Answers to the Chapter 6 QA Section

8 Given the IP address 200.1.1.130 and the mask 255.255.255.224, what is the subnet number?

The answer is 200.1.1.128. The table that follows shows the subnet chart used in this chapter to help you learn the way to calculate the subnet number without binary math. The magic number is 256 - 224 = 32.

9 Given the IP address 220.8.7.100 and the mask 255.255.255.240, what is the subnet number?

The answer is 220.8.7.96. The table that follows shows the subnet chart used in this chapter to help you learn the way to calculate the subnet number without binary math. The magic number is 256-240=16.

10 Given the IP address 140.1.1.1 and the mask 255.255.255.248, what is the subnet number?

The answer is 140.1.1.0. The table that follows shows the subnet chart used in this chapter to help you learn the way to calculate the subnet number without binary math. The magic number is 256 - 248 = 8.

11 Given the IP address 167.88.99.66 and the mask 255.255.255.192, what is the subnet number?

The answer is 167.88.99.64. The table that follows shows the subnet chart used in this chapter to help you learn the way to calculate the subnet number without binary math. The magic number is 256-192=64

12 Given the IP address 134.141.7.11 and the mask 255.255.255.0, what is the subnet broadcast address?

The broadcast address is 134.141.7.255. The binary algorithm is shown in the table that follows.

13 Given the IP address 193.193.7.7 and the mask 255.255.255.0, what is the broadcast address?

The broadcast address is 193.193.7.255. Because this is a Class C address and the mask used is 255.255.255.0 (the default), there is no subnetting in use. The binary algorithm is shown in the table that follows.

14 Given the IP address 10.5.118.3 and the mask 255.255.0.0, what is the broadcast address?

The broadcast address is 10.5.255.255. The binary algorithm math is shown in the table that follows.

16 Given the IP address 200.1.1.130 and the mask 255.255.255.224, what is the broadcast address?

The broadcast address is 200.1.1.159. The binary algorithm math is shown in the table that follows. The easy decimal algorithm is shown in the answer to an earlier question.

17 Given the IP address 220.8.7.100 and the mask 255.255.255.240, what is the broadcast address?

The broadcast address is 220.8.7.111. The binary algorithm is shown in the table that follows.

19 Given the IP address 167.88.99.66 and the mask 255.255.255.192, what is the broadcast address?

The broadcast address is 167.88.99.127. The binary algorithm math is shown in the table that follows.

20 Given the IP address 134.141.7.11 and the mask 255.255.255.0, what are the assignable IP addresses in this subnet?

The subnet number is 134.141.7.0, and the subnet broadcast address is 134.141.7.255. The assignable addresses are all the addresses between the subnet and broadcast addresses, namely 134.141.7.1 to 134.141.7.254.

21 Given the IP address 193.193.7.7 and the mask 255.255.255.0, what are the assignable IP addresses in this subnet?

The subnet number is 193.193.7.0, and the subnet broadcast address is 193.193.7.255. The assignable addresses are all the addresses between the subnet and broadcast addresses, namely 193.193.7.1 to 193.193.7.254.

22 Given the IP address 10.5.118.3 and the mask 255.255.0.0, what are the assignable IP addresses in this subnet?

The subnet number is 10.5.0.0, and the subnet broadcast address is 10.5.255.255. The assignable addresses are all the addresses between the subnet and broadcast addresses, namely 10.5.0.1 to 10.5.255.254.

23 Given the IP address 190.1.42.3 and the mask 255.255.255.0, what are the assignable IP addresses in this subnet?

The subnet number is 190.1.42.0, and the subnet broadcast address is 190.1.42.255. The assignable addresses are all the addresses between the subnet and broadcast addresses, namely 190.1.42.1 to 190.1.42.254.

24 Given the IP address 200.1.1.130 and the mask 255.255.255.224, what are the assignable IP addresses in this subnet?

The subnet number is 200.1.1.128, and the subnet broadcast address is 200.1.1.159. The assignable addresses are all the addresses between the subnet and broadcast addresses, namely 200.1.1.129 to 200.1.1.158.

25 Given the IP address 220.8.7.100 and the mask 255.255.255.240, what are the assignable IP addresses in this subnet?

The subnet number is 220.8.7.96, and the subnet broadcast address is 220.8.7.111. The assignable addresses are all the addresses between the subnet and broadcast addresses, namely 220.8.7.97 to 220.8.7.110.

26 Given the IP address 140.1.1.1 and the mask 255.255.255.248, what are the assignable IP addresses in this subnet?

The subnet number is 140.1.1.0, and the subnet broadcast address is 140.1.1.7. The assignable addresses are all the addresses between the subnet and broadcast addresses, namely 140.1.1.1 to 140.1.1.6.

27 Given the IP address 167.88.99.66 and the mask 255.255.255.192, what are the assignable IP addresses in this subnet?

The subnet number is 167.88.99.64, and the subnet broadcast address is 167.88.99.127. The assignable addresses are all the addresses between the subnet and broadcast addresses, namely 167.88.99.65 to 167.88.99.126.

28 Given the IP address 134.141.7.7 and the mask 255.255.255.0, what are all the subnet numbers if the same (static) mask is used for all subnets in this network?

The answer is 134.141.1.0, 134.141.2.0, 134.141.3.0, and so on, up to 134.141.254.0. 134.141.0.0 is the zero subnet, and 134.141.255.0 is the broadcast subnet.

29 Given the IP address 10.5.118.3 and the mask 255.255.255.0, what are all the subnet numbers if the same (static) mask is used for all subnets in this network?

The answer is 10.0.1.0, 10.0.2.0, 10.0.3.0, and so on, up to 10.255.254.0. The Class A network number is 10.0.0.0. The mask implies that the entire second and third octets, and only those octets, comprise the subnet field. The first subnet number, called the zero subnet (10.0.0.0), and the last subnet number, called the broadcast subnet (10.255.255.0), are reserved.

30 Given the IP address 220.8.7.100 and the mask 255.255.255.240, what are all the subnet numbers if the same (static) mask is used for all subnets in this network?

The answer is not as obvious in this question. The Class C network number is 220.8.7.0. The mask implies that the bits 25 through 28, which are the first 4 bits in the fourth octet, comprise the subnet field. The answer is 220.8.7.16, 220.8.7.32, 220.8.7.48, and so on, through 220.8.7.224. 220.8.7.0 is the zero subnet, and 220.8.7.240 is the broadcast subnet. The following table outlines the easy decimal algorithm to figure out the subnet numbers.

31 Given the IP address 220.8.7.1 and the mask 255.255.255.240, what are all the subnet numbers if the same (static) mask is used for all subnets in this network?

The Class C network number is 220.8.7.0. The mask implies that the bits 25 to 28, which are the first 4 bits in the fourth octet, comprise the subnet field. Essentially, the subnet numbers have the same numbers in the network portion and the same (all binary 0) value in the host portion of the number. Each individual subnet number has a unique value in the subnet portion of the number.

32 How many IP addresses could be assigned in each subnet of 134.141.0.0, assuming that a mask of 255.255.255.0 is used? If the same (static) mask is used for all subnets, how many subnets are there?

There will be 2hostbits, or 28 hosts per subnet, minus two special cases. The number of subnets will be 2subnetbits, or 28, minus 2 special cases.

33 How many IP addresses could be assigned in each subnet of 10.0.0.0, assuming that a mask of 255.255.255.0 is used? If the same (static) mask is used for all subnets, how many subnets are there?

There will be 2hostbits, or 28 hosts per subnet, minus two special cases. The number of subnets will be 2subnetbits, or 216, minus 2 special cases.

34 How many IP addresses could be assigned in each subnet of 220.8.7.0, assuming that a mask of 255.255.255.240 is used? If the same (static) mask is used for all subnets, how many subnets are there?

There will be 2hostbits, or 24 hosts per subnet, minus two special cases. The number of subnets will be 2subnetbits, or 24, minus 2 special cases.

35 How many IP addresses could be assigned in each subnet of 140.1.0.0, assuming that a mask of 255.255.255.248 is used? If the same (static) mask is used for all subnets, how many subnets are there?

There will be 2hostbits, or 23 hosts per subnet, minus two special cases. The number of subnets will be 2subnetbits, or 213, minus 2 special cases.

36 You design a network for a customer, and the customer insists that you use the same subnet mask on every subnet. The customer will use network 10.0.0.0 and needs 200 subnets, each with 200 hosts maximum. What subnet mask would you use to allow the largest amount of growth in subnets? Which mask would work and would allow for the most growth in the number of hoses per subnet?

Network 10.0.0.0 is a Class A network, so you have 24 host bits with no subnetting. To number 200 subnets, you will need at least 8 subnet bits because 28 is 256. Likewise, to number 200 hosts per subnet, you will need 8 host bits. So, you need to pick a mask with at least 8 subnet bits and 8 host bits. 255.255.0.0 is a mask with 8 subnet bits and 16 host bits. That would allow for the 200 subnets and 200 hosts, while allowing the number of hosts per subnet to grow to 216-2—quite a large number. Similarly, a mask of 255.255.255.0 gives you 16 subnet bits, allowing 216-2 subnets, each with 28-2 hosts per subnet.

37 Create a minimal configuration enabling IP on each interface on a 2501 router (two serial, one Ethernet). The NIC assigned you network 8.0.0.0. Your boss says that you need, at most, 200 hosts per subnet. You decide against using VLSM. Your boss also says to plan your subnets so that you can have as many subnets as possible rather than allow for larger subnets later. When choosing the actual IP address values and subnet numbers, you decide to start with the lowest numerical values. Assume that point-to-point serial links will be attached to this router and that RIP is the routing protocol.

router rip network 8.0.0.0

interface ethernet 0

ip address 8.0.1.1 255.255.255.0

interface serial 0

ip address 8.0.2.1 255.255.255.0

interface serial 1

ip address 8.0.3.1 255.255.255.0

The zero subnet was not used in this solution. If desired, the ip subnet-zero global command could have been used, enabling subnet 8.0.0.0 as well as the subnets 8.0.1.0, 8.0.2.0, and 8.0.3.0 to be used in the configuration.

38 In the previous question, what would be the IP subnet of the link attached to serial 0? If another user wanted to answer the same question but did not have the enable password, what command(s) might provide this router's addresses and subnets?

The attached subnet is 8.0.2.0, 255.255.255.0. The show interface, show ip interface, and show ip interface brief commands would supply this information, as would show ip route. The show ip route command would show the actual subnet number instead of the address of the interface.

39 Describe the question and possible responses in setup mode when a router wants to know the mask used on an interface. How can the router derive the correct mask from the information supplied by the user?

When using versions of Cisco IOS Software earlier than Release 12.0, the question asks for the number of subnet bits. The router creates a subnet mask with x more binary 1s than the default mask for the class of network of which the interface's IP

address is a member. (x is the number in the response.) "Number of subnet bits" from the setup question uses the definition that there are three parts to an address—network, subnet, and host. The size of the network field is based on the class of address; the interface's address was typed in response to an earlier setup question. The mask simply has binary 1s in the network and subnet fields, and binary 0s in the host field.

With Cisco IOS Software Release 12.0 and later, setup prompts for the subnet mask in canonical decimal format—for example, 255.255.255.0.

40 Name the three classes of unicast IP addresses and list their default masks, respectively. How many of each type could be assigned to companies and organizations by the NIC?

Class A, B, and C, have default masks 255.0.0.0, 255.255.0.0, and 255.255.255.0,

7 14

respectively. 27 Class A networks are mathematically possible, 214 Class B networks 21

are possible, and 221 Class C networks are possible. There are two reserved network numbers in each range.

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

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. A full explanation is provided in the section "Error Recovery (Reliability)."

42 Define the purpose of an ICMP redirect message.

The ICMP redirect message enables a router to tell a host to use a different router than itself because that other router has a better route to the subnet to which the host sent a packet. The redirect also implies that the router sending the redirect, the host sending the original packet, and the better router all have interfaces attached to this same subnet.

43 Define the purpose of the trace command. What type of messages does it send, and what type of ICMP messages does it receive?

The trace command learns the current route to a destination address. It uses IP packets with UDP as the transport layer protocol, with TTL values beginning at 1 and then incrementing by 1 in successive messages. The result is that intervening routers will find that the TTL is exceeded and will send ICMP Time Exceeded messages back to the originator of the packet, which is the router where the trace command is being executed. The source addresses of the Time Exceeded packets identify each router on the path. By sending successive packets with TTL = 2, then 3, and so on, eventually the packet is received by the destination host. The host returns a Port Unreachable ICMP message, which lets the trace command know that the endpoint host has been reached.

44 What does IP stand for? What does ICMP stand for? Which protocol is considered to be a Layer 3 protocol when comparing TCP/IP to the OSI protocols?

Internet Protocol and Internet Control Message Protocol. Both protocols are considered to be part of TCP/IP's protocols equivalent to OSI Layer 3. ICMP also is considered a Layer 3 protocol because it is used for control and management of IP. However, an IP header precedes an ICMP header, so it is common to treat ICMP as another Layer 4 protocol, like TCP and UDP. ICMP does not provide services to a higher layer, however, so it is really an adjunct part of Layer 3.

45 What causes the output from an IOS ping command to display "UUUUU?"

U is an indication that an unreachable message was received. The type of unreachable message is not implied by the U.

46 Describe how to view the IP ARP cache in a Cisco router. Also describe the three key elements of each entry.

show ip arp displays the IP ARP cache in a Cisco router. Each entry contains the IP address, the MAC address, and the interface from which the information was learned. The encapsulation type is also in the table entry.

47 How many hosts are allowed per subnet if the subnet mask used is 255.255.255.192? How many hosts are allowed for 255.255.255.252?

255.255.255.192 has 6 bits of value 0, giving 26 hosts, minus the two reserved numbers, for 62. The 255.255.255.252 mask leaves 22 hosts, minus the two reserved numbers, for two hosts. 255.255.255.252 often is used on serial links when using VLSM; point-to-point links need only two IP addresses.

48 How many subnets could be created if using static-length masks in a Class B network when the mask is 255.255.255.224? What about when the mask is 255.255.252.0?

With a Class B network, the first 16 bits are network bits. With mask 255.255.255.224, there are 5 host bits, leaving 11 subnet bits. 211 is 2,048, minus 2 special cases, which leaves 2,046. For the mask 255.255.252.0, there are 10 host bits, leaving 6 subnet bits. 26 is 64, minus 2 special cases, which leaves 62.

49 Name the two commands typically used to create a default gateway for a router.

The ip default-network command and the ip route 0.0.0.0 0.0.0.0 command. Both accomplish the goal of having the router use a known route as the default for packets that are not matched in the routing table. The ip route 0.0.0.0 0.0.0.0 uses the fact that network 0.0.0.0 is used by Cisco IOS Software to represent the default network.

50 Assume that subnets of network 10.0.0.0 are in the IP routing table in a router but that no other network and subnets are known, except that there is also a default route (0.0.0.0) in the routing table. A packet destined for 192.1.1.1 arrives at the router. What configuration command determines whether the default route will be used in this case?

The packet will be routed using the default route, regardless of other configuration commands. In this particular scenario, in which the Class A, B, or C network is known, there is no match for the destination in the known subnets, and a default exists, the default must be used.

51 Assume that subnets of network 10.0.0.0 are in the IP routing table in a router but that no other network and their subnets are known, except that there is also a default route (0.0.0.0) in the routing table. A packet destined for 10.1.1.1 arrives at the router, but there is no known subnet of network 10 that matches this destination address. What configuration command determines whether the default route will be used in this case?

If the command ip classless is configured, the packet will be routed using the default route. If no ip classless is configured, the packet will be discarded.

52 What does the acronym CIDR stand for? What is the original purpose of CIDR?

Classless Interdomain Routing is the concept of grouping multiple sequential network numbers into a single routing table entry, for the purpose of improving scalability of Internet routers by reducing the size of the IP routing table.

53 Define the term private addressing as defined in RFC 1918.

Some hosts will never need to communicate with other hosts across the Internet. For such hosts, assignment of IP addresses from registered networks wastes IP addresses. To conserve IP addresses, a set of network numbers, called private addresses, has been reserved and can be used in these cases to help conserve IP addresses for use over the Internet.

54 Define the acronym NAT and the basics of its operation.

Network Address Translation is a mechanism for allowing hosts with private addresses or addresses that conflict with IP addresses from a registered network to communicate with hosts over the Internet. The basic operation involves the NAT router changing the IP addresses in packets to and from these hosts so that only legitimately registered IP addresses are used in flows through the Internet.

55 Which requires more lines of source code, FTP or TFTP? Justify your answer.

TFTP requires less code. It was designed to be simple (that is, trivial), with the small amount of memory needed to load the code ranking as an advantage to using TFTP. FTP is much more robust, with many more features and more code.

56 Does FTP or TFTP perform error recovery? If so, describe the basics of how they perform error recovery.

Both FTP and TFTP perform error recovery. FTP relies on TCP, whereas TFTP performs application layer recovery one block of data at a time.

57 Describe the process used by IP routers to perform fragmentation and reassembly of packets.

Answers to the Chapter 7 "Do I Know This Already?" Quiz 897

When a packet must be forwarded but the packet is larger than the maximum transmission unit (MTU) size for the outgoing interface, the router fragments the packet, as long as the Don't Fragment bit is not set. No IP router reassembles the fragments; fragments are reassembled at the final destination host.

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

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

59 How many Class B-style networks are reserved by RFC 1918 private addressing?

Sixteen Class B networks are reserved for use as private networks in RFC 1918, networks 172.16.0.0 to 172.31.0.0.

Continue reading here: Answers to the Chapter 7 Do I Know This Already Quiz

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