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| 2.1 |  | IPv4 Addressing |
| 2.1.1 |  | Address architecture of the Internet |

When TCP/IP was first introduced in the 1980s, it relied on a two level addressing scheme. At the time, this scheme offered adequate scalability. The 32-bit long IPv4 address identifies a network number and a host number.

Together, the network number and the host number uniquely identify all hosts connected by way of the Internet. It is possible that the needs of a small networked community, such as a LAN, could be satisfied with just host addresses. However, network addresses are necessary for end systems on different networks to communicate with each other. Routers use the network portion of the address to make routing decisions and facilitate communication between hosts that belong to different networks.

Unlike routers, humans find working with strings of 32 ones and zeros tedious and clumsy. Therefore, 32-bit IP addresses are written using dotted decimal notation. Each 32-bit address is divided into four groups of eight, called octets. Each octet is converted to decimal and then separated by decimal points, or dots.

In the dotted decimal address, 172.30.128.17, which of these four numbers represents the network portion of the address? Which numbers are the host numbers? Finding the answers to these questions is complicated by the fact that IP addresses are not really four numbers. They actually consist of 32 different numbers or 32 bits.

In the early days of TCP/IP, a class system was used to define the network and host portions of the address. IPv4 addresses were grouped into five distinct classes. This was done according to the value of the first few bits in the first octet of the address. Although the class system can still be applied to IP addresses, networks today often ignore the rules of class in favor of a classless IP scheme.

In the next few sections, all of the following topics related to IP addressing will be covered:

- The limitations of the IP address classes
- The subsequent addition of the subnet mask
- The addressing crisis that led to the adoption of a classless system

| Web Links IP Addressing and Subnetting for New Users http://www.cisco.com/warp/public/701/3.pdf