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Department of Computer Science DCS COMSATS Institute of Information Technology Network Layer Advanced Computer Networks Rab Nawaz Jadoon Assistant Professor COMSATS University, Lahore Pakistan

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Page 1: CS Presentation template - WordPress.com · Department of Computer Science 3 Network Layer IPv4 Addresses An IPv4 address is 32 bits long. It defines the connection of a device (for

Department of Computer Science

DCS

COMSATS Institute of Information Technology

Network Layer

Advanced Computer Networks

Rab Nawaz Jadoon

Assistant Professor

COMSATS University, Lahore

Pakistan

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Department of Computer Science 22

Network Layer

Main Functions of Network layer Source-to-destination delivery of a Packet

(host to host communication)

Possibly across multiple networks (links).

Logical addressing , IPv4 addressing

The Internet addresses are 32 bits in length;

This gives us a maximum of 232 addresses.

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Department of Computer Science 3

Network Layer

IPv4 Addresses An IPv4 address is 32 bits long.

It defines the connection of a device (for example, a computer or a router) to the Internet.

Two devices on the Internet can never have the same address at the same time.

Address Space Total number of addresses used by the protocol.

IPv4 uses 32-bit addresses.

The address space is 232 or

4,294,967,296 (more than 4 billion).

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Department of Computer Science 4

Network Layer

Notations

Binary notation

01110101 10010101 00011101 00000010

Dotted decimal notation.

117.149.29.2

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Department of Computer Science 5

Network Layer

Find the error, if any, in the following IPv4 addresses. 111.56.045.78

221.34.7.8.20

75.45.301.14

11100010.23.14.67

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Department of Computer Science 6

Network Layer

Classful Addressing

The address space is divided into five classes: A, B, C, D, and E.

We can find the class of an address when given the address in binary notation or dotted-decimal notation.

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Department of Computer Science 7

Network Layer

Netid and Hostid

In class A, one byte defines the netid and three bytes define the hostid.

In class B, two bytes define the netid and two bytes define the hostid.

In class C, three bytes define the netid and one byte defines the hostid.

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Department of Computer Science 8

Issues in Addressing

A large corporate/campus environment

Large number of Local Area Networks

Some with fewer than 256 hosts

Some with more than 256 hosts

If each physical network is assigned a network number:

Immense administrative overhead to manage a large number of network addresses

Routing tables in routers become extremely large (one entry for each physical network)

Insufficient number of class B prefixes to cover medium sized networks (having more than 256 hosts)

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Department of Computer Science 9

Subnetting

Solution: Provide the campus with a single class B network

Give freedom to the campus network admin to allocate host numbers to hosts

From outside, the whole campus is simply known by the class B network ID

Inside, there may be a hierarchy that remains transparent to the outside world

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Department of Computer Science 10

Subnetting

Consider a class B network

How to allocate host numbers to hosts?

A single LAN is out of question

If host numbers are assigned randomly, i.e., without any hierarchy, the routers inside the network will have to deal with large tables –one entry per host

Thus, a hierarchical structure is required

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Department of Computer Science 11

Subnetting

Physical Network(Subnet 1)

Physical Network(Subnet 2)

Physical Network(Subnet 3)

Physical Network(Subnet 4)

R

RR R

R

H H

H H

H

H HHH

H

H

H

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Department of Computer Science 12

Subnetting

RInternetH1 H2

H4H3

Network 138.10.1.0

Network 138.10.2.0

138.10.1.1 138.10.1.2

138.10.2.2138.10.2.1

H1 wants to send an IP datagram to H3:Old addressing dictates it is a “direct delivery”With Subnetting, it may become “indirect”

Subnet 1

Subnet 2

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Department of Computer Science 13

Subnetting

We previously divided IP addresses in a network portion and a host portion

More generally, think of a 32-bit IP address as having an Internet part and a Local part

Internet part of the IP address identifies a site (possibly with many physical networks)

The local portion identifies a physical network and host at that site (note: physical network == extended LAN)

Internet Part Local Part

Internet Part Subnet Host

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Department of Computer Science 14

Subnetting

Examples: Class-B IP address

Internet Part Subnet Host

16bits 8bits 8bits

Internet Part Subnet Host

16bits 3bits 13bits

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Department of Computer Science 15

Subnet Implementation

Subnet Mask:Specifies the bits of the IP address used to identify

the subnet

Internet Part of Address Subnet Host

16bits 8bits 8bits11111111 11111111 11111111 00000000

Internet Part of Address Subnet Host

Subnet Mask(32bits)

16bits 3bits 13bits11111111 11111111 111 00000 00000000

255. 255. 255. 0

255. 255. 224. 0

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Department of Computer Science 16

Example

140.179.220.200

Written in binary form:

140 .179 .220 .200

10001100.10110011.11011100.11001000

We see the address in the decimal form

Your computer sees it in the binary form

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Department of Computer Science 17

Binary Octet:

An octet is made up of eight “1”s and/or “0”s, representing the following values:

128 64 32 16 8 4 2 1

So the value of 140 (the first octet of our example) looks like this:

1 0 0 0 1 1 0 0

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Department of Computer Science 18

Network Layer

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Department of Computer Science 19

Network vs. Host

Every IP address has 2 parts:

1 identifying the network it resides on

1 identifying the host address on the network

The class of the address and the subnet mask determine which part belongs to the network address and which part belongs to the host address

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Department of Computer Science 20

IP Address Breakdowns:

The class of the address determines, by default, which part is for the network (N) and which part belongs to the host (H)

Class A: NNNNNNNN.HHHHHHH.HHHHHHHH.HHHHHHHH

Class B: NNNNNNNN.NNNNNNNN.HHHHHHH.HHHHHHHH

Class C: NNNNNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH

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Department of Computer Science 21

140.179.220.200

Our example is a Class B address

By default, the Network part of the address is defined by the first 2 octets: 140.179.x.x

By default, the Host part of the address is defined by the last 2 octets: x.x.220.200

*Note:

The network part of the address is also known as the Network Address

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Department of Computer Science 22

Two Reserved Addresses on a Subnet:

In order to specify the Network Address of a given IP address, the Host portion is set to all “0”s:

140.179.0.0

If all the bits in the Host portion are set to “1”s, then this specifies the broadcast address that is sent to all hosts on the network:

140.179.255.255

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Department of Computer Science

SUBNETTING

The highly dreaded….

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Department of Computer Science 24

Subnetting

Subnetting an IP network can be done for various reasons including:

Preservation of address space

Security

Control network traffic

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Department of Computer Science 25

Example Class A

Millions of Addresses Available

Over 16,000,000

Efficiency

Non-subnetted networks are wasteful

Division of networks not optimal

Smaller Network

Easier to manage

Smaller broadcast domains

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Department of Computer Science 26

Subnet Mask

Subnet masks are applied to an IP address to identify the Network portion and the Hostportion of the address.

Your computer performs a bitwise logical AND operation between the address and the subnet mask in order to find the Network Address or number.

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Department of Computer Science 27

Default Subnet Masks

Class A - 255.0.0.0

11111111.00000000.00000000.00000000

Class B - 255.255.0.0

11111111.11111111.00000000.00000000

Class C - 255.255.255.0

11111111.11111111.11111111.00000000

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Department of Computer Science 28

Logical Bitwise AND Operation

Remember our example?

140.179.240.200

It’s a Class B, so the subnet mask is:

255.255.0.0

We need to look at this as our computer does so

we can perform the bitwise AND...

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Department of Computer Science 29

Logical Bitwise AND Operation

140.179.240.200 Class B address

255.255.0.0 Subnet Mask

In Binary:

10001100.10110011.11110000.11001000

11111111.11111111.00000000.00000000

10001100.10110011.00000000.00000000

By doing this, the computer has found that our Network Address is 140.179.0.0

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Department of Computer Science 30

Another Example:

Suppose we have the address of: 206.15.143.89?

Class C

255.255.255.0

206.15.143.0

0.0.0.89

What class is it?

What is the subnet mask?

What is the Network Address?

What is the host portion of the address?

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Department of Computer Science 31

Why Do We Care!?

You can manipulate your subnet mask in order to create more network addresses. Why?

If you have a Class C network, how many individual host addresses can you have?

1 to 254

Remember, you can’t have all “0”s and all “1”s in the host portion of the address.

So we cannot use 206.25.143.0 (all “0”s) or 206.25.143.255 (all “1”s) as a host address.

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Department of Computer Science 32

Why Do We Care!?

So we have 1 Class C Network (206.15.143.0)

And we have 254 host address (1 to 254)

But what if our LAN has 5 networks in it and each network has no more than 30 hosts on it?

Do we apply for 4 more Class C licenses, so we have one for each network?

We would be wasting 224 addresses on each network, a total of 1120 addresses!

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Department of Computer Science 33

Subnetting

Subnetting is a way of taking an existing class license and breaking it down to create more Network Addresses.

This will always reduce the number of hostaddresses for a given network.

Subnetting makes more efficient use of the address or addresses assigned to you.

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Department of Computer Science 34

How Does Subnetting Work?

Additional bits can be added (changed from 0 to 1) to the subnet mask to further subnet, or breakdown, a network.

When the logical AND is done by the computer, the result will give it a new Network (or Subnet) Address.

Remember, an address of all “0”s or all “1”s cannot be used in the last octet (or host portion). All “0”s signify the Network Address and all “1”s signify the broadcast address

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Department of Computer Science 35

So How Does This Work?

We ask our ISP for a Class C license.

They give us the Class C bank of 206.15.143.0

This gives us 1 Network (206.15.143.0) with the potential for 254 host addresses (206.15.143.1 to 206.15.143.254).

But we have a LAN made up of 5 Networks with the largest one serving 25 hosts.

So we need to Subnet our 1 IP address...

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Department of Computer Science 36

So How Does This Work?

To calculate the number of subnets(networks) and/or hosts, we need to do some math:

Use the formula 2n-2 where the n can represent either how many subnets (networks) needed OR how many hostsper subnet needed.

Magic

Formula

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Department of Computer Science 37

So How Does This Work?

We know we need at least 5 subnets. So 23-2 will give us 6 subnet addresses (Network Addresses).

We know we need at least 25 hosts per network. 25-2will give us 30 hosts per subnet (network).

This will work, because we can steal the first 3 bits from the host’s portion of the address to give to the network

portion and still have 5 (8-3) left for the host portion:

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Department of Computer Science 38

Break it down:

Let’s go back to what portion is what:

We have a Class C address:

NNNNNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH

With a Subnet mask of:

11111111.11111111.11111111.00000000

We need to steal 3 bits from the host portion to give it to the Network portion:

NNNNNNNN.NNNNNNNN.NNNNNNNN.NNN HHHHH

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Department of Computer Science 39

Break it down:

NNNNNNNN.NNNNNNNN.NNNNNNNN.NNNHHHHH

This will change our subnet mask to the following:

11111111.11111111.11111111.11100000

Above is how the computer will see our new subnet mask, but we need to express it in decimal form as well:

255.255.255.224 128+64+32=224

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Department of Computer Science 40

What address is what?

Which of our 254 addresses will be a Subnet (or Network) address and which will be our hostaddresses?

Because we are using the first 3 bits for our subnet mask, we can configure them into eight different ways (binary form):

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Department of Computer Science 41

What address is what?

Which of our 254 addresses will be a Subnet (or Network) address and which will be our host addresses?

Because we are using the first 3 bits for our subnet mask, we can configure them into eight different ways (binary form):

000 001010 011100 101110 111

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Department of Computer Science 42

What address is what?

We cannot use all “0”s or all “1”s000 001010 011100 101110 111

•We are left with 6 useable network

numbers.

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Department of Computer Science 43

Network (Subnet) Addresses

Remember our values:

128 64 32 16 8 4 2 1 Equals

Now our 3 bit configurations:

0 0 1 H H H H H 32

0 1 0 H H H H H 64

0 1 1 H H H H H 96

1 0 0 H H H H H 128

1 0 1 H H H H H 160

1 1 0 H H H H H 192

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Department of Computer Science 44

Network (Subnet) Addresses

0 0 1 h h h h h 32

0 1 0 h h h h h 64

0 1 1 h h h h h 96

1 0 0 h h h h h 128

1 0 1 h h h h h 160

1 1 0 h h h h h 192

Each of these numbers becomes the

Network Address of their subnet...

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Department of Computer Science 45

Network (Subnet) Addresses

206.15.143.32

206.15.143.64

206.15.143.96

206.15.143.128

206.15.143.160

206.15.143.192

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Department of Computer Science 46

host Addresses

The device assigned the first address will receive the first number AFTER the network address shown before.

206.15.143.33 or 32+1

0 0 1 0 0 0 0 1

And the last address in the Network will look like this:

206.15.143.62

0 0 1 1 1 1 1 0

*Remember, we cannot use all “1”s, that is the broadcast address (206.15.143.63)

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Department of Computer Science 47

Host Addresses

The next network will start at 206.15.143.64

The first IP address on this subnet network will receive:

206.15.143.65

0 1 0 0 0 0 0 1

And the last address in the Network will receive: 206.15.143.94

0 1 0 1 1 1 1 0

*Remember, the broadcast address (206.15.143.95)

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Department of Computer Science 48

Can you figure out the rest?

Network: Host Range

206.15.143.32 206.15.143.33 to 206.15.143.62

206.15.143.64 206.15.143.65 to 206.15.143.94

206.15.143.96 206.15.143.97 to 206.15.143.126

206.15.143.128 206.15.143.129 to 206.15.143.158

206.15.143.160 206.15.143.161 to 206.15.143.190

206.15.143.192 206.15.143.193 to 206.15.143.222

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Department of Computer Science 49

How the computer finds the Network Address:

200.15.143.89 An address on the subnet

225.225.225.224 The new subnet mask

When the computer does the Logical Bitwise AND Operation it will come up with the following Network Address (or Subnet Address):

11001000.00001111.10001111.01011001 = 200.15.143.89

11111111.11111111.11111111.11100000 = 255.255.255.224

11001000.00001111.10001111.01000000 = 200.15.143.64

This address falls on our 2nd Subnet (Network)

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Department of Computer Science 50

Review

We have one class C license.

We need to subnet that into 12 possible networks.

Each network needs a maximum of 10 hosts.

How many bits do we need to take?

24-2=14

4 bits need to be taken from the host portion and given to the network portion.

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Department of Computer Science 51

Review

Will that leave enough bits for the host portion? We need a maximum of 10 on each network…

24-2=14

If we take 4 away, that leaves us with 4. That is enough for our individual networks of 10 hosts each.

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Department of Computer Science 52

Review Our new subnet mask will look like this:

11111111.11111111.11111111.11110000

255.255.255.240 128+64+32+16= 240

Our subnet, or network addresses will be:

206.15.143.16 206.15.143.32 206.15.143.48

206.15.143.64 206.15.143.80 206.15.143.96

206.15.143.112 206.15.143.128 206.15.143.144

206.15.143.160 206.15.143.176 206.15.143.192

206.15.143.208 206.15.143.224

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Department of Computer Science 53

Subnetting

It is recommended that sites use contiguous subnet masks

Avoid masks such as

11111111 11111111 11000010 11000000

When choosing a subnet mask, balance:

Size of networks

Number of networks

Expected growth

Ease of maintenance

It is possible to use different masks in different parts of the network

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Department of Computer Science 54

Review

Let's say that you need to subnet the network address 199.42.78.0 using the subnet mask 255.255.255.224.

What are the valid subnets that you can obtain from the given subnet mask?

What is the broadcast address of each subnet?

What are the valid hosts in each of the subnets?

What subnet mask should you use if you need x subnets and y hosts?

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Department of Computer Science 55

Solution

Subnet 1 Subnet 2 Subnet 3 Subnet 4 Subnet 5 Subnet 6

Subnet address

32 64 96 128 160 192

First Valid Host

33 65 97 129 161 193

Last Valid Host

62 94 126 158 190 222

Broadcast address

63 95 127 159 191 223

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Department of Computer Science 56

Review

Using the subnet mask 255.255.255.224, what is the subnet that the host 199.42.78.133 belongs to and the broadcast for this address?

Solution:

Formula 256 – subnet mask = base number.

256-224=32, which is your first subnet.

Add the 32 to itself until you reach the subnet that contains the host 133.

32+32=64, 64+32=96, 96+32=128, 128+32=160.

Stop here because 133 is between 128 and 160.

Address 199.42.78.133 belongs within the 199.42.78.128.

The broadcast address is 199.42.78.159

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Department of Computer Science 57

Home Assignment

Given the network address 172.16.68.17 and the subnet mask 255.255.192.0, find the subnet it belongs to and its broadcast?

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Exercise

We have a class B network 150.5.0.0, number of sub network needed 100.

How the Subnetting you will done to form 100 networks each accommodating 500host

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Department of Computer Science 59

Subnet Routing

Conventional routing table entry

(network address, next hop address)

Network address format is predetermined for a given class (e.g., first 16 bits for class B addresses!)

With subnetting, routing table entry becomes

(subnet mask, network address, next hop address)

Then compare with network address field of entries to find next hop address

Subnet mask indicates the network address!

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Department of Computer Science 60

Subnet Routing The use of mask generalizes the subnet routing

algorithm to handle all the special cases of the standard algorithm

Routes to individual hosts

Default route

Routes to directly connected networks

Routes to conventional networks (that do not use subnet addressing)

Merely combine the 32-bit mask field with the 32-bit IP address

Example: To install a route for:

Individual host (Mask of all 1’s, Host IP address)

Default Route (Mask of all 0’s, network address all 0’s)

Class B network address (Mask of two octets of 1’s and two of 0’s)

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Subnet Routing

Algorithm

Extract destination IP (D) from datagram

Compute IP address of destination network N

If N matches any directly connected network address

Send datagram over that network (obviously encapsulated in a frame)

Else

For each entry in the routing table, do

N* = bitwise-AND of D and subnet mask

If N* equals the network address field of the entry, then route the datagram to the specified next hop

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