Cisco ICND1 Section 1 Networking Fundamentals

1. Name the seven layers of the OSI model.

1. Answer: Application (Layer 7), presentation (Layer 6), session (Layer 5), transport (Layer 4), network (Layer 3), data link (Layer 2), and physical (Layer 1). Some mnemonics to help you recall the names of the layers are: All People Seem To Need Data Processing (Layers 7 to 1), Please Do Not Take Sausage Pizzas Away (Layers 1 to 7), and the ever-popular Pew! Dead Ninja Turtles Smell Particularly Awful (Layers 1 to 7).

10. Name three benefits to layering networking protocol specifications.

10. Answer: Some examples of benefits to layering networking protocol specifications include reduced complexity, standardized interfaces, modular engineering, interoperable technology, accelerated evolution, and simplified teaching and learning. Questions such as this on the exam require some subjective interpretation of the wording on your part.

11. What header or trailer does a router discard as a side effect of routing?

11. Answer: A router discards the data-link header and trailer as a side effect of routing. This is because the network layer, where routing is defined, is interested in delivering the network layer (Layer 3) PDU from end to end. Routing uses intermediate data links (Layer 2) to transport the data to the next routers and eventually to the true destination. The data-link header and trailer are useful only to deliver the data to the next router or host, so the header and trailer are discarded by each router.

12. What OSI layer typically encapsulates using both a header and a trailer?

12. Answer: The data link layer typically encapsulates using both a header and a trailer. The trailer typically includes a frame check sequence (FCS), which is used to perform error detection.

13. What terms are used to describe the contents of the data encapsulated by the data link, network, and transport layers, respectively?

13. Answer: Frame, packet, and segment, respectively.

14. Explain the meaning of the term L5PDU.

14. Answer: PDU stands for protocol data unit. A PDU is the entity that includes the headers and trailers created by a particular networking layer, plus any encapsulated data. For instance, an L5PDU includes Layer 5 headers and the encapsulated data.

15. Explain how Layer x on one computer communicates with Layer x on another computer.

15. Answer: Each layer of a networking model works with the same layer on another computer with which it wants to communicate. The protocol defined by each layer uses a header that is transmitted between the computers to communicate what each computer wants to do.

16. List the terms behind the acronym TCP/IP.

16. Answer: Transmission Control Protocol and Internet Protocol.

17. List the terms behind the acronym OSI.

17. Answer: Open System Interconnection.

18. What is the main purpose(s) of Layer 2?

18. Answer: Layer 2 (the data link layer) defines addressing specific to a particular medium as part of the means of providing delivery of data across that medium. It also includes the protocols used to determine what device(s) accesses the media at any point in time.

19. What is the main purpose(s) of Layer 1?

19. Answer: Layer 1 (the physical layer) is responsible for encoding energy signals onto the medium and interpreting a received energy signal. Layer 1 also defines the connector and cabling details.

2. What is/are the main purpose(s) of Layer 7?

2. Answer: Layer 7 (the application layer) provides standardized services to applications. The definition for this layer is typically ambiguous because it varies. The key is that it does not define a user interface, but instead it is a sort of toolbox used by application developers. For example, a web browser is an application that uses HTTP, as defined as a TCP/IP application layer protocol, to transfer the contents of web pages between a server and client.

20. What does MAC stand for?

20. Answer: MAC stands for Media Access Control.

21. Name three terms popularly used as a synonym for MAC address.

21. Answer: NIC address, card address, LAN address, hardware address, Ethernet address, and burned-in address are all synonymous with MAC address. All of these names are used casually and in formal documents, and they refer to the same 6-byte MAC address concept as defined by IEEE.

22. What portion of a MAC address encodes an identifier representing the manufacturer of the card?

22. Answer: The first 3 bytes, called the Organizationally Unique Identifier (OUI), comprise the portion of a MAC address that encodes an identifier representing the manufacturer of the card.

23. Are MAC addresses defined by a Layer 2 or Layer 3 protocol?

23. Answer: MAC addresses are defined by a Layer 2 protocol. Ethernet MAC addresses are defined in the 802.3 specification.

24. How many bits are present in a MAC address?

24. Answer: MAC addresses have 48 bits. The first 24 bits for burned-in addresses represent a code that identifies the manufacturer.

25. Name the two main parts of a MAC address. Which part identifies which "group" this address is a member of?

25. Answer: There are no parts, and nothing defines a grouping concept in a MAC address. This is a trick question. Although you might have guessed that the MAC address has two parts—the first part dictated to the manufacturer, and the second part made up by the manufacturer—there is no grouping concept.

26. What OSI layer typically encapsulates using both a header and a trailer?

26. Answer: The data link layer typically encapsulates using both a header and a trailer. The trailer typically includes a frame check sequence (FCS), which is used to perform error detection.

27. If a Fast Ethernet NIC currently is receiving a frame, can it begin sending a frame?

27. Answer: Yes, if the NIC is operating in full-duplex mode.

28. What are the two key differences between a 10-Mbps NIC and a 10/100-Mbps NIC?

28. Answer: The obvious benefit is that the 10/100-Mbps NIC can run at 100 Mbps. The other benefit is that 10/100-Mbps NICs can autonegotiate both speed and duplex between itself and the device that it is cabled to, typically a LAN switch.

29. How fast is Fast Ethernet?

29. Answer: 100 million bits per second (100 Mbps).

3. What is/are the main purpose(s) of Layer 6?

3. Answer: Layer 6 (the presentation layer) defines data formats, compression, and possibly encryption.

30. How many bytes long is a MAC address?

30. Answer: 6 bytes long, or 48 bits.

31. Define the difference between broadcast and multicast MAC addresses.

31. Answer: Both identify more than one device on the LAN. Broadcast always implies all devices on the LAN, whereas multicast implies some subset of all devices. Devices that intend to receive frames addressed to a particular multicast address must be aware of the particular multicast address(es) that they should process. These addresses are dependent on the applications used. For example, the broadcast address is FFFF.FFFF.FFFF, and one sample multicast address is 0100.5e00.0001.

32. Explain the function of the loopback and collision-detection features of an Ethernet NIC in relation to half-duplex and full-duplex operations.

32. Answer: The loopback feature copies the transmitted frame back onto the receive pins on the NIC interface. The collision-detection logic compares the received frame to the transmitted frame during transmission; if the signals do not match, a collision is occurring. With full-duplex operation, collisions cannot occur, so the loopback and collision-detection features are purposefully disabled, and concurrent transmission and reception is allowed.

33. Are DLCI addresses defined by a Layer 2 or Layer 3 protocol?

33. Answer: DLCI addresses are defined by a Layer 2 protocol. Although they are not covered in detail for this book, Frame Relay protocols do not define a logical addressing structure that can usefully exist outside a Frame Relay network; by definition, the addresses would be OSI Layer 2-equivalent.

34. Define the terms DCE and DTE in the context of the physical layer and a point-to-point serial link.

34. Answer: At the physical layer, DTE refers to the device that receives clocking from the device on the other end of the cable on a link. The DCE supplies that clocking. For example, the computer is typically the DTE, and the modem or CSU/DSU is the DCE. At the data link layer, both X.25 and Frame Relay define a logical DTE and DCE. In this case, the customer premises equipment (CPE), such as a router and a CSU/DSU, is the logical DTE, and the service provider equipment (the Frame Relay switch and the CSU/DSU) is the DCE.

35. Which layer or layers of OSI are most closely related to the functions of Frame Relay? Why?

35. Answer: OSI Layer 2. Frame Relay depends on other well-known physical layer specifications. Frame Relay does define headers for delivery across the Frame Relay cloud, making it a Layer 2 protocol. Frame Relay does not include any routing or logical addressing specifications, so it is not a Layer 3 protocol.

36. True or false: "A leased line between two routers provides a constant amount of bandwidth—never more and never less." Defend your answer.

36. Answer: True. A leased line creates the cabling equivalent of having a cable between the two routers, with the speed (clock rate) defined by the telco. Even when the routers have no data to send, the full bandwidth is available to be used.

37. Do HDLC and PPP, as implemented by Cisco routers, support Protocol Type fields and error detection? Explain your answer.

37. Answer: Both protocols support a Protocol Type field and an FCS field to perform error detection. PPP defines both fields as part of the PPP standard; the HDLC standard includes the FCS field, but Cisco added a Protocol Type field to the standard HDLC header.

38. What are some of the main similarities between Frame Relay and ATM?

38. Answer: Both use an access link to access the service provider. Both use the concept of a virtual circuit between DTE devices. And both allow multiple VCs to cross a single access link.

39. What are the two main functions of each OSI Layer 3-equivalent protocol?

39. Answer: Path selection, which is also called routing, and logical addressing.

4. What is/are the main purpose(s) of Layer 5?

4. Answer: Layer 5 (the session layer) controls the conversation between two endpoints. Although the term used is session, the term conversation more accurately describes what is accomplished. The session layer ensures that not only communication, but also useful sets of communication between endpoints is accomplished.

40. Assume that PC1 sends data to PC2, and PC2 is separated from PC1 by at least one router. Are the IP addresses of the PCs in the same IP subnet? Explain your answer.

40. Answer: They must be in different subnets. IP addressing rules require that IP hosts separated by a router be in different subnets.

41. How many bits are present in an IP Version 4 address?

41. Answer: IPv4 addresses have 32 bits: a variable number in the network portion, and the rest of the 32 in the host portion. IP Version 6 uses a 128-bit address.

42. Name the two main parts of an IP address. Which part identifies which group this address is a member of?

42. Answer: Network and host are the two main parts of an IP address. When subnetted, there are three portions of the IP address: network, subnet, and host. However, because most people think of the network and subnet portions as one portion, another correct answer to this question, using popular terminology, would be subnet and host. In short, without subnetting, the network part identifies the group; with subnetting, the network and subnet part together identifies the group.

43. PC1 sends data to PC2 using TCP/IP. Three routers separate PC1 and PC2. Explain why the statement "PC1 sends an Ethernet frame to PC2" is true or false.

43. Answer: The statement is false. Packets are delivered from end to end across a network, whereas frames simply pass between devices on each common physical network. The intervening routers discard the original Ethernet header, replacing it with other data-link headers as needed. A truer statement would be "PC1 sends an IP packet to PC2."

44. In IP addressing, how many octets are in 1 byte?

44. Answer: One. Octet is a generic word to describe a single byte. Each IP address is 4 bytes, or four octets, long.

45. Describe the differences between a routed protocol and a routing protocol.

45. Answer: The routed protocol defines the addressing and Layer 3 header in the packet that actually is forwarded by a router. The routing protocol defines the process of routers exchanging topology data so that the routers know how to forward the data. A router uses the routing table created by the routing protocol when choosing where to route a packet.

46. Imagine an IP host on an Ethernet, with a single router attached to the same segment. In which cases does an IP host choose to send a packet to this router instead of directly to the destination host, and how does this IP host know about that single router?

46. Answer: Typically an IP host knows to what router to send a packet based on its configured default router. If the destination of the packet is in another subnet, the host sends the packet to the default router. Otherwise, the host sends the packet directly to the destination host because it is in the same subnet and, by definition, must be on the same data link.

47. Name three items in an entry in any routing table.

47. Answer: A number that identifies a group of addresses, the interface out which to forward the packet, and the Layer 3 address of the next router to send this packet to are three items that you will always find in a routing table entry. For instance, IP routes contain subnet numbers, the outgoing interface, and the IP address of the next-hop router.

48. Name the parts of an IP address when subnetting is used.

48. Answer: Network, subnet, and host are the three parts of an IP address. However, many people commonly treat the network and subnet parts of an address as a single part, leaving only two parts, the subnet and host parts. On the exam, the multiple-choice format should provide extra clues as to which terminology is used.

49. How many valid IP addresses exist in an unsubnetted Class A network? (You may refer to the formula if you do not know the exact number.)

49. Answer: 16,777,214, derived by the formula 224 - 2.

5. What is/are the main purpose(s) of Layer 4?

5. Answer: Layer 4 (the transport layer) provides end-to-end error recovery, if requested.

50. How many valid IP addresses exist in an unsubnetted Class B network? (You may refer to the formula if you do not know the exact number.)

50. Answer: 65,534, derived by the formula 216 - 2.

51. How many valid IP addresses exist in an unsubnetted Class C network? (You may refer to the formula if you do not know the exact number.)

51. Answer: 254, derived by the formula 28 - 2.

52. What values can a Class A network have in the first octet?

52. Answer: 1 through 126, inclusive.

53. What values can a Class B network have in the first octet?

53. Answer: 128 through 191, inclusive.

54. What values can a Class C network have in the first octet?

54. Answer: 192 through 223, inclusive.

55. When subnetting a Class B network, do you create the subnet field by taking bits from the network part of the address or the host part?

55. Answer: Host part.

56. When subnetting a Class B network, using the entire third octet for the subnet part, describe the number of possible subnets created.

56. Answer: The subnet part consists of a full octet, which is 8 bits long. You can number 28 things with 8 bits, or 256.

57. When subnetting a Class A network using the entire second octet for the subnet part, describe the number of hosts in each subnet.

57. Answer: The host part consists of two entire octets in this case, which is 16 bits long. You can number 216 things with 16 bits, or 65,536.

58. When a router hears about multiple routes to the same subnet, how does it choose which route to use?

58. Answer: Routing protocols use a metric to describe how good each route is. The lower the metric is, the better the route is.

59. What is the primary purpose of a routing protocol?

59. Answer: Routing protocols discover the routes in a network and build routing tables.

6. What is/are the main purpose(s) of Layer 3?

6. Answer: Layer 3 (the network layer) defines logical addressing and routing as a means of delivering data across an entire network. IP and IPX are two examples of Layer 3-equivalent protocols.

60. True or false: "Routing protocols are required to learn routes of directly connected subnets."

60. Answer: False. Routers add routes to directly connected subnets when the interfaces initialize. No routing protocols are needed.

61. List the similarities and differences between ARP and DNS.

61. Answer: Both protocols send messages with one piece of information, hoping to learn another piece of information. The similarities do not go beyond that fact. DNS requests are unicast IP packets sent specifically to the DNS server, whereas ARP uses a LAN broadcast frame. DNS queries supply a name, expecting to hear the corresponding IP address back from the server. ARP requests supply an IP address, hoping to hear a corresponding MAC address not from a server, but from the host that uses that IP address.

62. Describe the features required for a protocol to be considered connectionless.

62. Answer: Connectionless protocols allow communication to occur without any previous configuration or dynamic protocol messages between the two devices.

63. Describe the features required for a protocol to be considered connection oriented.

63. Answer: Either the protocol must exchange messages with another device before data is allowed to be sent, or some pre-established correlation between the two endpoints must be defined. TCP is an example of a connection-oriented protocol that exchanges messages before data can be sent; Frame Relay is a connection-oriented protocol for which a pre-established correlation between endpoints is defined.

64. In a particular error-recovering protocol, the sender sends three frames, labeled 2, 3, and 4. On its next sent frame, the receiver of these frames sets an Acknowledgment field to 4. What does this typically imply?

64. Answer: Frames through number 3 were received successfully. The receiver might have not received Frame 4, or Frame 4 might not have passed the FCS check.

65. Describe how TCP performs error recovery. What role do the routers play?

65. Answer: TCP numbers the first byte in each segment with a sequence number. The receiving host uses the Acknowledgment field in segments that it sends back to acknowledge receipt of the data. If the receiver sends an acknowledgment number that is a smaller number than the sender expected, the sender believes that the intervening bytes were lost, so the sender resends them. The router plays no role unless the TCP connection ends in the router—for example, a Telnet into a router.

66. How many TCP segments are exchanged to establish a TCP connection? How many are required to terminate a TCP connection?

66. Answer: A three-way connection-establishment sequence is used, and a four-way connection-termination sequence is used.

67. Describe the purpose of the Port Number field in a TCP header. Give one example.

67. Answer: The port numbers are used to help computers multiplex received data. For instance, a PC with two web browsers open can receive an IP packet. The destination TCP port number identifies which of the two browsers should receive the data.

68. How many UDP segments must be sent to establish a UDP connection? How many are used with normal UDP connection termination?

68. Answer: UDP does not establish connections because it is not connection oriented.

7. What is/are the main purpose(s) of Layer 2?

7. Answer: Layer 2 (the data link layer) defines addressing specific to a particular medium as part of the means of providing delivery of data across that medium. It also includes the protocols used to determine what device(s) accesses the media at any point in time.

8. What is/are the main purpose(s) of Layer 1?

8. Answer: Layer 1 (the physical layer) is responsible for encoding energy signals onto the medium and interpreting a received energy signal. Layer 1 also defines the connector and cabling details.

9. Describe the process of data encapsulation as data is processed from creation until it exits a physical interface to a network. Use the OSI model as an example.

9. Answer: Data encapsulation represents the process of a layer adding a header (and possibly a trailer) to the data as it is processed by progressively lower layers in the protocol specification. In the context of OSI, each layer could add a header so that, other than the true application data, there would be six other headers (Layers 2 to 7) and a trailer for Layer 2, with this L2PDU being encoded by the physical layer onto the network media.