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Solution Manual Of Computer Networks And Internets, 6th Edition By Douglas E.Comer

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  • ISBN-10 ‏ : ‎ 0133587932
  • ISBN-13 ‏ : ‎ 978-0133587937

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Solution Manual Of Computer Networks And Internets, 6th Edition By Douglas E.Comer

PART IV
Internetworking Using TCP/IP
Chapter 20 – Internetworking: Concepts, Architecture, and Protocols

20.1 Will the Internet be replaced by a single networking technology? Why or why not?

Answer: Incompatibilities make it impossible to form a large network merely by interconnecting the wires among networks. The beauty of the Internet is interconnection of wide range of technologies from various manufacturers. The diversity of the products and solutions is more of a richness than a limitation as long as they all adopt the same set of protocols.

20.2 What is the chief difficulty in providing universal service?

Answer: The main difficulty in providing universal service is that incompatibilities among network hardware, frames, and addresses prevent a bridged network from including arbitrary technologies.

20.3 What are the two reasons an organization does not use a single router to connect all its networks?

Answer: An organization seldom uses a single router to connect all of its networks. There are two major reasons:
• Because the router must forward each packet, the processor in a given router is insufficient to handle the traffic passing among an arbitrary number of networks.
• Redundancy improves internet reliability. To avoid a single point of failure, protocol software continuously monitors internet connections and instructs routers to send traffic along alternative paths when a network or router fails.

20.4 If a given router can connect to at most K networks, how many routers, R, are required to connect N networks? Write an equation that gives R in terms of N and K.

Answer: A router requires an interface of each network to be connected. Since N networks to be connected by a router having K, then number of routers required will be N/K.

20.5 Users view the Internet as a single network. What is the reality, and to what does a user’s computer attach?

Answer: Internet is a virtual network system because the communication system is an abstraction. That is, although a combination of hardware and software provides the illusion of a uniform network system, no such network exists. Internet protocol software hides the details of physical network connections, physical addresses, and routing information. Neither users nor application programs are aware of the underlying physical networks or the routers that connect them. Internet software provides the appearance of a single, seamless communication system to which many user computers attach. The system offers universal service: each computer is assigned an address, and any computer can send a packet to any other computer.

20.6 In the 5-layer reference model used with the TCP/IP Internet protocols, what is the purpose of each of the five layers?

Answer: The 5-layers of TCP/IP and their purposes are given as follows:
• Layer 1 (Physical): Protocols in the Physical layer specify details about the underlying transmission medium and the associated hardware.
• Layer 2 (Network Interface): Protocols in the Network Interface layer specify details about communication between higher layers of protocols and the underlying network.
• Layer 3 (Internet): Protocols in the Internet layer form the fundamental basis for the Internet. Layer 3 protocols specify communication between two computers across the Internet
• Layer 4 (Transport): Protocols in the Transport layer provide for communication from an application program on one computer to an application program on another.
• Layer 5 (Application): Protocols in the top layer of the TCP/IP stack specify how a pair of applications interacts when they communicate.

Chapter 21- IP: Internet Addressing

21.1 Could IP be redesigned to use hardware addresses instead of the IP addresses it currently uses? Why or why not?

Answer: MAC addresses cannot be used, because the Internet can include multiple network technologies and each technology can define its own MAC addresses. But if the Internet were to be designed from the beginning, for sure so many things would be different, probably much easier. Backward compatibility and various tries by different companies made the system complicated.

21.2 What does the Internet address hierarchy allow a local manager to do?

Answer: It allows a local manager to divide the network into subnetworks for better management in terms of security and traffic segregation.

21.3 In the original IPv4 classful address scheme, was it possible to determine the class of an address from the address itself? Explain.

Answer: Yes, since in the classful addressing scheme initial bit(s) gives indication about the class being used.

21.4 Write a computer program that accepts a dotted decimal address as input and displays a string of 32 bits.

21.5 Write a computer program that accepts a colon-hex address as input and displays a string of 128 bits.

21.6 Write a computer program that reads an IPv4 address in dotted decimal form and determines whether the address is a multicast address.

21.7 Write a computer program that translates between CIDR slash notation and an equivalent dotted decimal value.

21.8 If an ISP assigned you a /28 IPv4 address block, how many computers could you assign an address?

Answer: When an organization is assigned /28 CIDR address, it means 28 bits out of 32 bits are fixed, so 32-28 = 4 bits available for user space. So number of users 24-2 = 4, since the all 0s and all 1s address are having special use and can’t be assigned to a user.

21.9 If an ISP assigned you a /28 IPv6 address block, how many computers could you assign an address?

21.10 If an ISP offers a / 17 address block for N dollars per month and a / 16 address block for 1.5 N dollars per month, which has the cheapest cost per computer?

Answer: Number of addresses in /17 block 232-17 = 215  Price per address: 215/N = 215/N
Number of addresses in /16 block 232-16 = 216  Price per address: 216/1.5N = 2×215/1.5N = 1.33×215/N

So /16 address block will be cheaper in comparison with the price given for /17 block.

21.11 Is the CIDR prefix 1.2.3.4/29 valid? Why or why not?

21.12 Suppose you are an ISP with a / 24 IPv4 address block. Explain whether you accommodate a request from a customer who needs addresses for 255 computers. (Hint: consider the special addresses.)

Answer: For /24 address block, number of available addresses will be 232-24 = 28 = 256. However, a suffix with all 0s address is reserved for network ID and a suffix with all 1s address is reserved for broadcast address, so number of addresses that can be assigned to computers/hosts will be 256 -2 = 254.

21.13 Suppose you are an ISP that owns a / 22 IPv4 address block. Show the CIDR allocation you would use to allocate address blocks to four customers who need addresses for 60 computers each.

Answer: The /22 address block can be assigned as follows:

ddd.ddd.ddd.00/26
ddd.ddd.ddd.01/26
ddd.ddd.ddd.10/26
ddd.ddd.ddd.11/26

21.14 Suppose you are an ISP that owns a / 22 IPv4 address block. Can you accommodate requests from six customers who need addresses for 9, 15, 20, 41, 128, and 260 computers, respectively? If so, how? If not, explain why.

Answer: If an ISP owns /22 address block, then number of addresses available will be 232-22 = 210 = 1024

We should look at the distribution of these available addresses in a very efficient way. For example, if we want to assign a block for 260 addresses, 28 = 256 is less and 29 =512 is a lot (lots of addresses are wasted). The reader is expected to analyze the situation accordingly and answer the remaining of the question.

21.15 Write a computer program that reads an IPv4 address in CIDR notation and prints the resulting address and mask in binary.

21.16 Write a computer program that reads as input an IPv4 network prefix in CIDR notation and a request for a number of hosts. Assume the request has been given to the ISP that owns the prefix, and assign a CIDR prefix that accommodates the request without wasting addresses.

21.17 Write a computer program that reads a 32-bit IPv4 host address and a 32-bit mask in CIDR notation and tells whether the address is one of the special addresses.

21.18 Write a computer program that reads a 128-bit IPv6 host address and a 128-bit mask in CIDR notation, and tells whether the address is a multicast address. (Hint: the IETF publishes standards that specify IPv6 address assignments.)

21.19 What is a Berkeley broadcast address?

Answer: The University of California at Berkeley developed and distributed an early implementation of TCP/IP protocols as part of BSD UNIX. The BSD implementation contained a nonstandard feature that has affected many subsequent implementations. Instead of using a host suffix of all ones to represent a directed broadcast address, the Berkeley implementation uses a host suffix that contains all zeroes (i.e., identical to the network address). The address form is known informally as Berkeley broadcast.

21.20 Does IPv6 use broadcast addresses? Explain.

21.21 How many IPv4 addresses are assigned to a router that connects to N networks? Explain

21.22 How many IPv6 addresses can be assigned to a router that connects to N networks? Explain.

21.23 Can a host have more than one IP address? Explain.

Answer: Yes. A host computer with multiple network connections is said to be multi-homed. Multi-homing is sometimes used to increase reliability. Namely, if one network fails, the host can still reach the Internet through the second connection. Also multi-homing may be used to increase performance connections to multiple networks can make it possible to send traffic directly and avoid routers, which are sometimes congested.

21.24 If an IPv6 host attaches to five networks, what term is used to describe the host?

21.25 When might an anycast address be useful?

Chapter 22- Datagram Forwarding

22.1 What are the two basic communication paradigms that designers consider when designing an internet?

Answer: Two basic communication paradigms that designers consider when designing an internet are:
• Connection-oriented service
• Connectionless service

22.2 How does the Internet design accommodate heterogeneous networks that each have their own packet format?

Answer: To overcome heterogeneity, the Internet Protocol defines a packet format that is independent of the underlying hardware. The result is a universal, virtual packet that can be transferred across the underlying hardware intact. The Internet packet format is not tied directly to any hardware. The underlying hardware does not understand or recognize an Internet packet.

22.3 Write a computer program to extract the source and destination addresses from an IPv4 datagram, and print them in dotted decimal notation.

22.4 Write a computer program to extract the source and destination addresses from an IPv6 datagram, and print them in colon hex notation.

22.5 Write a program to extract all fields from an IPv4 or IPv6 datagram header. Print the values in hexadecimal, dotted decimal, or colon hex notation as appropriate.

22.6 What is the maximum length of an IP datagram?

Answer: In the current version of the Internet Protocol (IP version 4), a datagram can contain at most 64 K (65535) octets, including the header.

22.7 Write a computer program that takes as input an IP forwarding table as in Figure 22.3b and a sequence of destination addresses. For each destination address, search the table sequentially to find the correct next hop, and output the results.

22.8 If the payload of an IPv4 datagram contains one 8-bit data value and no header options, what values will be found in header fields H. LEN and TOTAL LENGTH?

22.9 If two prefixes in a forwarding table both match a given destination address, which will the forwarding algorithm use?

22.10 Does a destination address in an IP datagram ever refer to an intermediate router? Explain.

22.11 Assume two routers are misconfigured to form a routing loop for some destination, D. Explain why a datagram destined for D will not go around the loop forever.

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