. • The result is a large amount of network bandwidth being used to retransmit packets that have been destroyed by collisions. • Segmentation, which is the breaking down of a single heavily populated network segment into smaller segments populated by fewer computers, is the answer to this problem.

  17. Segmentation

  18. Repeaters • Although they don't segment networks, repeaters do play an important role. • Repeaters function at the Physical layer of OSI model and amplify signals that lose strength while travelling along the networking media due to attenuation or electromagnetic interference. • Repeaters do not filter traffic based upon physical or logical addresses. • Repeaters do not decrease the number of collisions that occur on a network segment.

  19. Repeaters Continued • Repeaters are used to increase the length of a cable run beyond its specified limit. Therefore, repeaters extend the catchment area of the network. • For example, by using a repeater with cat 5 cable the signal strength can be boosted to allow an additional 100 meters before attenuation is a problem. • Active hubs connect nodes together and work like repeaters to boost the network signals. • Sometimes hubs are referred to as multiport repeaters.

  20. Bridges • Bridges are networking devices that function at the Data Link layer of the OSI model. Bridges are used to segment networks, using bridging tables of MAC addresses. • Bridges learn the MAC address of every computer attached to their ports by examining frame headers on the data packets. • The bridging tables are kept in RAM and map MAC addresses to bridge ports. In this way a bridge can filter network traffic based on the segment it is on.

  21. Bridge with Bridging Table Figure 1-4

  22. Bridges Continued • If Station A on segment 1 sends a message to Station B on the same segment, the bridge will not pass the packet over to segment 2 because Station B's MAC is listed in the bridging table as residing on segment 1. • This MAC address filtering reduces collisions and the size of collision domains. A collision domain is an area in which collisions are possible. Bridges define collision domains.

  23. Bridges Continued • One of the key disadvantages to using bridges is that a broadcast packet will be passed through the bridge to all nodes on segments connected to the bridge. • A broadcast message has a destination MAC address of FFFF:FFFF:FFFF. Bridges cannot stop broadcasts or limit broadcast storms.

  24. Switches • Switches operate at layer 2 like bridges and are sometimes referred to as multiport bridges. • The key advantage to using switches instead of bridges is that switches use virtual circuits to communicate with the destination node. • The entire bandwidth is dedicated to the connection between the source and destination nodes. • Switches are the preferred device for improving performance on a network.

  25. Routers • In large and complex networks, routers are used for segmentation often in combination with bridges and/or switches. • Routers function at the Network layer of the OSI reference model and segment a large network into smaller subnetworks. • Routers route packets from one network or subnet to another using the best available path.

  26. Routers Continued • Routers build routing tables that map logical network addresses to router ports. • Logical addresses are typically IP or IPX addresses. • Unlike bridges and switches, which use MAC addresses, routers forward packets based on source and destination network addresses. • Routers will not pass broadcast packets. • Bridges and switches define collision domains, but routers define broadcast domains.

  27. Quick Quiz • Which device segments a network into subnetworks? • Which device segments a network based on MAC addresses? • Which device operates at the physical layer to extend the usable distance of the media? • Which devices discussed this far can be used to segment a network? • Which device defines collision domains and which device defines broadcast domains?

  28. IP Addressing • IP addresses are 32-bit logical Network layer addresses consisting of a network ID, an optional subnet ID, and a host id; an example is the IP address 192.168.2.3. • The IP address above is represented in dotted decimal notation. However, each address is actually a binary number consisting of four octets of eight bits each. • The number 192.168.2.3 can be written as 11000000.10101000.00000010.00000011. • Networking devices use the binary number in calculating information about IP addresses.

  29. IP Addressing Continued • IP addresses are grouped into five classes, based on the starting decimal values and bit boundaries in the first octet. • Network administrators interact mostly with Class A, B and C addresses. • Once a network is assigned to an organization by the InterNIC agency, the local administrator ensures that an IP addressing scheme is put in place. • Each node on the network should have a unique IP address, a subnet mask, and, if necessary, a default gateway.

  30. IP Address Classes

  31. IP Addressing Continued • A default gateway is an address used by a device when the destination address is not known. • Usually, the default gateway is an interface on a router. • A default gateway is not required on a node if the network does not have any paths to external networks. • A small LAN without a router will not need a default gateway configured on the hosts.

  32. Subnet Masks • Each IP address consists of a network address and a node address. All devices on an IP network require a subnet mask. • Subnet masks are 32-bit addresses that normally appear in dotted decimal notation. • Subnet masks hide the portion of the address that is the network or subnetwork from the node address. • Networking devices use the subnetwork mask to determine which portion of the address is the network ID and if a node is local or remote.

  33. Subnet Masks • Default subnet masks that place all ones in the network portion of the IP address, are already defined.

  34. Subnetting • Class A, B and C addresses have limits on the number of networks or hosts they can support.

  35. Subnetting Class C • You may encounter a LAN that requires more networks than the default subnet mask of an address will allow. • A Class C address of 192.168.2.0 would allow one network with 254 hosts. This would not work for the network shown below.

  36. Class C With No Subnetting

  37. Subnetting Class C Continued • The network in the previous figure requires three networks; one for the shared serial connection and one each for the Ethernet ports connected via E0. • To allow for the 3 networks in the figure, you must create subnetworks from the network number 192.168.2.0. • Borrow bits from the available host bits to increase the number of network bits. • These borrowed bits will comprise the subnetwork ID.

  38. Subnetting Class C Continued • Once you have determined the network class and default subnet mask, determine how many subnets you need. In this example, you need 3. • You may borrow more bits than you need, however, more subnets equal fewer hosts per subnet. • Use the following formula to determine the number of usable subnets. • Number of Usable Subnets = 2y-2, where y equals the number of bits borrowed from the host portion.

  39. Subnetting Class C Continued • To obtain 3 subnets, you must borrow 3 bits: 23-2=8-2 = 6 • This yields 6 subnets. That is more subnets than necessary in this example but borrowing 2 bits will yield only 2 usable subnets. 22-2=2 • Borrowing 3 bits from the node portion changes the subnet mask from the default of 255.255.255.0 to 255.255.255.224. • Looking at the subnet mask numbers in binary illustrates more clearly how the mask changes.

  40. Subnetting Class C Continued • The default subnet mask is: • 255.255.255.0 or 11111111.11111111.11111111.00000000 • Once you borrow 3 bits the subnet mask becomes: • 255.255.255.224 or 11111111.11111111.11111111.11100000 • Now you must find the range of IP addresses within each subnet. • You can determine the multiplier by looking at the lowest significant bit in the octet in which you borrowed bits. • In this example, the last bit borrowed is the bit representing the 32 value in the octet. Therefore, 32 is the multiplier.

  41. Class C - Borrowing Three Bits

  42. Subnetting Class C Continued • The table below shows the subnet IP address ranges for the network 192.168.2.0 with a subnet mask of 255.255.255.224. Notice 8 subnets are listed instead of 6.

  43. Subnetting Class C Continued • For the purposes of the CCNA exam, the first and last subnetworks are designated as the network and broadcast subnetworks, respectively and unusable. • To calculate the number of hosts per subnet, use the formula: 2x-2, where x equals the number of host bits after borrowing for subnetting • In this example 5 bits are left after 3 are borrowed for subnetting, so the formula to calculate hosts per subnet is 25-2=30. • Once again, two is subtracted in the formula. The first address is reserved for the subnet number and the last for the broadcast address of that subnet.

  44. Subnetting Class B • The ranges for a Class B address are calculated in exactly the same way as for Class C, however, it is slightly more complicated. • In this Class B example we use the same network layout but the Class B network number of 131.107.0.0 and a default subnet mask of 255.255.0.0. • Three networks are still required so 3 bits is still the number you must borrow. This yields 6 usable networks as before. • The Class B subnet mask will change to 255.255.224.0. In this case, the borrowing takes place in the third octet so the final octet will not be affected by the borrowing

  45. Class B - Borrowing Three Bits

  46. Subnetting Class B Continued • The resulting range of addresses for this Class B example is shown in the table on the next slide. • There are the same number of subnets as in the Class C example but the usable hosts per subnet is now 213-2=8190 • The 13 comes from 13 bits left in the host portion after borrowing (16-3=13). • Remember, for the purposes of the CCNA exam, you can not use the first and last subnet.

  47. Subnetting Class B Continued

  48. Quick Quiz • What is the usable decimal value range of the first octet for Class A addresses? • What is the usable decimal value range of the first octet for Class B addresses? • What is the usable decimal value range of the first octet for Class C addresses? • For what is a subnet mask used? • What do the variables x and y represent in the formulas 2x-2=usable hosts per subnet, and 2y-2=usable number of subnets?

  49. Router Components and Configuration • As you learned earlier, routers are internetworking devices that function at the Network layer of the OSI model. • Routers route packets across the best path, among multiple available paths. • Routers do not pass broadcasts and are used in many organizations to limit broadcast domains. • For all their fancy functions, routers are just specialized hardware configured with specialized software to perform the task of routing packets.

  50. Router Components • Cisco routers are powered by the Cisco Internetwork Operating System (IOS). The IOS provides the operating system that allows the routers to be configured to perform certain tasks. • Since each IOS version may implement a feature in a slightly different way, you must be aware of what IOS is in use on your routers. • The show version command displays IOS information including the IOS filename and version.

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