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Lesson 02
WHAT IS A NETWORK
A network can be anything from a simple collection of computers at one location connected through a
connectivity media to the internet (a global network of networks). Local Area Network (LAN) is a server-
based network confined to a particular area/place. Most LANs consist of many clients and a few servers.
Fig. 1 below shows a simple LAN setup:
Fig. 1
Why networking your computer
We network our computers to share resources and communicate. We can do networking for:
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File sharing
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Hardware sharing ­ printer sharing, for example
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Program sharing
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User communication through a machine called e-mail server
Network protocol
Network protocols are those standard rules using which computers on a network communicate and
exchange data with each other. A group of protocols that prepare the data for communication on the
network is called the Protocol stack.
International organization for standard's (ISO) model
In 1970's came ISO's OSI model ­ a conceptual model for network communications. OSI stands for Open
System Interconnection Reference Model and it proposes a 7 layer architecture. Each layer (except physical
layer) at the sending machine sends instructions through its header to the receiving machine as to how the
accompanying data be interpreted or treated by the receiving machine. Header is a piece of information
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which is attached to the data at its beginning by each layer except the physical layer. The process of moving
the data down the OSI Protocol stack at the sending machine is called Encapsulation, and the process of
moving the data up the OSI stack at the receiving side is called De-encapsulation. Fig. 2 below shows names
of the 7 layers of the OSI model which are numbered from bottom up.
Fig. 2
Application layer
It sits at top of the OSI model. Requests related to file transfer and database queries are handled by this
layer. Two very important protocols, namely, HTTP and FTP (file transfer protocol) operate at this layer.
Presentation layer
It is the translator of the OSI model. It provides instructions through its header that how the accompanying
data should be formatted by the receiving machine. MIME-multipurpose internet mail extensions protocol
operates at this layer to define file formats and data types
Session layer
It provides instructions about the nature of communication link between the sending and receiving machine
during a session. A combination of protocols called Session Protocol Data Units work at this layer. Three
modes of communication are simplex, half-duplex and full-duplex. Simplex means communication in one
direction only. Half-duplex means communication in two directions but one party can send data at a time.
Full-duplex means communication in two directions while both parties are able to send data simultaneously.
It also places special checkpoints on data packets to trace any lost packets.
Transport layer
TCP (Transmission Control Protocol) or UDP (User Datagram Protocol) operate at this layer. It has two
functions. It converts the data into data packets. Secondly, it is responsible for flow control of data. TCP is
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more reliable as it is acknowledgment based as opposed to UDP which does not use any system of
acknowledgment for the delivery of data packets.
Network layer
It is responsible for providing IP addresses on data packets using IP protocol. Routing Information
Protocol (RIP) also operates here which enables routers to build their routing table. Another protocol,
Address Resolution Protocol (ARP) is also designed to operate at network layer.
Data link layer
It places data packets into data frames. Network Interface Card Drivers/Protocols operate at this layer. It is
used to identify MAC or hardware address of computer machines. A mathematical calculation, Cyclical
Redundancy Check (CRC), takes place here to confirm integrity of data frames.
Physical layer
It has got no header. All information including the header information is converted into binary data at this
layer. It results into the generation of electric signals as 1s and 0s are queued up and travel along the
connectivity media to the receiving side.
Internet layers- TCP/IP stack
Internet communication model (TCP/IP Stack) proposes a 4 layer architecture. Following are the names of
internet layers that map with the OSI model's layers:
Application layer is equivalent to OSI model's Application, Presentation and Session layers
Host-to-Host Layer is equivalent to OSI model's Transport layer
Internet layer is equivalent to OSI model's Network layer
Network Access layer is equivalent to OSI model's data link layer
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This mapping of layers is also shown in Fig. 3 below:
Fig. 3
In the following table (Fig. 4) there is a list of important protocols that operate at the TCP/IP stack.
Fig. 4
IP addressing
It is the logical addressing scheme used to identify computer machines on the internet. Each computer has a
unique IP address provided by IP protocol for a given session. It is represented by four decimal numbers
separated by a period e.g, 140.57.220.200 (see Fig. 5 below). Computers see this information as a stream of
32 bits.
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Fig. 5
You can look at figures 6 to 8 below to revise the basic concept of conversion of decimal numbers into
binary and from binary into decimal.
Fig. 6
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Fig. 7
Fig. 8
Classes of IP address
There are five classes of IP addresses, namely, A, B, C, D and E. Classes A, B and C are for general public
use, whereas Classes D and E are used by people belonging to certain special groups. To find which class a
particular IP address belongs to, the rule is to look at the number in the first byte. If this number falls
within the calculated range of a specific class, then we can say that this IP address belongs to such particular
class.
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Fig. 9 below explains the logic how we can calculate these ranges:
Fig. 9
By rule, in class A networks first byte would refer to the network address and the remaining three bytes to
the host machines on that network providing a very large network. In class B, the first two bytes indicate
the network address and the last two bytes indicate the host machines on that network giving rise to a
medium-sized network. In class C, first three bytes refer to the network address and the last byte to the host
machines on that network, which would therefore be a very small network. (See Fig. 10 below)
Fig. 10
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