Internet Technology Unit-II

Internet Technology Unit-II

Unit-II: IP Addressing

IP Addressing Scheme - Class A, Class B, and Class C Addressing Scheme -Subnetting - Custom Subnetting - IPV4 Header

Addressing

Four levels of addresses are used in an internet employing the TCP/IP protocols. Each address is related to a specific layer in the TCP/IP architecture.

Sr.
No. / Type of Address / TCP/IP Layer
1 / Physical (link) addresses / Physical Layer
2 / Logical (IP) addresses / Network Layer
3 / Port addresses / Transport Layer
4 / Specific addresses / Application Layer

Physical Addresses

The physical address, also known as the link address, is the address of a node as defined by its LAN or WAN. It is included in the frame used by the data link layer. It is the lowest-level address.

The size and format of physical addresses vary depending on the network (LAN or WAN). For example, Ethernet uses a 6-byte (48-bit) physical address that is imprinted on the network interface card (NIC). LocalTalk (Apple), however, has a 1-byte dynamic address that changes each time the station comes up.

Most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below:

07:01:02:01:2C:4B

A 6-byte (12 hexadecimal digits) physical address

Logical Addresses

Logical addresses are necessary for universal communications that are independent of underlying physical networks. Physical addresses are not adequate in an internetwork environment where different networks can have different address formats. A universal addressing system is needed in which each host can be identified uniquely, regardless of the underlying physical network.

The logical addresses are designed for this purpose. A logical address in the Internet is currently a 32-bit address that can uniquely define a host connected to the Internet. No two publicly addressed and visible hosts on the Internet can have the same IP address.

The physical addresses will change from hop to hop, but the logical addresses usually remain the same.

Port Addresses

The IP address and the physical address are necessary for a quantity of data to travel from a source to the destination host.But, today’s computers can run multiple processes at the same time. The end objective of Internet communication is a process communicating with another process. For example, computer A can communicate with computer C by using TELNET. At the same time, computer A communicates with computer B by using the File Transfer Protocol (FTP). For these processes to receive data simultaneously, we need a method to label the different processes. In other words, they need addresses. In the TCP/IP architecture, the label assigned to a process is called a port address. A port address in TCP/IP is 16 bits in length.

The physical addresses change from hop to hop, but the logical and port addresses usually remain the same.

A port address is a 16-bit address represented by one decimal number as shown below.

753

A 16-bit port address represented as one single number

Specific Addresses

Some applications have user-friendly addresses that are designed for that specific address. Examples include the e-mail address (for example, ) and the Universal Resource Locator (URL) (for example, The first defines the recipient of an e-mail; the second is used to find a document on the World Wide Web. These addresses, however, get changed to the corresponding port and logical addresses by the sending computer.

Internet Address

Communication at the network layer is host-to-host (computer-to-computer); a computer somewhere in the world needs to communicate with another computer somewhere else in the world. Usually, computers communicate through the Internet. The packet transmitted by the sending computer may pass through several LANs or WANs before reaching the destination computer.

For this level of communication, we need a global addressing scheme; called logical addressing. Today, we use the term IP address to mean a logical address in the network layer of the TCP/IP protocol suite.

The Internet addresses are 32 bits in length; this gives us a maximum of 232 addresses. These addresses are referred to as IPv4 (IP version 4) addresses or simply IP addresses.

The need for more addresses, in addition to other concerns about the IP layer, motivated a new design of the IP layer called the new generation of IP or IPv6 (IP version 6). In this version, the Internet uses 128-bit addresses that give much greater flexibility in address allocation. These addresses are referred to as IPv6 (IP version 6) addresses.

IPv4 addressesare currently being used in the Internet. The IPv6 addressesmay become dominant in the future.

IPv4 ADDRESSES

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.

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. However, 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.

If a device operating at the network layer has m connections to the Internet, it needs to have m addresses. For example, a router is such a device.

The IPv4 addresses are universal in the sense that the addressing system must be accepted by any host that wants to be connected to the Internet.

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. If a protocol uses N bits to define an address, the address space is 2Nbecause each bit can have two different values (0 or 1) and N bits can have 2Nvalues.

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.

The address space of IPv4 is 232or 4,294,967,296

Notations

There are two prevalent notations to show an IPv4 address:

1. Binary notation and
2. Dotted decimal notation.

Binary Notation

In binary notation, the IPv4 address is displayed as 32 bits. Each octet (collection of 8 bits) 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

Figure 19.1 shows an IPv4 address in both binary and dotted-decimal notation. Note that because each byte (octet) is 8 bits, each number in dotted-decimal notation is a value ranging from 0 to 255.

Classful Addressing

IPv4 addressing, at its inception, used the concept of classes. This architecture is called classful addressing. (Note that this scheme is becoming obsolete and replaced with 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. Class A consumes 50%, class B consumes 25%, class C consumes 12.5%, and Class D & E consumes 12.5%.

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 following Figure.

/ Class A: 0 to 127(0) (50% of 256)
Class B: 128 to 191(0+128) (25%)
Class C: 192 to 223(128+64) (12.5%)
Class D: 224 to 239(192+32) (6.25%)
Class E: 240 to 255(224+16) (6.25%)

Example 19.4

Find the class of each address.

a. 00000001 00001011 00001011 11101111

b. 11000001 10000011 00011011 11111111

c. 14.23.120.8

d. 252.5.15.111

Solution

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 (between 0 and 127); the class is A.

d. The first byte is 252 (between 240 and 255); the class is E.

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. The Figure below shows some netid and hostid bytes. 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.

Classes and Blocks

One problem with classful addressing is that each class is divided into a fixed numberof blocks with each block having a fixed size. Let us look at each class.

Class A

Since only 1 byte in class A defines the netid and the leftmost bit should be 0, the next7 bits can be changed to find the number of blocks in this class. Therefore, class A isdivided into 27 = 128 blocks that can be assigned to 128 organizations (the number isless because some blocks were reserved as special blocks, e.g., 10.x.x.x). However, each block in thisclass contains 16,777,216 addresses, which means the organization should be a reallylarge one to use all these addresses. Many addresses are wasted in this class. Figure 5.9 shows the block in class A.

Class B

Since 2 bytes in class B define the class and the two leftmost bit should be 10 (fixed), the next 14 bits can be changed to find the number of blocks in this class. Therefore, class B is divided into 214 = 16,384 blocks that can be assigned to 16,384 organizations (the number is less because some blocks were reserved as special blocks). However, each block in this class contains 65,536 addresses. Not so many organizations can use so many addresses. Many addresses are wasted in this class. Figure 5.10 shows the blocks in class B.

Class C

Since 3 bytes in class C define the class and the three leftmost bits should be 110 (fixed), the next 21 bits can be changed to find the number of blocks in this class. Therefore, class C is divided into 221= 2,097,152 blocks, in which each block contains 256 addresses that can be assigned to 2,097,152 organizations (the number is less because some blocks were reserved as special blocks). Each block contains 256 addresses. However, not so many organizations were as small as to be satisfied with a class C block. Figure 5.11 shows the blocks in class C.

Class D

There is just one block of class D addresses. It is designed for multicasting. Each address in this class is used to define one group of hosts on the Internet. When a group is assigned an address in this class, every host that is a member of this group will have a multicast address in addition to its normal (unicast) address. Figure 5.12 shows the block.

Class E

There is just one block of class E addresses. It was designed for use as reserved addresses, as shown in Figure 5.13.

The Classes and Blocks in classful addressing are summarized in Table 19.1.

Class / Net Id Bytes / No. of Blocks/Networks / Host Id Bytes / No. of Hosts
A / 1 (128) / 128 / 3 / 256*256*256=1,67,77,216
B / 2 (64) / 64 * 256=16,384 / 2 / 256*256=65,536
C / 3 (32) / 32*256*256=20,97,152 / 1 / 256
D / - (16) / 1 / - / 16*256*256*256=26,84,35,456
E / - (16) / 1 / - / 16*256*256*256=26,84,35,456

Let us examine the table. Previously, when an organization requested a block of addresses, it was granted one in class A, B, or C. Class A addresses were designed for large organizations with a large number of attached hosts or routers. Class B addresses were designed for midsize organizations with tens of thousands of attached hosts or routers. Class C addresses were designed for small organizations with a small number of attached hosts or routers.

We can see the flaw in this design. A block in class A address is too large for almost any organization. This means most of the addresses in class A were wasted and were not used. A block in class B is also very large, probably too large for many of the organizations that received a class B block. A block in class C is probably too small for many organizations. Class D addresses were designed for multicasting as we will see in a later chapter. Each address in this class is used to define one group of hosts on the Internet. The Internet authorities wrongly predicted a need for 268,435,456 groups. This never happened and many addresses were wasted here too. And lastly, the class E addresses were reserved for future use; only a few were used, resulting in another waste of addresses.

Thus, in c1assfnl addressing, a large part of the available addresses were wasted.

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 1s followed by contiguous 0s. 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 of Table 19.2 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) notation. The notation is used in classless addressing, normally. But, it can also be applied to classful addressing as classful addressing is a special case of classless addressing.

Finding Addresses using Mask

To be added for more information (Please read from Book)

Subnetting

During the era of classful addressing, subnetting was introduced. If an organization was granted a large block in class A or B, it could divide the addresses into several contiguous groups and assign each group to smaller networks (called subnets) or, in rare cases, share part of the addresses with neighbors. Subnetting increases the number of 1s in the mask.

Two-Level Addressing

The whole purpose of IPv4 addressing is to define a destination for an Internet packet (at the network layer). When classful addressing was designed, it was assumed that the whole Internet is divided into many networks and each network connects many hosts. In other words, the Internet was seen as a network of networks. A network was normally created by an organization that wanted to be connected to the Internet. The Internet authorities allocated a block of addresses to the organization (in class A, B, or C).

Since all addresses in a network belonged to a single block, each address in classful addressing contains two parts: netid and hostid. The netid defines the network; the hostid defines a particular host connected to that network. Figure 5.14 shows an IPv4 address in classful addressing. If n bits in the class defines the net, then 32 −n bits defines the host. However, the value of n depends on the class the block belongs to. The value of n can be 8, 16 or 24 corresponding to classes A, B, and C respectively.

Two-Level Hierarchy: No Subnetting

An IP address can define only two levels of hierarchy when not subnetted. The n leftmost bits of the address x.y.z.t/n define the network (organization network); the 32 - n rightmost bits define the particular host (computer or router) to the network. The two common terms are prefix and suffix. The part of the address that defines the network is called the prefix; the part that defines the host is called the suffix. Figure 19.6 shows the hierarchical structure of an IPv4 address.

The prefix is common to all addresses in the network; the suffix changes from one device to another.

Three-Levels of Hierarchy: Subnetting

An organization that is granted a large block of addresses may want to create clusters of networks (called subnets) and divide the addresses between the different subnets. The rest of the world still sees the organization as one entity; however, internally there are several subnets. All messages are sent to the router address that connects the organization to the rest of the Internet; the router routes the message to the appropriate subnets. The organization, however, needs to create small subblocks of addresses, each assigned to specific subnets. The organization has its own mask; each subnet must also have its own.

As an example, suppose an organization is given the block 17.12.14.0/26, which contains 64 addresses (2(32-26) = 64). The organization has three offices and needs to divide the addresses into three subblocks of 32, 16, and 16 addresses. We can find the new masks by using the following arguments:

1. Suppose the mask for the first subnet is n1, then 232- n1 must be 32, which means that n1 =27.
2. Suppose the mask for the second subnet is n2, then 232- n2 must be 16, which means that n2 = 28.
3. Suppose the mask for the third subnet is n3, then 232- n3 must be 16, which means that n3 =28.

This means that we have the masks 27, 28, 28 with the organization mask being 26.

Figure 19.7 shows one configuration for the above scenario.

More Levels of Hierarchy

The structure of classless addressing does not restrict the number of hierarchical levels. An organization can divide the granted block of addresses into subblocks. Each subblock can in turn be divided into smaller subblocks, and so on. One example of this is seen in the ISPs. A national ISP can divide a granted large block into smaller blocks and assign each of them to a regional ISP. A regional ISP can divide the block received from the national ISP into smaller blocks and assign each one to a local ISP. A local ISP can divide the block received from the regional ISP into smaller blocks and assign each one to a different organization. Finally, an organization can divide the received block and make several subnets out of it.