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Answers to the Chapter 6 Do I Know This Already Quiz

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 (Layer 3) of TCP/IP.

2 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.

3 Does FTP or TFTP perform error recovery? If so, describe the basics of how error recovery is performed.

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

4 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 performed, and a four-way connection-termination sequence is used.

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

The subnet is 134.141.7.0. The binary algorithm is shown in the following table.

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

6 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 following table.

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

7 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 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	The interesting octet is the fourth octet.
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 ast octet.
Broadcast	200	1	1	159	Subnet + magic - 1.
Last valid address	200	1	1	158	Subtract 1 from broadcast.

8 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 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

Answers to the Chapter 6 "Do I Know This Already?" Quiz 879

	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; magic number is 256 -240 = 16.
Subnet zero	220	8	7	0	Copy down network the number; it's the zero subnet.
First valid subnet	220	8	7	16	Add magic to 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 to here . . .
Broadcast subnet	220	8	7	240	. . . and to here, the broadcast subnet, because the next one is 256, which is invalid.

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 says to plan your subnets so that you can have as many subnets as possible rather than allowing 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.

10 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 the IOS before version 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 IOS version 12.0 and beyond, setup prompts for the subnet mask in canonical decimal format—for example, 255.255.255.0.

11 Define the purpose of the trace command. What type of messages does it send, and what type of 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 find that the TTL is exceeded and 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.

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

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

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