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Chapter 2

• Router computers act as receive-and-forward devices; they do not retain
information about the packets that they handle.
70 • No global control exists over the network.

The open architecture approach has contributed to the success of the Internet
because computers manufactured by different companies (Apple, Dell, Hewlett-Packard,
and so on) can be interconnected. The ARPANET and its successor, the Internet, use
routers to isolate each LAN or WAN from the other networks to which they are
connected. Each LAN or WAN can use its own set of protocols for packet traffic within the
LAN or WAN, but must use a router (or similar device) to move packets onto the Internet
in its standard format (or protocol). Following these simple rules makes the connections
between the interconnected networks operate effectively.

TCP/IP
The Internet uses two main protocols: the Transmission Control Protocol (TCP) and the
Internet Protocol (IP). Developed by Internet pioneers Vinton Cerf and Robert Kahn,
these protocols are the rules that govern how data moves through the Internet and how
network connections are established and terminated. The acronym TCP/IP is commonly
used to refer to the two protocols. TCP/IP was technologically superior to other protocols
available at the time (such as the NCP). Once its use became pervasive on the Internet,
the network effects, which you learned about in Chapter 1, have prevented any other
protocol from challenging TCP/IP.
The TCP controls the disassembly of a message or a file into packets before it is
transmitted over the Internet, and it controls the reassembly of those packets into their
original formats when they reach their destinations. The IP specifies the addressing details
for each packet, labeling each with the packet’s origination and destination addresses.
Soon after the new TCP/IP protocol set was developed, it replaced the NCP that ARPANET
originally used.
In addition to its Internet function, TCP/IP is used today in many LANs. The TCP/IP
protocol is provided in most personal computer operating systems commonly used today,
including Linux, Macintosh, Microsoft Windows, and UNIX.

IP Addressing
The version of IP that has been in use since 1981 on the Internet is Internet Protocol
version 4 (IPv4). It uses a 32-bit number to identify computers connected to the Internet.
This address is called an IP address. Computers do all of their internal calculations using
a base 2 (or binary) number system in which each digit is either a 0 or a 1, corresponding
to a condition of either off or on. IPv4 uses a 32-bit binary number that allows for more
than 4 billion different addresses (2 32 = 4,294,967,296).
When a router breaks a message into packets before sending it onto the Internet, the
router marks each packet with both the source IP address and the destination IP address
of the message. To make them easier to read, IP numbers (addresses) appear as four
numbers separated by periods. This notation system is called dotted decimal notation.
An IPv4 address is a 32-bit number, so each of the four numbers is an 8-bit number
(4 × 8 = 32). In most computer applications, an 8-bit number is called a byte; however,

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