Datasheet

“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 418 — #444

21.1.2 IP Addresses and Routing

Note

The discussion in the following sections is limited to IPv4 net-

works. For information about IPv6 protocol, the successor to

IPv4, refer to Section 21.2 on page 422.

Note

IP Addresses

Every computer on the Internet has a unique 32-bit address. These 32 bits

(or 4 bytes) are normally written as illustrated in the second row in Ta-

ble 21.1.

Example 21.1: How an IP Address is Written

IP Address (binary): 11000000 10101000 00000000 00010100

IP Address (decimal): 192. 168. 0. 20

In decimal form, the four bytes are written in the decimal number system,

separated by periods. The IP address is assigned to a host or a network in-

terface. It cannot be used anywhere else in the world. There are certainly

exceptions to this rule, but these play a minimal role in the following pas-

sages.

The ethernet card itself has its own unique address, the MAC, or media ac-

cess control address. It is 48 bits long, internationally unique, and is pro-

grammed into the hardware by the network card vendor. There is, how-

ever, an unfortunate disadvantage of vendor-assigned addresses — MAC

addresses do not make up a hierarchical system, but are instead more or

less randomly distributed. Therefore, they cannot be used for addressing

remote machines. The MAC address still plays an important role in com-

munication between hosts in a local network and is the main component of

the protocol header of layer 2.

The points in IP addresses indicate the hierarchical system. Until the 1990s,

IP addresses were strictly categorized in classes. However, this system has

proven too inﬂexible so was discontinued. Now, classless routing (CIDR,

classless interdomain routing) is used.

418

21.1. TCP/IP — The Protocol Used by Linux