chap 6 addressing study questions

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CCNA Exploration Chapter 6 Addressing. Study questions.Answers in white font. Select and change colour to see them.

Reminder just in case.21 (2^1 or 2 to the power 1) = 2

22 (2^2 or 2 to the power 2) = 2 x 2 = 423 (2^3 or 2 to the power 3) = 2 x 2 x 2 = 824 (2^4 or 2 to the power 4) = 2 x 2 x 2 x 2 = 1625 (2^5 or 2 to the power 5) = 2 x 2 x 2 x 2 x 2 = 3226 (2^6 or 2 to the power 6) = 2 x 2 x 2 x 2 x 2 x 2 = 6427 (2^7 or 2 to the power 7) = 2 x 2 x 2 x 2 x 2 x 2 x 2 = 12828 (2^8 or 2 to the power 8) = 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 256

6.1

How is an IPv4 address normally written?

As dotted decimal four decimal numbers separated by dots. E.g.192.168.1.3

What is an octet?

A group of 8 bits (binary digits) in an IP address. Each octet is converted toone decimal number so that the address can be written conveniently.

The IP address is in two parts. The bits on the left (high order) represent the:

Network part

The bits on the right (low order) represent the:

Host part.

What is the largest number that can be held in one octet (8 bits)?

255 (11111111 in binary)

Convert the binary number 10011101 to decimal.

157

Convert the binary number 11110000 to decimal.

240

Convert the decimal number 224 to binary.

11100000

Convert the decimal number 200 to binary.

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11001000

Convert the IP address 192.168.20.15 to binary.

11000000101010000001010000001111

6.2

What are the three types of IP address in the range of a network?

The first (lowest number) address is the network address. The last (highest)number address is the broadcast address. The other addresses are hostaddresses.

What information does the network prefix give?

The number of bits in the network portion of an IP address.

If a host IP address is 192.168.3.55 and the prefix length is /24, what are thenetwork address and broadcast address of the hosts network?

Network address 192.168.3.0, broadcast address 192.168.3.255

If a host IP address is 172.16.29.104 and the prefix length is /16, what are the

network address and broadcast address of the hosts network?


If a host IP address is 172.16.29.104 and the prefix length is /24, what are thenetwork address and broadcast address of the hosts network?


A host IP address is 201.37.16.9 and the prefix length is /28. How many bitsof the last octet are in the network part of the address?

4

The host address and prefix are the same as in the previous question. Writethe last octet in binary showing all 8 bits.

00001001

The host address and prefix are the same as before. The network addresshas all the bits in the host part set to 0. What is the last octet of the networkaddress written in binary?

00000000

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The host address and prefix are the same as before. What is the networkaddress written as four octets of decimal?

201.37.16.0

The host address and prefix are the same as before. The network addresshas all the bits in the host part set to 1. What is the last octet of the networkaddress written in binary?

00001111

The host address and prefix are the same as before. What is the broadcastaddress written as four octets of decimal?

201.37.16.15

A host IP address is 201.37.16.19 and the prefix length is /28. What are thenetwork address and the broadcast address of the hosts network?

Network address 210.37.16.16, broadcast address 210.37.16.31.

A host IP address is 201.37.16.19 and the prefix length is /28. What are thefirst and last host addresses of the network?

First host 210.37.16.17, last host 210.37.16.30.

A host IP address is 150.90.163.12 and the prefix length is /22. What are thenetwork address and the broadcast address of the hosts network?

Network address 150.90.160.0 Broadcast address 150.90.163.255

What is a unicast packet?

A packet sent to one IP address a single host.

What is a broadcast packet?

A packet sent to all hosts on a network.

What is a multicast packet?

A packet sent to a selected group of hosts.

What destination IP address would a host use for a limited broadcast to hostson its own network?

255.255.255.255

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A host is using the broadcast address 192.168.1.255 to send a directedbroadcast to hosts on the 192.168.1.0 network, which is the other side of arouter. Will the broadcast reach the hosts?

Only if the router is configured to forward the broadcast message. By default it

will not.

Give three examples of the use of broadcast messages.

Mapping upper layer addresses to lower layer addressesRequesting an addressExchanging routing information by routing protocolsWhich IPv4 addresses are used for multicasts?

224.0.0.0 to 239.255.255.255

Which IPv4 addresses are not currently used to address hosts, but arereserved for experimental purposes?

240.0.0.0 to 255.255.255.254.

What is the difference between public addresses and private addresses?

Public addresses can be used over the Internet. A public address must beunique in the world, so their use is controlled. Private addresses are notrouted over the Internet. Their use is unrestricted on networks that do notrequire Internet access.

What are the blocks of private addresses?

10.0.0.0 to 10.255.255.255 (10.0.0.0 /8)172.16.0.0 to 172.31.255.255 (172.16.0.0 /12)192.168.0.0 to 192.168.255.255 (192.168.0.0 /16)

What is the name of the service that can allow hosts on a network usingprivate addresses to have the use of a public address or addresses to accessthe Internet?

Network Address Translation.

Why are hosts never given IP addresses with the first octet 0?

0.0.0.0 is used in default routes to mean any address. This use reserves thewhole of the 0.0.0.0 network so that it cannot be used for hosts.

What is special about the address 127.0.0.1?

This is the loopback address, used by hosts to direct messages to

themselves. This use reserves the whole 127.0.0.0 network so that no hostscan be given addresses starting with 127.

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Which other addresses have special uses and are not routed over theInternet?

Link-Local Addresses 169.254.0.0 to 169.254.255.255 (169.254.0.0 /16) and

TEST-NET Addresses 192.0.2.0 to 192.0.2.255 (192.0.2.0 /24).

What is classful network addressing?

Originally IPv4 addresses were organised into classes A, B and C, givingdifferent sizes of network.

In class A, how much of the IP address is in the network part, and what is therange of numbers in the first octet?

8 bits (one octet), first octet 1 - 127

In class B, how much of the IP address is in the network part, and what is therange of numbers in the first octet?

16 bits (two octets) , first octet 128 - 191

In class C, how much of the IP address is in the network part, and what is therange of numbers in the first octet?

24 bits (three octets) , first octet 192 - 223

What are the subnet masks for class A, class B and class C?

Class A 255.0.0.0, class B 255.255.0.0, class C 255.255.255.0.

We now use classless addressing, so why is it still useful to learn aboutclassful addressing?

Operating systems on PCs and routers still assume that the default subnetmasks for the classes will be used unless they are configured differently.

6.3

Give three reasons why the allocation of IP addresses in a company networkshould be planned and documented carefully.

Preventing duplication of addressesProviding and controlling accessMonitoring security and performance

What is static IP addressing and which devices are normally given staticaddresses?

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The address is configured manually by an administrator. Servers, printers,routers, switches and generally any hosts that need to be contacted by otherdevices should have static addresses because static addresses stay thesame.

What is dynamic IP addressing and why is it used for the majority ofworkstations?

The IP address is assigned automatically by a DHCP server. The address isgiven on a temporary lease and returned for re-use if the lease is notrenewed. It is used because static addressing is time consuming and canresult in duplication of addresses unless records are kept very carefully.Dynamic addressing needs no further input from the administrator once it hasbeen set up, and it checks to avoid duplicate addresses. It is also usefulwhere workstations e.g. laptops are regularly added to the network andremoved from it.

Why should the host addresses within a network be split into blocks ofadjacent addresses, with one block for workstations, one block for servers,one block for network devices etc?

Blocks of addresses can be excluded from the DHCP pool and used for staticaddresses. Blocks of addresses make it easier to monitor traffic to differenttypes of device. It is easier to configure translation to public addresses for useover the internet. It is easier to configure security.

Which network address should be given to the default gateway?

The default gateway (router interface) is normally given either the first or thelast host address on a network. The choice of first or last should be consistentthroughout an organisation that has several networks.

What is IANA?

Internet Assigned Numbers Authority (IANA) is in charge of allocating IPaddresses. It used to allocate them directly to organisations. Now it allocatesthem to regional registries which then allocate them to organisations, usually

via an ISP.

What was the main reason for developing IPv6?

The shortage of IPv4 addresses.

How many bits are in an IPv6 address, and how are they written?

128 bits, written as 8 hexadecimal values separated by conons.

What are the advantages of IPv6 over IPv4?

Much larger address space (more addresses)

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Simpler packet header to improve and speed up packet handlingSupports extensions and options which should increase scalability andlongevityBetter quality of service mechanismsSecurity capabilities integrated.

6.4

The subnet mask gives the same information as:

The prefix

What does a binary 1 in the subnet mask mean?

The corresponding bit in the IP address is in the network part.

What does a binary 0 in the subnet mask mean?

The corresponding bit in the IP address is in the host part.

A host is configured with a subnet mask 255.255.0.0. Write the subnet maskin binary and write down the prefix that gives the same information.

11111111 11111111 00000000 00000000/16 (because the first 16 bits of the IP address are in the network part.)

Which decimal numbers can appear in subnet masks, and why?

0, 128, 192, 224, 240, 248, 252, 254, 255These are the only numbers that can be made from 8 bits where all the 1scome at the beginning and all the 0s come at the end.

What results do you get from the following logical operations on bits?0 AND 0 =1 AND 0 =0 AND 1 =1 AND 1 =

0 AND 0 = 01 AND 0 = 00 AND 1 = 01 AND 1 = 1

If you take the IP address of a host and perform a bit-by-bit logical AND withthe hosts subnet mask, what do you get?

The network address of the hosts network.

Why does the host that originates a message carry out the logical ANDoperation?

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It finds its own network address and checks to see if the destination host is onthe same network. If so, then it sends the message direct to the destinationhost. If not, then it sends the message to the default gateway for forwarding.

Which networking device carries out the logical AND, and why?

A router does a logical AND on the destination IP address of each incomingmessage and the appropriate subnet mask. It does this to find out whichnetwork the message should go to.

A host has IP address 172.16.98.29 and subnet mask 255.255.255.0. What isthe network address of the hosts network?

172.16.98.0


172.16.0.0

A host has IP address 172.16.98.29 and subnet mask 255.255.255.240. Whatis the network address of the hosts network?

172.16.98.16


172.16.96.0

There are many good subnet calculators available. Why are you learning tosubnet and to carry out logical AND without a calculator?

A network administrator needs to understand the principles involved. You willnot be able to use any kind of calculator in the CCNA exam.

6.5

You start with the network 201.73.1.0/24. You borrow 2 bits from the host partfor subnetting.How many subnets can you have?How many hosts can you have on each subnet?Give the network address with its prefix, subnet mask, broadcast address andrange of host addresses for each subnet.(Try to work this out. Do not just copy answers from the curriculum.)

4 subnets62hosts on each subnet

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Network address Subnet mask Broadcast address Host addresses

201.73.1.0/26 255.255.255.192

201.73.1.63 201.73.1.1 to201.73.1.62

201.73.1.64/26 255.255.255.192

201.73.1.127 201.73.1.65 to201.73.1.126

201.73.1.128/26 255.255.255.192

201.73.1.191 201.73.1.129 to201.73.1.190

201.73.1.192/26 255.255.255.192

201.73.1.255 201.73.1.193 to201.73.1.254

You start with the network 199.30.57.0/24. You need 5 subnets. How manybits will you borrow? How many subnets will this give you? How many hostson each subnet?

Borrow 3 bits, giving 8 subnets. (2 bits would give 4 subnets, so not enough.)5 bits remain for host addressing. 25= 32 but take two off for network andbroadcast address, so 30 host addresses on each subnet.

An organisation has 80 hosts in the London office, 20 hosts in Cardiff, 15hosts in Belfast and 150 hosts in Edinburgh. There are 5 WAN linksconnecting the offices. You need to plan the addressing. There will be onesubnet for each office and one for each WAN link. Where will you start whenyou divide up your address space into subnets?

Start with the Edinburgh subnet because it has most hosts. (Then do London,Cardiff, Belfast, and the WAN links last.)

Warning. Page 6.5.2 of the curriculum has a misprint.The formula for the number of usable hosts should be:Usable hosts = 2n 2 (or 2^n 2) and not Usable hosts = 2 n - 2

Suppose that you have borrowed 2 host bits for subnetting to create 4subnets. You now want to split one of the subnets into 2 smaller subnets. Howcan you do this?

Borrow an extra bit (adds 1 to the prefix, converts another bit of the (binary)subnet mask to 1.)

What is another name for subnetting the subnets?

Variable Length Subnet mask (VLSM)

Why was this technique of subnetting the subnets introduced?

Classical subnetting has all the subnets the same size. This is very inefficientif some subnets have a lot of hosts but others are point to point links with only2 hosts. VLSM allows subnets to be different sizes.

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What is the most efficient prefix for a point to point WAN link? And what is theequivalent subnet mask?

/30255.255.255.252

This allows two host addresses.

Use the online activities to practice calculations.

6.6

You ping 127.0.0.1 and get a reply. What does this tell you?

IP is properly configured on the host. (This is the loopback address.)

You ping the default gateway and get no reply. Does this mean that thedefault gateway address is wrong?

No necessarily. The router might be configured not to respond to pings forsecurity reasons.

What does traceroute tell you that ping does not?

The address of every router along the route.

What is the purpose of ICMP?

It provides control and error messages for the TCP/IP suite of protocols.

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chap 6 addressing study questions

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