Answers to the Chapter 3 QA Section

1 Name the seven layers of the OSI model.

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 (Layer 7 to 1), Please Do Not Take Sausage Pizzas Away (Layer 1 to 7), and the ever-popular Pew! Dead Ninja Turtles Smell Particularly Awful (Layer 1 to 7).

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

Layer 7 (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 HTML to format text, as defined by the TCP/IP application layer, to describe the graphics to be displayed onscreen.

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

Layer 6 (presentation layer) defines data formats, compression, and possibly encryption.

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

Layer 5 (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.

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

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

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

Layer 3 (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.

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

Layer 1 (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.

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.

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

Unordered low-overhead delivery of data from one host to another is the service provided in most connectionless protocol services.

11 Name at least three connectionless protocols.

LLC Type 1, UDP, IPX, IP, and PPP are some examples of connectionless protocols. Remember, Frame Relay, X.25, and ATM are connection-oriented, regardless of whether they define error recovery.

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

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 may be sent; Frame Relay is a connection-oriented protocol for which pre-established correlation between endpoints is defined.

13 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?

Frames up through number 3 were received successfully. Most windowing, error-recovery protocols use forward acknowledgment.

14 Name three connection-oriented protocols.

TCP, SPX, LLC Type 2, and X.25 are some examples of connection-oriented protocols that provide error recovery. ATM and Frame Relay are also connection-oriented, but without error recovery.

15 What does MAC stand for?

MAC stands for Media Access Control.

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

NIC address, card address, LAN address, hardware address, Ethernet address, Token Ring address, FDDI 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.

17 Are IP addresses defined by a Layer 2 or Layer 3 protocol?

IP addresses are defined by the IP part of TCP/IP, which is the second layer of TCP/ IP. However, compared to OSI, IP most closely matches OSI Layer 3 in function, so the popular (and CCNA exam) answer is Layer 3.

18 Are IPX addresses defined by a Layer 2 or Layer 3 protocol? IP addresses are defined by a Layer 3 protocol.

19 Are OSI NSAP addresses defined by a Layer 2 or Layer 3 protocol?

OSI NSAP addresses are defined by a Layer 3 protocol. Of course, they are truly Layer 3 because they are defined by OSI. The number of bits in the address is variable. However, it is highly unlikely that questions about NSAPs would be on the exam because they are not mentioned in any objective and are not covered in any class.

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

The first 3 bytes comprise the portion of a MAC address that encodes an identifier representing the manufacturer of the card.

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

MAC addresses are defined by a Layer 2 protocol. Ethernet and Token Ring MAC addresses are defined in the 802.3 and 802.5 specifications.

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

DLCI addresses are defined by a Layer 2 protocol. Although they are not specifically covered in this chapter, 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.

23 Name two differences between Layer 3 addresses and Layer 2 addresses.

Layer 3 addresses can be used regardless of media type, whereas Layer 2 addresses are useful only on a particular medium. Layer 3 addresses are designed with a minimum of two parts, the first of which creates a grouping concept. Layer 2 addresses do not have a grouping concept that allows the setup interfaces on the same medium to share the same value in a portion of the data-link address, which is how Layer 3 addresses are structured.

24 How many bits are present in an IP address?

IP 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 much larger address. Stay tuned!

25 How many bits are present in an IPX address?

IPX addresses have 80 bits: 32 bits in the network portion and 48 bits in the node portion.

26 How many bits are present in a MAC address?

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

27 Name the two main parts of an IPX address. Which part identifies which "group" this address is a member of?

Network number and node number are the two main parts of an IPX address. Addresses with the same network number are in the same group. On LAN interfaces, the node number is made to have the same value as the LAN MAC address.

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

Network and host are the two main parts of an IP address. As described in Chapter 5, technically 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.

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

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.

30 Name three benefits to layering networking protocol specifications.

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 will require some subjective interpretation of the wording on your part.

31 What header and/or trailer does a router discard as a side effect of routing?

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.

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

The routed protocol defines the addressing and Layer 3 header in the packet that is actually 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 to choose how to forward a routed protocol packet.

33 Name at least three routed protocols.

TCP/IP (IP), Novell (IPX), OSI (CLNP), DECnet (CLNP), AppleTalk (DDP), and VINES are some examples of routed protocols.

34 Name at least three routing protocols.

IP RIP, IP IGRP, IP/IPX/AppleTalk EIGRP, IP OSPF, OSI NLSP, AppleTalk RTMP, VINES VTP, and OSI IS-IS are some examples of routing protocols.

35 How does an IP host know what router to send a packet to? In which cases does an IP host choose to send a packet to this router instead of directly to the destination host?

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.

Answers to the Chapter 4 "Do I Know This Already?" Quiz 865

36 How does an IPX host know which router to send a packet to? In which case does an IPX host choose to send a packet to this router instead of directly to the destination host?

An IPX host knows which router to send a packet to by broadcasting a RIP request to locate any servers or routers on the attached IPX network that have a route to the destination network. If the destination is an IPX address on the attached network, a router is not needed and the node forwards the packet directly instead of sending a RIP request.

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

The group identifier, the interface by 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.

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

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.

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    How many bit are present in an ipx address?
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