Computer Networks I
COS2626
Module 1: Introduction to Networking
Loosely defined, a network is a group of computers and other devices (such as printers) that are
connected by some type of transmission media.
Network Models
A topology describes how the parts of a whole work together. When studying networking, you need to
understand both the physical topology and the logical topology of a network, as follows:
• Physical topology—Mostly refers to a network’s hardware and how computers, other devices,
and cables or radio signals work together to form the physical network
• Logical topology—Mostly refers to how software controls access to network resources
(including how users and software initially gain access to the network) and how specific resources
such as applications and databases are shared on the network
Controlling how users and programs get access to resources on a network is a function of the operating
systems used on the network. Each OS (operating system) is configured to use one of two models to
connect to network resources: the peer-to-peer model or the client-server model. The peer-to-peer model
can be achieved using any assortment of desktop, mobile, or tablet operating systems, but the client-
server model requires a NOS (network operating system), which controls access to the entire network.
Peer-to-Peer Network Model
In a P2P (peer-to-peer) network model, the operating system of each computer on the network is
responsible for controlling access to its resources without centralized control. The computers, called
nodes or hosts on the network, form a logical group of computers and users that share. Each computer
on a P2P network controls its own administration, resources, and security.
The following are advantages of using peer-to-peer networks:
• They are simple to configure. For this reason, they may be used in environments in which time or
technical expertise is scarce. They are often less expensive to set up and maintain than other
types of
• networks. A network operating system, such as Windows Server 2019, is much more expensive
than a desktop operating system, such as Windows 10.
The following are disadvantages of using traditional peer-to-peer networks:
• They are not scalable, which means, as a peer-to-peer network grows larger, adding or changing
significant elements of the network may be difficult.
• They are not necessarily secure, meaning that in simple installations, data and other resources
shared by network users can be easily discovered and used by unauthorized people.
• They are not practical for connecting more than a few computers because it’s too time consuming
to manage the resources on the network. For example, suppose you want to set up a file server
using P2P file sharing. A server is any computer or program that provides a service, such as data
or other resources, to other devices.
Client-Server Network Model
In the client-server network model (which is sometimes called the client-server architecture or client-
server topology), resources are managed by the NOS via a centralized directory database. The database
can be managed by one or more servers, so long as they each have a similar NOS installed.
,When Windows Server controls network access to a group of computers, this logical group is called a
Windows domain. The centralized directory database that contains user account information and security
for the entire group of computers is called AD (Active Directory). Each user on the network has their
own domain-level account assigned by the network administrator and kept in Active Directory. This
account might be a local account, which is specific to that domain, or a Microsoft account, which links
local domain resources with Microsoft cloud resources. A user can sign on to the network from any
computer on the network and get access to the resources that Active Directory allows. This process is
managed by AD DS (Active Directory Domain Services).
A computer making a request from another is called the client. Clients on a client-server network can run
applications installed on the desktop and store their own data on local storage devices. Clients don’t
share their resources directly with each other; instead, access is controlled by entries in the centralized
domain database.
In summary, the NOS is responsible for the following:
• Managing data and other resources for clients
• Ensuring that only authorized users access the network
• Controlling which types of files a user can open and read
• Restricting when and from where users can access the network
• Dictating which rules computers will use to communicate
• In some situations, supplying applications and data files to clients
Although client-server networks are typically more complex in their design and maintenance than peer-
to-peer networks, they offer many:
• User accounts and passwords to the network are assigned in one place.
• Access to multiple shared resources (such as data files or printers) can be centrally granted to a
single user or groups of users.
• Problems on the network can be monitored, diagnosed, and often fixed from one location.
• Client-server networks are more scalable than peer-to-peer networks. In other words, it’s easier
to add users and devices to a client-server network.
Client-Server Applications
The resources a network makes available to its users include applications and the data provided by these
applications. Collectively, these resources are usually referred to as network services. In this section,
you’ll focus on applications typically found on most networks. These applications involve at least two
endpoint devices, such as computers or smartphones, and are known as client-server applications. The
first computer, a client computer, requests data or a service from the second computer, which is the
server.
Network Services and Their Protocols
How does the client know how to make the request in a way the server can understand and respond to?
These networked devices use methods and rules for communication known as protocols. To handle the
request for a web page, the client computer must first find the web server. Then, the client and server
must agree on the protocols they will use to communicate. Finally, the client makes the request, and the
server sends its response in the form of a web page. Hardware, the operating systems, and the
applications on both computers are all involved in this process.
The computers on a network communicate with each other via the protocols they have in common. The
two primary network protocols are TCP (Transmission Control Protocol) and IP (Internet Protocol), and
the suite of all the protocols an OS uses for communication on a network is the TCP/IP suite of protocols.
,Here’s a brief list of several popular client-server applications and their protocols
• Web service
o A web server serves up web pages to clients. Many corporations have their own web
servers, which are available privately on the corporate network. Other web servers are
public, accessible from anywhere on the Internet. The primary protocol used by web
servers and browsers (clients) is HTTP (Hypertext Transfer Protocol). When HTTP is
layered on top of an encryption protocol, such as SSL (Secure Sockets Layer) or TLS
(Transport Layer Security), the result is HTTPS (HTTP Secure), which gives a secure
transmission. The most popular web server applications are Apache (see apache.org)
and Nginx (pronounced engine-x, see nginx.com), both of which are free and open
source and primarily run on Linux systems. Open source is the term for software whose
code is publicly available for use and modification. Open source applications are often
considered more secure because users can evaluate the source code of the software to
ensure there are no loopholes left open for attackers to exploit.
• Email services
o Email is a client-server application that involves two servers. The client uses SMTP
(Simple Mail Transfer Protocol) to send an email message to the first server, which is
sometimes called the SMTP server. The first server sends the message on to the
receiver’s mail server, where it’s stored until the recipient requests delivery. The
recipient’s mail server delivers the message to the receiving client using one of two
protocols: POP3 (Post Office Protocol, version 3) or IMAP4 (Internet Message Access
Protocol, version 4). Using POP3, email is downloaded to the client computer and
typically removed from the server. Using IMAP4, the client application manages the email
while it remains stored on the server
• DNS service
o DNS (Domain Name System) helps clients find web servers over a network such as the
Internet.
• FTP service
o FTP service is a client-server application that transfers files between two computers, and
it primarily uses FTP (File Transfer Protocol). FTP does not provide encryption and is,
therefore, not secure.
• Database services
o Not all data is stored in individual files. Databases serve as a container for massive
amounts of data that can be organized into tables and records. Users and applications
can then access and interact with the data that is stored on a database server. A DBMS
(database management system) is software installed on the database server. It is
responsible for making requested changes to the data and organizing the data for
viewing, reporting, or exporting. Many DBMSs use the programming language SQL
(Structured Query Language, pronounced S-Q-L or just sequel) to configure and
interact with the database’s objects and data. Popular examples of SQL database
software include Microsoft SQL Server, Oracle Database, and the open-source
MySQL.
• Remote access service
o Some protocols allow an administrator or other user to “remote in,” that is, to access a
remote computer from the technician’s local device to control the computer remotely. The
Telnet protocol is a command-line application included in many operating systems, but
transmissions in Telnet are not encrypted. This has caused Telnet to be largely replaced
by other, more secure programs, such as the ssh command in the Linux operating
system. The ssh command in Linux uses the SSH (Secure Shell) protocol, which creates
an encrypted channel or tunnel between two computers and provides security that Telnet
lacks. In Windows operating systems, RDP (Remote Desktop Protocol) also provides
secure, encrypted transmissions. SSH or RDP might be used when a vendor supports
software on your corporate network.
, Network Hardware
Technically, two computers connected by an ad hoc Wi-Fi connection are a network. But let’s start this
discussion of networking hardware with the slightly more complex network shown in Figure 1-7. Keep in
mind that every node on a network needs a network address so that other nodes can find it.
LANs and Their Hardware
The network in Figure 1-7 is a LAN (local area network) because each node on the network can
communicate directly with others on the network. LANs are usually contained in a small space, such as
an office or building. The five computers and the network printer all connect to the switch by way of wired
connections. A switch (see Figure 1-8) receives incoming data from one of its ports and redirects (or
switches) it to another port or multiple ports that will send the data to its intended destination(s) within the
local network. The physical topology used by this network is called a star topology because all devices
connect to one central device, the switch. Compare this to the physical topology of the network where
each device connects to multiple other devices, which is called a mesh topology.
Another type of port is provided by a modular NIC (network interface card), also called a network
adapter (see Figure 1-10), installed in an expansion slot on the motherboard.
