## Answers to the Chapter 6 QA Section

1 What do TCP, UDP, IP, and ICMP stand for? Which protocol is considered to be Layer 3-equivalent when comparing TCP/IP to the OSI protocols?

Transmission Control Protocol, User Datagram Protocol, Internet Protocol, and Internet Control Message Protocol. Both TCP and UDP are Layer 4 protocols. ICMP is considered a Layer 3 protocol because it is used for control and management of IP. IP is the core part of the network layer of TCP/IP.

### 2 Name the parts of an IP address.

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.

3 Define the term subnet mask. What do the bits in the mask whose values are binary 0 tell you about the corresponding IP address(es)?

A subnet mask defines the number of host bits in an address. The bits of value 0 define which bits in the address are host bits. The mask is an important ingredient in the formula to dissect an IP address; along with knowledge of the number of network bits implied for Class A, B, and C networks, the mask provides a clear definition of the size of the network, subnet, and host parts of an address.

4 Given the IP address 134.141.7.11 and the mask 255.255.255.0, what is the subnet number?

 Address 134.141.7.11 1000 0110 1000 1101 0000 0111 0000 1011 Mask 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000 Result 134.141.7.0 1000 0110 1000 1101 0000 0111 0000 0000

5 Given the IP address 193.193.7.7 and the mask 255.255.255.0, what is the subnet number?

The network number is 193.193.7.0. 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.

 Address 193.193.7.7 1100 0001 1100 0001 0000 0111 0000 0111 Mask 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000 Result 193.193.7.0 1100 0001 1100 0001 0000 0111 0000 0000

6 Given the IP address 10.5.118.3 and the mask 255.255.0.0, what is the subnet number?

 Address 10.5.118.3 0000 1010 0000 0101 0111 0110 0000 0011 Mask 255.255.0.0 1111 1111 1111 1111 0000 0000 0000 0000 Result 10.5.0.0 0000 1010 0000 0101 0000 0000 0000 0000

7 Given the IP address 190.1.42.3 and the mask 255.255.255.0, what is the subnet number?

 Address 190.1.42.3 1011 1110 0000 0001 0010 1010 0000 0011 Mask 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000 Result 190.1.42.0 1011 1110 0000 0001 0010 1010 0000 0000

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.

 Octet 1 Octet 2 Octet 3 Octet 4 Comments Address 200 1 1 130 N/A Mask 255 255 255 224 Interesting octet is the fourth octet (magic = 256 - 224 = 32) Subnet number 200 1 1 128 128 is the closest multiple of magic not greater than 130. First valid address 200 1 1 129 Add 1 to the last octet of the subnet number. Broadcast 200 1 1 159 Subnet + magic - 1. Last valid address 200 1 1 158 Subtract 1 from broadcast.

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.

 Octet 1 Octet 2 Octet 3 Octet 4 Comments Address 220 8 7 100 N/A Mask 255 255 255 240 Interesting octet is the fourth octet. Subnet number 220 8 7 96 96 is the closest multiple of magic not greater than 100. First valid address 220 8 7 97 Add 1 to the last octet. Broadcast 220 8 7 111 Subnet + magic - 1. Last valid address 220 8 7 110 Subtract 1 from broadcast.

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.

 Octet 1 Octet 2 Octet 3 Octet 4 Comments Address 140 1 1 1 N/A Mask 255 255 255 248 Interesting octet is the fourth octet. Subnet number 140 1 1 0 0 is the closest multiple of magic not greater than 1. First valid address 140 1 1 1 Add 1 to the last octet. Broadcast 140 1 1 7 Subnet + magic - 1. Last valid address 140 1 1 6 Subtract 1 from broadcast.

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

 Octet 1 Octet 2 Octet 3 Octet 4 Comments Address 167 88 99 66 N/A Mask 255 255 255 192 Interesting octet is the fourth octet. Subnet number 167 88 99 64 64 is the closest multiple of magic not greater than 66. First valid address 167 88 99 65 Add 1 to the last octet. Broadcast 167 88 99 127 Subnet + magic - 1. Last valid address 167 88 99 126 Subtract 1 from broadcast.

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

 Address 134.141.7.11 1000 0110 1000 1101 0000 0111 0000 1011 Mask 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000 Result 134.141.7.0 1000 0110 1000 1101 0000 0111 0000 0000 Broadcast address 134.141.7.255 1000 0110 1000 1101 0000 0111 1111 1111

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.

 Address 193.193.7.7 1100 0001 1100 0001 0000 0111 0000 0111 Mask 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000 Result 193.193.7.0 1100 0001 1100 0001 0000 0111 0000 0000 Broadcast address 193.193.7.255 1100 0001 1100 0001 0000 0111 1111 1111

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

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.

 Address 200.1.1.130 1100 1000 0000 0001 0000 0001 1000 0010 Mask 255.255.255.224 1111 1111 1111 1111 1111 1111 1110 0000 Result 200.1.1.128 1100 1000 0000 0001 0000 0001 1000 0000 Broadcast address 200.1.1.159 1100 1000 0000 0001 0000 0001 1001 1111

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

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

 Address 167.8 18.99.66 1010 0111 0101 1000 0110 0011 0100 0010 Mask 255.255.255.192 1111 1111 1111 1111 1111 1111 1100 0000 Result 167.8 18.99.64 1010 0111 0101 1000 0110 0011 0100 0000 Broadcast address 167.8 18.99.127 1010 0111 0101 1000 0110 0011 0111 1111

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?

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?

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?

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?

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?

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?

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?

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?

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.

 Octet 1 Octet 2 Octet 3 Octet 4 Comments Network number 220 8 7 0 N/A Mask 255 255 255 240 The last octet is interesting; the magic number is 256 -240 = 16. Subnet zero 220 8 7 0 Copy the network number; it's the zero subnet. First valid subnet 220 8 7 16 Add magic to the last subnet number's interesting octet. Next valid subnet 220 8 7 32 Add magic to the previous one. Last valid subnet 220 8 7 224 You eventually get here . . . Broadcast subnet 220 8 7 240 . . . and then here, the broadcast subnet, because the next one is 256, which is invalid.

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.

 Octet 1 Octet 2 Octet 3 Octet 4 Comments Network number 220 8 7 0 N/A Mask 255 255 255 240 The last octet is interesting. Subnet zero 220 8 7 0 Copy the network number; it's the zero subnet. First valid subnet 220 8 7 16 Add magic to the last subnet number's interesting octet. Next valid subnet 220 8 7 32 Add magic to the previous one. Last valid subnet 220 8 7 224 You eventually get here . . . Broadcast subnet 220 8 7 240 . . . and then here, the broadcast subnet, because the next one is 256, which is invalid.

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.

 Number of Number of Network Network Number of Number of Hosts per Number of and Mask Bits Host Bits Subnet Bits Subnet Subnets 134.141.0.0, 16 8 8 254 254 255.255.255.0

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.

 Number of Number of Network Network Number of Number of Hosts per Number of and Mask Bits Host Bits Subnet Bits Subnet Subnets 10.0.0.0, 8 8 16 254 65,534 255.255.255.0

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.

 Number of Number of Number of Network and Network Number of Subnet Hosts per Number of Mask Bits Host Bits Bits Subnet Subnets 220.8.7.0, 24 4 4 14 14 255.255.255. 240

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.

 Number of Number of Network Network Number of Number of Hosts per Number of and Mask Bits Host Bits Subnet Bits Subnet Subnets 140.1.0.0 16 3 13 6 8190

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

interface serial 0

interface serial 1

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.

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### Responses

• Chica Banks
How to find out What is the IP address of 167.88.99.66?
2 years ago
• CORDELIA
What do the bits in the mask whose values are binary 0 tell u about the corresponding ip adress(es)?
1 year ago