A

20 Bytes

Bit 0

Bit 15 Bit 16

Bit 31

Version (4)

Header Length (4)

Priority & Type of Service (8)

Total Length (16)

Identification (16)

Fragment Offset (13)

Protocol (8)

Header Checksum (16)

Source IP Address (32)

Destination IP Address (32)

IP Options (0 Or 32 If Any)

Data (Varies If Any)

Note that each IP datagram carries this header, which includes a source IP address and destination IP address that identify the source and destination network and host.

An IP address is a hierarchical address, and it consists of two parts:

■ The high order, or leftmost, bits specify the network address component (network ID) of the address.

■ The low order, or rightmost, bits specify the host address component (host ID) of the address.

Every physical or virtual LAN on the corporate internetwork is seen as a single network that must be reached before an individual host within that company can be contacted. Each LAN has a unique network address. The hosts that populate that network share those same bits, but each host is identified by the uniqueness of the remaining bits. Like a group of houses along the same road, the street address is the same, but the house number is unique.

Figure 1-27 illustrates a sample IP addressing scheme in an internetwork. Figure 1-27 IP Addressing

Figure 1-27 illustrates a sample IP addressing scheme in an internetwork. Figure 1-27 IP Addressing

The IP address is 32 bits in length and is binary in nature, but it is expressed in a format that can be easily understood by the human brain. Basically, the 32 bits are broken into 4 sections of 8 bits each, known as octets or bytes. Each of these octets is then converted into decimal numbers between 0 and 255, and each octet is separated from the following one by dots. Figure 1-28 illustrates the format of an IP address using 172.16.122.204 as an example.

Figure 1-28 IP Address Format

Dotted Decimal

Maximum

Binary

Example Decimal

Example Binary

The IP address format is known as dotted decimal notation. Figure 1-28 shows how the dotted decimal address is derived from the 32-bit binary value:

■ Each bit in the octet has a binary weight (such as 128, 64, 32, 16, 8, 4, 2, and 1), and when all the bits are on, the sum is 255.

■ The minimum decimal value for an octet is 0; it contains all 0s.

■ The maximum decimal value for an octet is 255; it contains all 1s.

While many computers might share the same network address, combining the network address with a host address uniquely identifies any device connected to the network.

IP Address Classes

When IP was first developed, no classes of addresses existed, because it was assumed that 254 networks would be more than enough for an internetwork of academic, military, and research computers.

As the number of networks grew, the IP addresses were broken into categories called classes to accommodate different sizes of networks and to aid in identifying them. These classes are illustrated in Figure 1-29.

Assigning IP addresses to classes is known as classful addressing. The allocation of addresses is managed by a central authority, the American Registry for Internet Numbers (ARIN), which you can go to at http://www.arin.net for more information about network numbers.

Binary

Network

Host

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