Topics: IP address, Classless Inter-Domain Routing, IPv4 Pages: 48 (1511 words) Published: March 15, 2015
Network Layer:
Logical Addressing


An IPv4 address is a 32-bit address that uniquely and universally defines the connection of a device (for example, a computer or a router) to the Internet.

Topics discussed in this section:
Address Space
Classful Addressing
Classless Addressing
Network Address Translation (NAT)


An IPv4 address is 32 bits long.

The IPv4 addresses are unique
and universal.
The address space of IPv4 is
or 4,294,967,296.

Figure 4.1 Dotted-decimal notation and binary notation for an IPv4 address

Example 4.1

Change the following IPv4 addresses from binary notation to dotted-decimal notation.

We replace each group of 8 bits with its equivalent decimal number (see Appendix B) and add dots for separation.

Example 4.2

Change the following IPv4 addresses from dotted-decimal notation to binary notation.

We replace each decimal number with its binary equivalent (see Appendix B).

Example 4.3

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

a. There must be no leading zero (045).
b. There can be no more than four group of numbers.
c. Each number needs to be between 0 to 255.
d. A mixture of binary notation and dotted-decimal
notation is not allowed.


In classful addressing, the address space is divided into five classes: A, B, C, D, and E.

Figure 4.2 Finding the classes in binary and dotted-decimal notation

Example 4.4

Find the class of each address.
a. 00000001 00001011 00001011 11101111
b. 11000001 10000011 00011011 11111111
a. The first bit is 0. This is a class A address.
b. The first 2 bits are 1; the third bit is 0. This is a class C address.
c. The first byte is 14; the class is A.
d. The first byte is 252; the class is E.

Table 4.1 Number of blocks and block size in classful IPv4 addressing

In classful addressing, a large part of the available addresses were wasted.

Table4.2 Default Masks for classful

The mask can help us to find out the netid and
The last column in Table 4.2 shows the mask in
the form /n where n can be 8, 16 or 24 in classful
addressing. This notation is also called slash
notation or Classless InterDomain Routing (CIDR)

Classful addressing, which is almost obsolete, is replaced with classless addressing.

Example 4.5

Figure 4.3 shows a block of addresses, in both binary and dotted-decimal notation, granted to a small business that needs 16 addresses.

We can see that the restrictions are applied to this block. The addresses are contiguous. The number 4
of addresses is a power of 2 (16 = 2 ), and the first address is divisible by 16. The first address, when converted to a decimal number is 3,440,387,360 which when divided by 16 results in 215,024,210.

Figure 4.3 A block of 16 addresses granted to a small organization

In IPv4 addressing, a block of
addresses can be defined as
x.y.z.t /n
in which x.y.z.t defines one of the addresses and the /n defines the mask.

The first address in the block can be found by setting the rightmost 32 − n bits to 0s.

Example 4.6

A block of addresses is granted to a small organization. We know that one of the addresses is What is the first address in the block?

The binary representation of the given address is
11001101 00010000 00100101 00100111
If we set 32−28 rightmost bits to 0, we get


00100101 00100000
This is actually the block shown in Figure 4.3.


The last address in the block can be found by setting the rightmost 32 − n bits to 1s

Example 4.7

Find the last address for the block in Example 4.6.

The binary representation of the given address is




If we set 32 − 28 rightmost bits to 1, we get
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