the internet protocol - 1

159.334 Computer Networks159.334 Computer Networks

The Internet Protocol - 1

Professor Richard Harris

School of Engineering and AdvancedTechnology (SEAT)

Computer NetworksComputer Networks -- 1/1/22


Presentation Outline

Format of IP packets

Internet addressing schemes

Allocation of hosts and subnets to meet requirementsfor IP network design.



Learning Objectives

You will be able to:

Discuss the format of IP packets

Apply internet addressing to sample problems

Perform allocation of hosts and subnets to meetrequirements for IP network design.



References

Tanenbaum, “Computer Networks”, 4th Edition

Forouzan, “Data Communications and Networking”, 4th

Edition

Cisco CCNA1 Module 10 - part 1

Stallings, William 2000 ‘Data and ComputerCommunications’, Prentice Hall, Sixth Edition

Russell, Travis 1997 ‘Telecommunications Protocols’,McGraw Hill



3 Concepts Underpinning the Internet(Revision)

The key concepts that underpin the Internet are:

Addressing

People need to be able to send data to you through the network.This requires a method for identifying you as the intendedrecipient! Hence the need to have an address where you can bereached.

Routing

There needs to be a methodology to send messages around thenetwork based on prevailing network conditions, speeds of thelinks, availability of capacity etc.

Internet Protocol

All the devices in the network need to speak the same “language”and this is a protocol for ensuring that the messages can bedelivered correctly to their intended addressee.

The key concepts that underpin the Internet are:

Addressing

People need to be able to send data to you through the network.This requires a method for identifying you as the intendedrecipient! Hence the need to have an address where you can bereached.

Routing

There needs to be a methodology to send messages around thenetwork based on prevailing network conditions, speeds of thelinks, availability of capacity etc.

Internet Protocol

All the devices in the network need to speak the same “language”and this is a protocol for ensuring that the messages can bedelivered correctly to their intended addressee.

Note:Note: Each of these concepts is designed to be scalable to large networks



Addressing - 1

An Internet Protocol address (“IP address”) corresponds to a host-interface card - ie a device that connects a computer to a network.

If a computer has more than one interface card it will require more thanone IP address.

IP addresses are set up in a two-part hierarchy:

Network number

Interface number (also called a host number)

Both the network and interface numbers are unique so that we canuniquely identify the device/card that is connected to the network.



Addressing - 2

A central authority allocates a network operator with aunique network number range.

Once this number has been assigned, the operator canallocate a globally unique IP address with the prefix andthis allows decentralised control of the address space.

If addresses had not been based on this hierarchicalstructure, we would require a central authority to checkevery IP address for uniqueness!!!

Network Number102.54.94

Interface number97

rhino.acme.com



Address Classes - 1

An important aspect of addressing is to work out how toallocate bits to the network and interface numbers.

If the Internet had many networks but few interfaces then itwould be better to allocate more bits to the network address. Ifthe opposite case prevailed, then we would obviously reversethe situation.

In IP Version 4, the addresses are 3232 bits in length:

If we use 24 bits for the address of the network number and theremaining 8 bits for the interface number then we could have 224

networks (16,772,216) and 28 = 256 interfaces per network.

A total of 4,294,967,296 computer cards/interfaces are possible!



Address Classes - 2

Internet designers initially thought there would only be 256networks so the situation described would have beenreversed!

In fact, a rather more flexible scheme was adopted, viz:

Partition the address space into Class A, Class B, Class C andClass D addresses with different numbers of bits assigned tothe network and interface numbers in each class.

A Class A address has 8 bits for the network number and 24bits for the interface number.

A single large part of the Internet would use a Class Aaddress.

But by contrast, a Class C address has 24 bits of networknumber and only 8 bits of interface number. This means thatonly 256 hosts can be accommodated in a Class C network.



Address Classes - 3

The network can distinguishbetween the 4 classes of addressby the first few bits of the networknumber:

Class A - first bit is ‘0’

Class B - first two bits are ‘10’

Class C - first three bits are ‘110’

Class D - first four bits are ‘1110’

Class E - first four bits are ‘1111’Used for experimental purposesonly.

• Using these bits from the network number has the effect of reducing the overallnumber of networks in each of the classes.

• Class A networks are only 7 bits long so there can only be 128 large class networks.• Class B networks consists of 14 bits so there can only be 16,384 medium class

networks.

0

0

01

1

Class A

Class B

Class C

Class D

1

0

Class E

1



Address Classes - 4

As mentioned above, there are 5 classes of addressformat in the Internet.

The following table shows the allocations of bits forthese classes:

High order bits010

11011101111

Format7 bits net, 24 bits host14 bits net, 16 bits host21 bits net, 8 bits host28 bits multicast group #reserved for experiments

ClassABCDE



Exercise

Compute the number of Class C networks possible.

Hint: A class C address consists of 21 bits for thenetwork number and 8 bits for the interface number.Refer to previous slide…..

Your Answer:Your Answer:



Address Class Problems - 1

We have stated that IP Version 4 addresses are 32 bits long andcan theoretically address a total of 4,294,967,296 computercards/interfaces.

In practice, we find that after the central authority hands out theaddress space allocations, there is little control that can beexercised over how much of that space is used. In earlier times,Class A and B spaces were rather freely allocated and rathersparsely used and cannot be reclaimed.

To solve this problem, IP Version 6 - or IPng has been proposed -this will be discussed at another time.



Address Class Problems - 2

If a campus network had more than 256 nodes, it would need aClass B address. This provides 65,536 nodes in the address space,but this is a very large number and typically the campus networkwould not have this many hosts.

Thus, there is considerable wastage if we adopt this approach.

A solution is to use Classless Inter Domain Routing addresses(CIDR).

In this form of addressing, the network number can be an arbitrarynumber of bits long so that the size of the network can be bounded bysome power of 2.



Internet Addresses - 2

Addresses are usually represented as four decimalnumbers separated using dots.

Each number represents an octet of the address.For example, consider 10.0.0.1

The binary representations are

00001010 00000000 00000000 00000001

High order bits0

10110

11101111

Format7 bits net, 24 bits host14 bits net, 16 bits host21 bits net, 8 bits host28 bits multicast group #reserved for experiments

ClassABCDE




The two level hierarchy (network and host numbers)were initially thought to be sufficient, but by 1984, itbecame clear that a third hierarchical level was neededand so the “subnet” was added to the hierarchy at thattime.

This leads to the following format:

Network number Subnet Host




Consider a general B-class address as shown below. Byusing a more flexible arrangement when we want more than256 but fewer than 65,536 hosts, we could flexibly rearrangethe address like this:




More subnets and fewer hosts

Fewer subnets and more hosts

This flexibility is enabled using a device known as a subnet mask




Here are some example B-class network options:

Network number 14 bits 2 bits



16,384 subnets 4 hosts




64 subnets 1,024 hosts

10

10

10

10

……………………………………………………




The subnet field can have any length - it isspecified by a 32-bit “mask”.

You find an address belongs to a subnet bycomparison using a mask operation:

This means that all bits of the address for which thecorresponding mask bit is null are zeroed and the resultcompared to the subnet identifier.

Mask Address Net Subnet Host

0xFFFF00000xFFFFFE00

0xFFFFFFC0

10.27.32.100136.27.33.100136.27.34.141193.27.32.197

A: 10 27 32.100B: 136.27 16 (32) 1.100

136.27 17 (34) 0.141C: 193.27.32 3 (192) 5




Here are three examples of the masks given on theprevious slide in their Hex, Binary and “Dot”formats: FFFF0000

1111 1111 1111 1111 0000 0000 0000 0000

255 . 255 . 0 . 0

Hex Format

Binary Format

Dot Format

FFFFFE00

255 . 255 . 254 . 0

1111 1111 1111 1111 1111 1110 0000 0000

Hex Format

Binary Format

Dot Format

FFFFFFC0

255 . 255 . 255 . 192

1111 1111 1111 1111 1111 1111 1100 0000

Hex Format

Binary Format

Dot Format



Exercise

Use the masks shown on the previous slide to show theNet/Subnet/Host results on the right hand side of thetable are correct.

Answer:Answer:



Answers - 1

First we convert the address into binary form:

10.27.32.100

0000 1010 0001 1011 0010 0000 0110 0100 Then we add the mask in binary form:

1111 1111 1111 1111 0000 0000 0000 0000 (Mask)

0000 1010 0001 1011 0000 0000 0000 0000 Then we perform the masking:

A-class address!

Result:

Net: 10 Subnet: 27 Comparison

Host: 32.100

We note that the length of the subnet mask here was the samelength as the length of a normal A-class address in this case.

We note that the length of the subnet mask here was the samelength as the length of a normal A-class address in this case.



Answers - 2


136.27.33.100


1111 1111 1111 1111 1111 1110 0000 0000 (Mask: FFFFFE00)


B-class address!

Result:

Net: 136.27 Comparison

Host: 1.100Subnet: 16

Normal B-class address specification: 10 + 14 bit net address + 16 bits for hostsArrangement illustrated above:

10 + 14 bit net address + 7 bit subnet address + 9 bit host addressLeads to 128 subnet addresses and 512 hosts per subnet possible.





Answers - 3


136.27.34.141


1111 1111 1111 1111 1111 1110 0000 0000 (Mask: FFFFFE00)


B-class address!

Result:

Net: 136.27 Comparison

Host: 0.141Subnet: 17







Answers - 4


193.27.32.197


1111 1111 1111 1111 1111 1111 1100 0000 (Mask: FFFFFFC0)


C-class address!

Result:

Net: 193.27.32 Comparison

Host: 5Subnet: 3

Normal C-class address specification: 110 + 21 bit net address + 8 bits for hostsArrangement illustrated above:


Normal C-class address specification: 110 + 21 bit net address + 8 bits for hostsArrangement illustrated above:




One more for you to try!

130.50.15.6

1000 0010 0011 0010 0000 1111 0000 0110

1111 1111 1111 1111 1111 1100 0000 0000 (Mask: FFFFFC00)

?-class address?

You do the rest!You do the rest!



Special Purpose Addresses - 1

It can be the case that a host does not know its IPaddress or the IP address of its neighbour - forexample when the host boots up.

Several addresses have been established forhandling this type of situation

When the network number is unknown, a hostcan use ‘0’ as a substitute.

The special address 0.0.0.0 means “the host onthis network”.

The special address can only be used as a sourceaddress - eg when the host is booting

An address of 0.X.Y.Z means the host X.Y.Z on thisnetwork.

the internet protocol - 1

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