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CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY

Chabot CollegeChabot College

ELEC 99.05ELEC 99.05IP AddressingIP Addressing


Logical Address CompositionLogical Address Composition

Though the exact length and format of a logical (layer 3) address differs depending on the protocol, all logical addresses share this basic formula:

NETWORK NUMBER

NETWORK NUMBER HOST NUMBERHOST NUMBER


IP AddressesIP Addresses

• IP addresses are the most common logical addresses. (Everyone on the Internet has one.)

• 32 - bit numbers (IP version 4)


32 bits not enough32 bits not enough

• 32 - bits yields 232 unique numbers

• 232 = 4,294,967,2964,294,967,296– there are over 4 billion possible IPv4

addresses– but many are “wasted” due to the

allocation scheme


IPv6: The Next GenerationIPv6: The Next Generation

The newest version of IP (version 6, or IPng) uses 128 bits, yielding

2128 unique combinations

That’s over 340,000,000,000,000,000,000,000,000,000,000,000,000 possible addresses!


IPv4 vs. IPv6IPv4 vs. IPv6

• IPv6 is slowly be integrated in the existing Internet.

• IPv4’s 32 bits continues to be the dominant form of IP addressing.


IP AddressesIP Addresses

We use dotted notation to represent the value of each byte (octet) of the IPv4 address in decimal.

10101100 00011100 1110110 00001010

172 . 28 . 118 . 10


Dotted Decimal NotationDotted Decimal Notation

Which addresses are legal?

201.165.321.1

12.1.2.2

198.261.34.2

645.250.2.4

209.254.130.4


Dotted Decimal NotationDotted Decimal Notation

The highest decimal number for any octet is 255, or 11111111 in binary.

201.165.321.1

12.1.2.2

198.261.34.2

645.250.2.4

209.254.130.4


IP Address ClassesIP Address Classes

• IPv4 uses a “class” system. • There are 5 classes of IP addresses:

– Class A– Class B– Class C– Class D– Class E


Address ClassesAddress Classes

Class A Used for Internet hosts

Class B Used for Internet hosts

Class C Used for Internet hosts

Class D Used for Internet multicasts

Class E Unused (used “experimentally”)

Computers on the Internet can only be addressed using Class A, Class B, or Class C addresses.


Determining Address ClassDetermining Address Class

Class A First octet is between 0 - 127

Class B First octet is between 128 - 191

Class C First octet is between 192 - 223

Class D First octet is between 224 - 239

Class E First octet is between 240 - 255

What is special about these numbers?


What class is 180.129.41.9?What class is 180.129.41.9?






Class BClass B








Class DClass D








Class CClass C


Class-what’s the difference?Class-what’s the difference?

• The address class can be used to determine:– network number– host address

NETWORK NUMBER

NETWORK NUMBER HOST NUMBERHOST NUMBER



Class A

Class B

Class C

Network Host Host Host

Network Network Host Host

Network Network Network Host

1st octet 2nd octet 3rd octet 4th octet



Class A

Class B

Class C

85 45 31 158

168 65 114 201

210 144 235 56

Network Host


Which part is network?Which part is network?

199.46.36.5

199.46.36.5

111.211.11.1

111.211.11.1

7.141.30.89

7.141.30.89


Which part is network?Which part is network?

222.8.56.107

222.8.56.107

192.168.16.2

192.168.16.2

163.100.5.1

163.100.5.1


Network NumbersNetwork Numbers

• A network number is the address of the network itself.

• It is not the address of any host on the network.

• Network numbers are reserved and cannot be assigned to any host.


Network Numbers by ClassNetwork Numbers by Class

Class A

Class B

Class C

63 0 0 0

142 56 0 0

209 126 155 0


Following are examples of network numbers.Notice that the entire host portion is 0.

Network numbers are reserved, and cannot beassigned to any workstation.


Network NumbersNetwork Numbers

How do you write a network number?

Set the entire host portion to all zeros.

84.124.51.1 (host address)

84.0.0.0 (network number)


170.98.0.0 (network number)


Broadcast AddressBroadcast Address

• Packets sent to a broadcast address will be sent to all hosts on the network.

• A broadcast address is not the address of any host on the network.

• Broadcast addresses are reserved and cannot be assigned to any host.


Broadcast Addresses by ClassBroadcast Addresses by Class

Class A

Class B

Class C

63 255 255 255

142 56 255 255

209 126 155 255


Following are examples of broadcast addresses.Notice that the entire host portion is 255.

Broadcast addresses are reserved, and cannot beassigned to any workstation.


Broadcast AddressesBroadcast Addresses

How do you write a broadcast address?

Set the entire host portion to all ones.


84.255.255.255 (broadcast address)


170.98.255.255 (broadcast address)


The Network NumberThe Network Number

• Because the network number provides logical order, it can not be randomly assigned.

• One organization administrates IP addressing.


InterNICInterNIC

• The management of IP addresses has been the responsibility of an organization known as InterNIC.

• Currently, Network Solutions (Verisign) controls InterNIC.


Class A addressesClass A addresses

Network Host Host Host

First octet is between 0 - 127

Number between 0 - 127

8 bits 8 bits 8 bits

With 24 bits available for hosts, there are 224 possible addresses. That’s 16,777,216 nodes!



• There are 126 class A addresses.– 0 and 127 have special meaning and are

not used.• Only large organizations such as the military,

government agencies, universities, and large corporations have class A addresses.

• Cable Modem ISPs have 24.0.0.0• Pacbell DSL users have 63.0.0.0



• Class A addresses account for 2,147,483,648 of the possible IPv4 addresses.

• That’s 50 % of the total unicast address space!


Class B addressesClass B addresses




8 bits 8 bits

With 16 bits available for hosts, there are 216 possible addresses. That’s 65,536 nodes!


Class B addressesClass B addresses

• There are 16,384 (214) class B networks.• Class B addresses represent 25% of the

total IPv4 unicast address space.• Class B addresses are assigned to large

organizations including corporations (such as Cisco, government agencies, and school districts).


Class C addressesClass C addresses

Network Network Network Host



8 bits

With 8 bits available for hosts, there are 28 possible addresses. That’s 256 nodes!


Class C addressesClass C addresses

• There are 2,097,152 possible class C networks.

• Class C addresses represent 12.5% of the total IPv4 unicast address space.


IP address shortageIP address shortage

• In the early days of the Internet, IP addresses were allocated to organizations based on request rather than actual need.


No Medium SizeNo Medium Size

• 16 million• 65,536• 256

For most organizations, 256 is too small a limit on hosts, yet 65,536 is far too many.


The Subnet MaskThe Subnet Mask

• The solution to the IP address shortage was thought to be the subnet mask.

• Formalized in 1985, the subnet mask breaks a single class A, B or C network in to smaller pieces.


Class B Subnet ExampleClass B Subnet Example

255 255 0 0

172 28 69 137Address

Mask

What if 216, or 65,536, hosts is too many (it is)? This network could be broken up in to smaller pieces by creating subnets.

8 bits 8 bits

Network Network HostNetwork Network Host



255 255 255 0

172 28 69 137Address

Mask

Note that the subnet mask in the third octet is set to all “1”s. Those 8 “1”s mean that all 8 of those bits are used to determine subnetwork number.

8 bits 8 bits

Network Network SubnetNetwork Network Host


Class B Subnet ExampleClass B Subnet ExampleGiven the Class B address 172.28.0.0

172.28.1.2

172.28.2.2

172.28.3.2

Class B Network Network Host Host

All of these addresses are on the same network

Routers “see” this network as 172.28.0.0


Class B Subnet ExampleClass B Subnet ExampleUsing subnets...

172.28.1.2

172.28.2.2

172.28.3.2

Network Network Subnet Host

But internal routers think all these addresses are on different networks, called subnetworks

Internet routers still “see” this net as 172.28.0.0



Using the 3rd octet, 172.28.0.0 was divided into:

172.28.1.0 172.28.2.0 172.28.3.0 172.28.4.0

172.28.5.0 172.28.6.0 172.28.7.0 172.28.8.0

172.28.9.0 172.28.10.0 172.28.11.0 172.28.12.0

172.28.13.0 172.28.14.0 172.28.15.0 172.28.16.0

172.28.17.0 172.28.18.0 172.28.19.0 and so on ...




What’s happened to the host fields?


8 bits 8 bits

8 bits are now used to represent subnets. Only 8 bits remain for possible hosts.



What’s happened to the host fields?


8 bits 8 bits

8-bit subnet field = 28 subnets = 256 subnets. 8-bit host field = 28 hosts = 256 hosts. Remember, we started with 65,536 hosts!



• Run winipcfg to examine your curriculum PC’s IP address configuration.

• What is your IP address class?• B

The first octet is 172 Class B ranges from 128 - 191.



• The default subnet mask for a class B network is 255.255.0.0

• What subnet mask is assigned to your PC?


255 255 0 0



• The default subnet mask for a class B network is 255.255.0.0

• What subnet mask is assigned to your PC?


255 255 0 0


255 255 255 0



• 172.28.0.0 has been divided into 256 subnets, each with 256 hosts:

– 172.28.1.0– 172.28.2.0– 172.28.3.0– 172.28.4.0– 172.28.5.0– and so on...

(Actually, 256 minus 2. We’ll see why shortly...)


172.28.118.0(Cisco Lab)

172.28.1.0

172.28.2.0

172.28.3.0

172.28.4.0

172.28.5.0

172.28.100.0

172.28.101.0

172.28.102.0

172.28.103.0

172.28.104.0

etc...

etc...

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Chabot Network172.28.0.0Class BSubnetted255.255.255.0

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