IPv4 ADDRESSES

  IPv4 ADDRESSES-

 An IPv4addressis a 32-bitaddress that uniquely and universally defines the connection of a device (for example, a computer or a router) to the Internet.  IPv4 addresses are unique. They are unique in the sense that each address defines one, and only one, connection to the Internet. Two devices on the Internet can never have the same address at the same time. We will see later that, by using some strategies, an address may be assigned to a device for a time period and then taken away and assigned to another device.

Address Space -

A protocol such as IPv4 that defines addresses has an address space. An address space is the total number of addresses used by the protocol. Ifa protocol uses N bits to define an address, the address space is 2N because each bit can have two different values (0 or 1) and N bits can have 2N values. IPv4 uses 32-bit addresses, which means that the address space is 232 or 4,294,967,296 (more than 4 billion). This means that, theoretically, if there were no restrictions, more than 4 billion devices could be connected to the Internet. We will see shortly that the actual number is much less because of the restrictions imposed on the addresses. 

Notations -

There are two prevalent notations to show an IPv4 address: binary notation and dotted decimal notation. 

 Binary Notation -

In binary notation, the IPv4 address is displayed as 32 bits. Each octet is often referred to as a byte. So it is common to hear an IPv4 address referred to as a 32-bit address or a 4-byte address. 

The following is an example of an IPv4 address in binary notation: 

 01110101 10010101 00011101 00000010 

 Dotted-Decimal Notation- 

 To make the IPv4 address more compact and easier to read, Internet addresses are usually written in decimal form with a decimal point (dot) separating the bytes. The following is the dotted~decimal notation of the above address: 

 117.149.29.2

 Classful Addressing-

IPv4 addressing, at its inception, used the concept of classes. This architecture is called classful addressing. Although this scheme is becoming obsolete, we briefly discuss it here to show the rationale behind classless addressing. In classful addressing, the address space is divided into five classes: A, B, C, D, and E. Each class occupies some part of the address space.

 We can find the class of an address when given the address in binary notation or dotted-decimal notation. If the address is given in binary notation, the first few bits can immediately tell us the class of the address. If the address is given in decimal-dotted notation, the first byte defines the class. Both methods are shown in Figure 19.2

 Netid and Hostid -

 In classful addressing, an IP address in class A, B, or C is divided into netid and hostid. These parts are of varying lengths, depending on the class of the address. Figure 19.2 shows some netid and hostid bytes. The netid is in color, the hostid is in white. Note that the concept does not apply to classes D and E. In class A, one byte defines the netid and three bytes define the hostid. In class B, two bytes define the netid and two bytes define the hostid. In class C, three bytes define the netid and one byte defines the hostid.

 Mask-

  Although the length of the netid and hostid (in bits) is predetermined in classful addressing, we can also use a mask (also called the default mask), a 32-bit number made of contiguous Is followed by contiguous as. The masks for classes A, B, and C are shown in Table 19.2. The concept does not apply to classes D and E. 

 The mask can help us to find the netid and the hostid. For example, the mask for a class A address has eight 1s, which means the first 8 bits of any address in class A define the netid; the next 24 bits define the hostid. 

The last column ofTable 19.2 shows the mask in the form In where n can be 8, 16, or 24 in classful addressing. This notation is also called slash notation or Classless Interdomain Routing (CIDR) notation. The notation is used in classless addressing, which we will discuss later. We introduce it here because it can also be applied to class ful addressing. We will show later that classful addressing is a special case of classless addressing.

 Mask -

 A better way to define a block of addresses is to select any address in the block and the mask. As we discussed before, a mask is a 32-bit number in which the n leftmost bits are Is and the 32- n rightmost bits are Os. However, in classless addressing the mask for a block can take any value from 0 to 32. It is very convenient to give just the value of n preceded by a slash (CIDR notation).

 The address and the In notation completely define the whole block (the first address, the last address, and the number of addresses).

First Address -The first address in the block can be found by setting the 32- n right most bits in the binary notation of the address to Os.

 The first address in the block can be found by setting the rightmost 32- n bits to Os.

Last Address -The last address in the block can be found by setting the 32- n right most bits in the binary notation of the address to Is.

 The last address in the block can be found by setting the rightmost32- n bits to Is.

Number of Addresses- The number of addresses in the block is the difference between the last and first address. It can easily be found using the formula 2 32-n .