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138 Part 1 Introduction

In addition to the transmission media, components of the network are also required to
direct or route the packets or messages via the most efficient route. On the Internet these are
referred to as ‘routers’ or ‘hubs’, and are manufactured by companies such as Cisco and 3Com.
The routers are the equivalent of postal sorting offices which decide the best route for mail to
take. They do not plan the entire route of the message, but rather they direct it to the next
router that seems most appropriate given the destination and current network traffic.
Some addressing information goes at the beginning of your message; this information gives
IP address the network enough information to deliver the packet of data. The IP address of a receiving
The unique numerical server is usually in the form 207.68.156.58 (as shown in Figure 3.8) which is a numerical repre-
address of a computer.
sentation of a better-known form such as www.microsoft.com. Each IP address is unique to a
given organization, server or client, in a similar way to postal codes referring to a small
number of houses. The first number refers to the top-level domain in the network, in this case
.com. The remaining numbers are used to refer to a particular organization.
Once the Internet message is addressed, the postal analogy is not so apt since related
information is not sent across the Internet in one large message. For reasons of efficiency,
Packet information sent across IP networks is broken up into separate parts called packets. The
Each Internet message information within a packet is usually between 1 and 1,500 characters long. This helps to
such as an e-mail or
HTTP request is broken route information most efficiently and fairly with different packets sent by different people
down into smaller parts gaining equal priority. The transmission control protocol TCP performs the task of splitting
for ease of transmission.
up the original message into packets on dispatch and reassembling it on receipt. Combining
TCP and IP, you can think of an addressed IP envelope containing a TCP envelope which in
turn contains part of the original message that has been split into a packet (Figure 3.13).

The HTTP protocol

HTTP (Hypertext HTTP, the Hypertext Transfer Protocol is the standard used to allow web browsers and
Transfer Protocol) servers to transfer requests for delivery of web pages and their embedded graphics. When
HTTP is a standard
which defines the way you click on a link while viewing a web site, your web browser will request information from
information is transmitted the server computer hosting the web site using HTTP. Since this protocol is important for
across the Internet delivering the web pages, the letters http:// are used to prefix all web addresses. HTTP
between web browsers
and web servers. messages are divided into HTTP ‘get’ messages for requesting and web page and HTTP

or web page)
data data Data (part of e-mail

TCP packet breaks up
Bytes 1 to 500 and recombines data
into/from packets
TCP Packet

From: 192.112.36.5 IP packet (addressing
To: 128.174.5.6 information)
IP Packet

Figure 3.13 The TCP/IP protocol