IP was created as a way to hide the complexity of physical addressing by creating a virtual addressing scheme that is independent of the underlying network. IP does not ensure that data is delivered to the
application in the appropriate order; that responsibility is left to upper-layer protocols such as TCP and UDP.

An IP address is 32 bits long. The bits can be broken down into four bytes. Each byte is expressed in decimal form and separated from other bytes by a dot (that is, x.x.x.x). This is called dotted-decimal format. Each bit within a byte carries a binary weight (starting from left to right) of 128, 64, 32, 16, 8, 4, 2, 1. If you add up these values, you get a range of 0-255 for each byte.

 For example, one byte can be translated from binary format to decimal format as follows: 

 IP addressing has been broken down into five separate classes based on the number of maximum hosts required by the network.

IP Address Classes

                                                       8                             16                                24                                    32

You can see from Figure 8.2 that each address class contains a network portion and a host portion. The network portion identifies the data link that is in common with all the devices attached to that network. The host portion uniquely identifies an end device connected to the network.

 Table IP Address Classes 

                         Class            Decimal Value                      Purpose                                      Max, Hosts
                                                of First Byte

Class A        0-127                                     Large organizations                     16,777,214

Class B        128-191                                Medium-sized Organizations            65,543

Class C        192-223                                Small organizations                            254

Class D        224-247                                Multicast addresses                           n/a

Class E        248-255                                Experimental                                        n/a  Private IP Addresses

Starting Address                      Ending Address            

10.0.0.0                                        10.255.255.255

172.16.0.0                                   172.31.255.255

192.168.0.0                                 192.168.255.255 

Address Masks

The network mask is used in conjunction with an IP address to delineate the network portion of an IP address from the host portion. Each major network address within its designated class has a standard network mask: 

Address Class                           Network Mask

Class A                                        255.0.0.0

Class B                                        255.255.0.0

Class C                                       255.255.255.0

A major network address can be further divided into smaller networks by using a technique called subnetting. When a major network is subnetted, the address can be broken into three parts: 

ü        _ The network portion

ü        _ The subnet portion

ü        _ The host portion 

When a network mask is varied into further subnets like this, it is commonly referred to as a Variable Length Subnet Mask (VLSM). 

Cisco often represents the subnet mask by identifying the number of bits used as the mask. For example, 192.174.10.0/30 would represent network 192.174.10.0 255.255.255.252. The value of 30 represents the number of bits used for the network portion of the address; in binary format, 30 would be 

255.255.255.252 = 11111111.11111111.11111111.11111100 = 30 

Let's look at another example. Given the following 170.130.0.0/21, what is the subnet mask? 

21 = 11111111.11111111.11111100.00000000 

The network address and mask are 

170.130.0.0 255.255.248.0. 

Understanding how to derive the network address and broadcast address when given an IP address and mask is critical to passing the CCIE Written Exam. Let's say that we want to determine the network address, the broadcast address, and the available addresses that

correspond with the given IP address: 

150.34.74.53 255.255.240.0 

1. Convert the IP address and its address mask into binary format. 

150.34.74.53 =           10010110             00100010             01001010             00110101

255.255.240.0 =         11111111             11111111            11110000             00000000 

2. Perform a logical AND between the IP address and the mask.

A logical AND is a digital math operation that compares two bits of data to each other. The result of the operation is as follows: 

0 and 0 = 0

0 and 1 = 0

1 and 0 = 0

1 and 1 = 1

 So, 

Host Address:                      10010110             00100010            01001010             00110101

Mask:                                     11111111             11111111            11110000            00000000

Logical AND Result:           10010110             00100010             01000000             00000000 

3. Convert the results of the logical AND back into decimal format; this is the network address: 
10010110             00100010             01000000             00000000             =             150.34.64.0 

4. Calculate the broadcast address.

Remember that the network mask is used to delineate the network portion of an IP address from the host portion. Mask bits are set to 1 if the corresponding bit in the IP address should be considered part of the network address and 0 if part of the host address. 

150.34.74.53 =     10010110             00100010            0100       1010       00110101

255.255.240.0 =   11111111             11111111            1111       0000       00000000

Network Bits                                            Host Bits 

To determine the broadcast address, we need to replace each bit available within the host portion of the IP address with a value of 1. 

So, the broadcast address of the network for the host 150.34.74.53 is

150.34.79.255 = 10010110             00100010             0100      1111       11111111

Network Bits                                             Host Bits 

Summary: 

Given the IP address and address mask: 150.34.74.53 255.255.240.0, we have determined the following: 

Network Address = 150.34.64.0

Broadcast Address = 150.34.79.255

Available Addresses = 150.34.64.1-150.34.79.254 (for a total of 4,078 hosts)