Unfortunately, the huge growth of home and business computing was never anticipated. IPv6 has several trillion available addresses which should last a few years into the future.

IP addresses are broken into classes. Classes were used when IP addresses were first invented. Depending upon how large your organization was, depended upon which class of IP address you were given.

IN THE REAL WORLD:
IP addresses are assigned by a group called the INIA. You can also buy then from an ISP who has in turn bought a block form the INIA.

Class A Addresses

These were given to the very largest organisations who would need the most IP addresses since they owned more computers than everyone else.

Class A addresses only use the first octet to identify the network number. The remaining three are left for identifying the hosts on the network.

Network.Host.Host.Host

10.2.5.4

So the network is 10 and 2.5.4 is a host on that network.

IN THE REAL WORLD:
You would pronounce the above IP address as ten dot two dot five dot four. Never say point, period or full stop.

A class A address starts from 1 to 127. Network equipment identifies a class A address because the very first bit on the first octet has to have a 0 in the column. It cannot have a 1 in the column.

So the first network number is 1.



The last possible network number is 127 (check by adding all the values together).

Network number 127 cannot actually be used because the value 127.0.0.1 is reserved for troubleshooting. You can ping the address which will check if TCP/IP is working on your host.



For the hosts we can start at number one until every single possible value is used up.

10.0.0.1 is the first host or in binary:



10.0.0.2 is the second host or in binary:



10.255.255.254 is the last host or in binary:



Now you can see why we use decimal. It would take a long time to write out addresses in binary and be almost impossible to remember them. You can change the router to display all addresses in binary but there is no reason ever to do this.

Why can't we have 10.255.255.255 as a host? Because when all the binary values have a 1 on the host part of the address this tells the network that it is a broadcast.

Class B addresses

Class B addresses were reserved for large organisations who needed a lot of host numbers but not as many as the larger ones. Unfortunately, when a class B address was assigned to an organisation, it still resulted in thousands of wasted host numbers.

Class B addresses have to have the first two binary columns reserved with a 1 and a 0 next to it.

So the first network number is 128.



The last available class B network number is 191 (add the values).



For Class B addresses we use the first two octets for the network address. So for the address 130.24.5.2. 130.24 is the network number and 5.2 is a host on that network. The rule is still that the first number you see though will always be between 128 and 191.

If we use the powers of two rule for the first two octets we will see that we can have a possible 16384 networks. 2^16= 16384

We are not allowed to use the first two bits of the first octet because they are reserved for showing the 10 value remember? So this leaves us with 6+8 digits. 2^14 gives us 16384 networks.

We have the full two octets to use for hosts so 8+8 gives us 2^16 = 65534 hosts per class B network.

Class C Addresses

Theses were originally reserved for any other organization who was not large enough to warrant having a class A or B address.

A class C address has the first three bits reserved so the networking device can recognize it. The first three bits show as 110.

So the first network number is 192.



And the last is 223.



An example of a class C address is 200.2.1.4. 200.2.1 is the network address and .4 is a host on that network. So we can see that there are lots of available network numbers to assign to companies however, we have a limited amount of numbers free to use on our networks.

For networks we have to take the 011 from the first octet giving us 5+8+8= 21.

2^21 = 2097152

For the hosts we have 2^8 giving us 255. We will see in a short while that not every single number can be used because we need to reserve some for a special purpose.

Class D and Class E Addresses

Class D addresses are reserved for multicast traffic and cannot be used on your network. Multicast traffic is traffic send to multiple hosts using one IP address. A live webcast of a rock concert would be an example of multicasting.

Class E addresses are reserved for experimental use only.

Summary

So far we can condense what we know to:

Class A - first bit set to 0. Address range 1-126 (127 is reserved for testing)
Network.Host.Host.Host

Class B - first bits set to 10. Address range 128-191
Network.Network.Host.Host

Class C - first bits set to 110. Address range 192-223
Network.Network.Network.Host

We only need to look at the number in the first octet to recognize which class address we are dealing with.

10.1.2.1 = Class A

190.2.3.4 = Class B

220.3.4.2 = Class C




FIG 4.1 IP Addressing Classes Summarized

Reserved Addresses

In order to help prevent wastage of IP addresses certain addresses were reserved for use on private networks. Any individual can use these addresses on their network providing they do not try to get out to the internet using these addresses.

IN THE REAL WORLD:
In order to get out to the internet a technique known as NAT (network address translation) is used to swap your private address to a public address. This is outside the scope of the CCNA syllabus.

The reserved addresses are:

10.x.x.x - Any IP address beginning with 10
172.16.x.x - 173.31.x.x - Any IP address between 172.16 and 172.31 inclusive
192.168.x.x - Any IP address starting with 192.168

This idea proved to be a great way to avoid having to waste thousands of IP addresses.

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