Advanced Computer Networks Chapter 3

8/13/2019 Advanced Computer Networks Chapter 3

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Dr. Tareq Alhmiedat

Department of Information Technologys,

Faculty of Computers and Information Technology,

Tabuk University,

[email protected]

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To introduce the concept of an address spacein general and the address space of IPv4 inparticular

To discuss the classful architecture

To discuss the idea of hierarchical addressing

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The identifier used in the IP layer of theTCP/IP protocol package to identify eachdevice connected to the Internet address or IPaddress

An IPv4 address is a 32-bit address thatuniquely and universally defines theconnection of a host or a router to the

Internet

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A protocol like IPv4 that defines addresseshas an address space

An address space is the total number ofaddresses used by the protocol

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Binary notation10001111 10011001 10101000 11100011

Dotted-Decimal Notation128 . 11 . 3 . 31

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We often need to deal with a range ofaddresses instead of one single address

We sometimes need to find the number ofaddresses in a range if the first and lastaddress is given

Example: Find the number of addresses in arange if the first address is 146.102.32.0andthe last address is 146.102.32.255

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IP addresses used the concept of classes This architecture is called classful addressing In the mid-1990, a new architecture called

classless addressingwas introduced We start with classful addressing

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In classful addressing, the IP address space isdivided into five classes: A, B, C, D, and E

Each class occupies some part of the wholeaddress space

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Class A: 231, forms 50% Class B: 230, forms 25%

Class C: 229, forms 12.5%

Class D: 228

, forms 6.25% Class E: 228, forms 6.25%

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Class A:

10

Byte 4yte 3yte 2yte 10-127

Octet 4ctet 3ctet 2ctet 10..

Binary notation

Dotted-decimal notation


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Class B:

11



Binary notation



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Class C:

12



Binary notation



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Class D:

13



Binary notation



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Class E:

14



Binary notation



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Example: Find the class of each address:

a. 00000001 00001011 00001011 11101111

b. 11000001 10000011 00011011 11111111

c. 11110011 10011011 11111011 00001111

d. 10100111 11011011 11111011 01101111

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In classful addressing, an IP address inclasses A, B, and C is divided into netidandhostid

These parts are of varying lengths, dependingon the class of the address

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Byte 1 Byte 2 Byte 3 Byte 4

Class A Netid Hostid

Class B Netid Hostid

Class C Netid Hostid

Class D Multicast address

Class E Reserved for future use


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A block is a range of addresses. Given anyaddress in the block, we normally like toknow three pieces of information about theblock:

1. The number of addresses

2. The first address

3. The last address

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In order to extract these pieces ofinformation, we need to do the following:

1. The number of addresses in block N, can befound using N = 232-n

2. To find the first address, we keep the nleftmost bit and set the (32-n) rightmostbits all to 0s

3. To find the last address, we keep n leftmostbits and set the (32-n) rightmost bits all to1s

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netid hostid

n bits (32-n) bits

netid 0000netid 1111

First address Last address

Class A: n = 8

Class B: n =16

Class C: n=24


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Example 1: An address in a block is given as73.22.17.25. Find the number of addressesinthe block, the first address, and the lastaddress.

Example 2: an address in a block is given as180.8.17.9. Find the number of addressesinthe block, the first address, and the lastaddress.

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The network address is particularly importantbecause it is used in routing a packet to itsdestination network

The network address is actually the identifierof the network

Each network is identified by its networkaddress

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The routers in the Internet usually use analgorithm to extract the network address fromthe destination address of a packet, to do this,we need a network mask

A network maskor a default maskin classfuladdressing is a 32-bit number with nleftmostbits all set to 1s and (32-n) rightmost bits allset to 0s

Since nis different for each class in classfuladdress, we have three default masks inclassful addressing

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24

11111111 00000000 00000000 00000000

8 bits 24 bitsMask for class A

11111111 11111111 00000000 00000000

16 bits 16 bits

11111111 11111111 11111111 00000000

24 bits 8 bitsMask for class B

Mask for class C


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To extract the network address from thedestination address of a packet, a router usesthe AND operation

When the destination address is ANDed withthe default mask, the result is the networkaddress

The default mask can be used to find the

number of addresses in the block and the lastaddress in the block

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26

10010101101 10010101101

AND

100101010000

Destination

address DefaultMask

Network

address


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Example: A router receives a packet with thedestination address 201.24.67.32. Show howthe router find the network address of thepacket.

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The IP addresses were originally designed withtwo levels of addressing

To reach a host on the Internet, we must firstreach the network and then the host

There is a great demand to have more than twoaddresses

The idea of splitting a block to smaller blocks isreferred to as subnetting

In subnetting, a network is divided into severalsmaller subnetworks (subnets) with eachsubnetwork having its own subnetwork address

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Consider the following example shown belowwhich shows a network using class Baddresses before subnetting

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Switch Network: 141.14.0.0/16

141.14.0.1 141.14.0.2 141.14.0.27

141.14.255.253 141.14.255.254

Internet router


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Internet router

Site router

141.14.0.0/18 141.14.64.0/18

141.14.128.0/18 141.14.192.0/18

Subnet 1 Subnet 2

Subnet 3 Subnet 4

141.14.0.0/16

141.14.0.1 141.14.31.29 141.14.63.254 141.14.64.1 141.14.90.27 141.14.127.254

141.14.128.1 141.14.142.37 141.14.191.254 141.14.192.1 141.14.223.47 141.14.255.254


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The network mask discussed before is usedwhen a network is not subnetted

When we divide the network into severalsubnetworks, we need to create a subnetwork

mask (or subnet mask) for each subnetwork

A subnetwork has subnetid and hostid

Subnetting increases the length of the netid

and decreases the length of hostid We can divide the network into snumber of

subnetworks

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We can calculate the subnetid for eachsubnetwork as

in which nis the length of netid, nsubis thelength of each subnetid, and sis the numberof subnets which must be a power of 2

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netid hostid

subnetid hostid

n bits 32 - n bits

ChangeNetwork mask

subnetwork mask


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Example: consider a network is divided intofour subnetworks in class B. Show the formatof the subnetwork mask.

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When a network is subnetted, the firstaddress in the subnet is the identifier of thesubnet and is used by the router to route thepackets destined for that subnetwork

Given any address in the subnet, the routercan find the subnet mask using the sameprocedure discussed in the network mask

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Example: Assume a network is divided intofour subnets. Since the address in subnet 2 is141.14.120.77. Find the subnet address.

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In supernetting, an organization can combineseveral class C blocks to create a larger rangeof addresses

In other words, several networks are

combined to create a supernetwork

An organization can apply for several class Cblocks instead of just one

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A supernet mask is the reverse of a subnetmask

A subnet mask for class C has more 1s thanthe default mask for this class

A supernet mask for class C has less 1s thanthe default mask for this class

In supernetting, the number of class C

addresses that can be combined to make asupernet needs to be a power of 2

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Comparison of subnet, default mask, andsupernet mask

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11111111 11111111 11111111 111 00000

11111111 11111111 11111111 00000000

Divide 1 class block into 8 subblocksSubnet mask

Default maskSupernet mask

11111111 11111111 11111 000 00000000

Combine 8 class blocks into 1 superblock

nsub= 24 + 3 = 27

n= 24

nsuper= 24 3 = 21


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Supernetting provides two main problems:1. The number of blocks to combine needs to

be a power of 2, which means anorganization that needed seven blocks

should be granted at least eight blocks(address wasting)

2. Supernetting and Subnetting reallycomplicated the routing of packets in theInternet

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Subnetting and supernetting in classfuladdressing did not really solve the addressdepletion problem and made the distribution ofaddresses and the routing process more difficult

With the growth of the Internet, it was clear thata larger address space was needed as a long-term solution

The larger address space requires that the

length of the IP addresses to be increased That means, the format of the IP packets needs

to be changed

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The class privilege was removed from thedistribution to compensate for the addressdepletion

In 1996, the Internet authorities announced a

new architecture called classless addressing In classless addressing, variable-length

blocks are used that belong to no classes

We can have a block of 1 address, 2addresses, 4 addresses,128 addresses, andso on

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In classfull addressing the whole addressspace was divided into five classes

Classes D & E were reserved for specialpurpose

In classless addressing, the whole addressspace is divided into variable length blocks

Theoretically, we can have a block of 20, 21,

22

, 232

addresses The only restriction is that the number of

addresses in a block needs to be a power of 2

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In classless addressing the block is actuallydivided into two parts: the prefixand suffix

The prefix plays the same role as the netid

The suffix plays the same role as the hostid

All addresses in the block have the sameprefix, while each address has a differentsuffix

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In classful addressing, the length of the netidn depends on the class of the address, it canbe only 8, 16, 24

In classless addressing, the length of the

prefix n depends on the size of the block, itcan be 0, 1, 2, 3, .. 32

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Prefix Suffix

Network Host

n bits (32-n) bits


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Example: What is the prefix length and suffixlength if the whole Internet is considered as onesingle block with 4,294,967,296 addresses?

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In classful addressing, the netid length isinherent in the address. Given an address, weknow the class of the address

In classless addressing, the prefix length can

not be found if we are given only an addressin the block. The given address can belong toa block with any prefix length

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byte .byte .byte .byte /n

Prefix

length


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The slash notation is formally referred to asclassless interdomain routing or CIDR

In classless addressing, we need to know theone of the addresses in the block and the

prefix length to define the block

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The idea of network mask in classlessaddressing is the same as the one in classfuladdressing

A network mask is a 32-bit number with the

n leftmost bits all set to 1s and the rest of thebits all set to 0s

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Example: Give the network mask for thefollowing addresses:

a. 12.23.24.78/8

b. 130.11.232.156/16

c. 167.199.170.82/27

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An address in slash notation (CIDR) containsall information we need about the block: thefirst address(network address), the number ofaddresses, and the last address

1. The number of addresses in the block canbe found as:

N = 232-nwhere nis the prefix, and Nis the number ofaddresses in the block

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2. The first address (Network address) in theblock can be found by ANDing the addresswith the network mask:

First Address = any address) AND network mask)

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3. The last address in the block can be foundby either adding the first address with thenumber of addresses, or directly, by Oringthe address with the complement (NOTing) of

the network mask

Last address = any address) OR [NOT network mask)]

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Example 1: One of the addresses in a block is167.199.170.82/27. Find the number ofaddresses in the network, the first address,and the last address

Example 2: One of the addresses is17.63.110.114/24. Find the number ofaddresses, the first address, and the lastaddress in the block

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The responsibility of block allocation is givento a global authority called the InternetCorporation for Assigned Names andAddresses (ICANN)

However, ICANN does not normally allocateaddresses to individual Internet users. Itassigns a large block of addresses to an ISP

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For the proper operation of the CIDR, threerestrictions need to be applied to the allocatedblock.

1. The number of requested addresses, N, needsto be a power of 2

2. The value of the prefix length can be foundfrom the number of addresses in the block

3. The requested block needs to be allocatedwhere there are a contiguous number of

unallocated addresses in the address space.The beginning address must be divisible by thenumber of addresses

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Example: Which of the following can be thebeginning address of a block that contains 16addresses?

A. 205.16.37.36

B. 190.16.42.44

C. 17.17.33.80

D. 123.45.24.52

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Example: Which of the following can be thebeginning address of a block that contains256 addresses?

A. 205.16.37.36

B. 190.16.42.0

C. 17.17.33.0

D. 123.45.24.52

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Example: An ISP has requested a block of1000 addresses.

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All issues discussed for classless addressingcan be applied to classful addressing

The classful addressing is a special case ofthe classless addressing in which the blocks

in class A, B, and C have the prefix lengthnA=8, nB=16, nC=24

A block in classful addressing can be easilychanged to a block in class addressing

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Three levels of hierarchy can be created usingsubnetting

An organization that is granted a range ofaddresses may divide the range into several

subranges and assign each subrange to asubnetwork (or subnet)

A subnetwork can be divided into severalsub-subnetworks. A sub-subnetwork can bedivided into several sub-sub-subnetworks.And so on

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The subnetwork in a network should becarefully designed to enable the routing ofpackets

We assume the total number of addresses

granted to the organization is N, the prefixlength is n, the assigned number ofaddresses to each subnetwork is Nsub, theprefix length for each subnetwork is nsub,

and the total number of subnetworks is s

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The following steps need to be carefully followedto guarantee the proper operation of thesubnetworks:

1. The number of addresses in each subnetwork

should be a power of 22. The prefix length for each subnetwork should

be found using the following formula:

nsub= n + log2(N/Nsub)

3. The starting address in each subnetwork shouldbe divisible by the number of addresses in thatsubnetwork.

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Example: An organization is granted theblock 130.34.12.64/26. The organizationneeds four subnetworks, each with an equalnumber of hosts. Design the subnetworks

and find the information about each network.

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Example: An organization is granted a blockof addresses with the beginning address14.24.74.0/24. the organization needs tohave 3 subblocks of addresses to use in its

three subnets as shown below:One subblock of 120 addresses

One subblock of 60 addresses

One subblock of 10 addresses

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One of the advantages of CIDR architecture isaddress aggregation

ICANN assigns a large block of addresses toan ISP

Each ISP in turn divides its assigned blockinto smaller subblocks and grants thesubblocks to its customers

Many blocks of addresses are aggregated inone block and granted to one ISP.

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In classful addressing some addresses werereserved for special purposes

All Zeros address 0.0.0.0/32, which contains only one single

address, is reserved for communication whena host needs to send an IPv4 packet but itdoes not know its own address

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All Ones Address: Limited Broadcast Address The block 255.255.255.255/32, which

contains one single address, is reserved forlimited broadcast address in the current

network A host that wants to send a message to every

other host can use this address as adestination address in the IPv4 packet

However, a router will block a packet havingthis type of address to confine thebroadcasting to the local network

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Loopback Addresses The block 127.0.0.0/8 is used for the loopback

address, which is an address used to test thesoftware on a machine

When this address is used, a packet never leavesthe machine, it simply returns to the protocolsoftware

It can be used to test the IPv4 software

For example: an application such as ping can

send a packet with a loopback address as thedestination address to see if the IPv4 is capableof receiving and processing a packet.

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Private Addresses A number of blocks are assigned for private

use

They are not recognized globally

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Some addresses in a block are used forspecial addresses

These addresses are not assigned to any host

However, if a block (or subblock) is so small,

we cannot afford to use part of the addressesas special addresses

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A. Network Address The first address (with the suffix set all to 0s)

in a block defines the network address

It actually defines the network itself and not

any host in the network

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B. Direct Broadcast Address The last address in a block or subblock (with

the suffix set all to 1s) can be used as a directbroadcast address

This address is usually used by a router tosend a packet to all hosts in a specificnetwork

All hosts will accept a packet having this type

of destination address Note that this address can be used only as a

destination address in an IPv4 packet

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In most situations, only a portion of computers ina small network need access to the Internetsimultaneously. This means that the number ofallocated addresses does not have to match thenumber of computers in the network

For example, assume a small business with 20computers in which the maximum number ofcomputers that access the Internet simultaneouslyis only 5. Most of the computers are either doing

some task that does not need Internet access. Thissmall business can use the TCP/IP protocol forboth internal and universal communication.

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A technology that can provide that mappingbetween the private and universal addresses,and at the same time, support virtual privatenetworks is network address translation(NAT)

The technology allows a site to use a set ofprivate addresses for internal communicationand a set of global Internet addresses (at

least one) for communication with the rest ofthe world

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All of the outgoing packets go through theNAT router, which replaces the sourcesaddress in the packet with the global NATaddress

All incoming packets also pass through theNAT router, which replaces the destinationaddressin the packet with the appropriateprivate address

Advanced Computer Networks Chapter 3

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