Since, we are going to do a series
of tutorials on packet tracer. In this manner, we need to have a familiarity of
various networking components and devices. We are going to discuss some
important devices which are going to be used in networking.
All
networks are made up of basic hardware building blocks to interconnect network
nodes, such as Network Interface Cards (NICs), Bridges, Hubs, Switches, and
Routers etc. These devices also need cables to connect them. In this tutorial,
we are going to discuss these important devices.
Network
interface cards
A
NIC (network interface card) is a piece of computer hardware designed to
allow computers to communicate over a computer network. It provides physical
access to a networking medium and often provides a low-level addressing system
through the use of MAC addresses. It allows users to connect to each other
either by using cables or wirelessly.The NIC provides the transfer of data
in megabytes.
Every
device on a network that needs to transmit and receive data must have
a network interface card (NIC) installed. They are sometimes
called network adapters, and are usually installed into one of the
computer's expansion slots in the same way as a sound or graphics card. The NIC
includes a transceiver, (a transmitter and receiver combined). The
transceiver allows a network device to transmit and receive data via the
transmission medium. Each NIC has a unique 48-bit Media Access
Control (MAC) address burned in to its ROM during manufacture. The first 24
bits make up a block code known as the Organisationally Unique
Identifier (OUI) that is issued to manufacturers of NICs, and identify the
manufacturer. The issue of OUIs to organisations is administered by
the Institute of Electrical and Electronics Engineers (IEEE). The
last 24 bits constitute a sequential number issued by the manufacturer. The MAC
address is sometimes called a hardware address or physical address, and
uniquely identifies the network adapter. It is used by many data link layer communications
protocols, including Ethernet, the 802.11 wireless protocol and Bluetooth. The
use of a 48-bit adress allows for 248(281,474,976,710,656) unique
addresses. A MAC address is usually shown in hexadecimal format, with each
octet separated by a dash or colon,
For
example: 00-60-55-93-R2-N7
Repeaters
A
repeater is an electronic device that receives a signal and retransmits it at a
higher power level, or to the other side of an obstruction, so that the signal
can cover longer distances without degradation. In most twisted pair ethernet
configurations, repeaters are required for cable runs longer than 100 meters
away from the computer. As signals travel
along a transmission medium there will be a loss of signal strength i.e.
attenuation. A repeater is a non-intelligent network device that receives a
signal on one of its ports, regenerates the signal, and then retransmits the
signal on all of its remaining ports. Repeaters can extend the length of a
network (but not the capacity) by connecting two network segments. Repeaters
cannot be used to extend a network beyond the limitations of its underlying
architecture, or to connect network segments that use different network access
methods. They can, however, connect different media types, and may be able to
link bridge segments with different data rates.
Repeaters are used to boost signals
in coaxial and twisted pair cable and in optical fibre lines. An
electrical signal in a cable gets weaker the further it travels, due to
energy dissipated in conductor resistance and dielectric losses.
Similarly a light signal traveling through an optical fiber suffers attenuation
due to scattering and absorption. In long cable runs, repeaters are used to
periodically regenerate and strengthen the signal.
Hubs
A
hub contains multiple ports. When a packet arrives at one port, it is copied to
all the ports of the hub for transmission. In
a hub, a frame is passed along or "broadcast" to every one of its
ports. It doesn't matter that the frame is only destined for one port. The hub
has no way of distinguishing which port a frame should be sent to. Passing it
along to every port ensures that it will reach its intended destination. This
places a lot of traffic on the network and can lead to poor network response
times. Additionally, a 10/100Mbps hub must share its bandwidth with each and every one of its ports.
So when only one PC is broadcasting, it will have access to the maximum
available bandwidth. If, however, multiple PCs are broadcasting, then that
bandwidth will need to be divided among all of those systems, which will
degrade performance.
Bridges
A
network bridge connects multiple network segments at the data link layer (layer
2) of the OSI model. Bridges do not copy traffic to all ports, as hubs do, but
learn which MAC addresses are reachable through specific ports. Once the bridge
associates a port and an address, it will send traffic for that address only to
that port. Bridges do send broadcasts to all ports except the one on which the
broadcast was received.
Bridges
learn the association of ports and addresses by examining the source address of
frames that it sees on various ports. Once a frame arrives through a port, its
source address is stored and the bridge assumes that MAC address is associated
with that port. The first time that a previously unknown destination address is
seen, the bridge will forward the frame to all ports other than the one on
which the frame arrived.
Bridges don't know anything about
protocols, but just forward data depending on the destination address in the
data packet. This address is not the IP address, but the MAC (Media Access
Control) address that is unique to each network adapter card. The bridge is
basically just to connect two local-area networks (LANs), or two segments of
the same LAN that use the same protocol. Bridges can extend the length of
a network, but unlike repeaters they can also extend the capacity of
a network, since each port on a bridge has its own MAC address. When bridges
are powered on in an Ethernet network, they start to learn the
network's topology by analysing the source addresses of incoming frames from
all attached network segments (a process called backward learning ).
Over a period of time, they build up a routing table .
The bridge monitors all traffic on
the segments it connects, and checks the source and destination address of each
frame against its routing table. When the bridge first becomes operational, the
routing table is blank, but as data is transmitted back and forth, the bridge
adds the source MAC address of any incoming frame to the routing table and
associates the address with the port on which the frame arrives. In this way,
the bridge quickly builds up a complete picture of the network topology. If the
bridge does not know the destination segment for an incoming frame, it will forward
the frame to all attached segments except the segment on which the frame was
transmitted. Bridges reduce the amount of traffic on individual segments by
acting as a filter, isolating intra-segment traffic. This can greatly improve
response times.
Switches
The switch is a relatively new
network device that has replaced both hubs and bridges in LANs. A switch uses
an internal address table to route incoming data frames via the port associated
with their destination MAC address. Switches can be used to connect together a
number of end-user devices such as workstations, or to interconnect multiple
network segments. A switch that interconnects end-user devices is often called
a workgroup switch. Switches provide dedicated full-duplex links for every
possible pairing of ports, effectively giving each attached device its own
network segment This significantly reduces the number of intra-segment and
inter-segment collisions. Strictly speaking, a switch is not capable of
routing traffic based on IP address (layer 3) which is necessary for
communicating between network segments or within a large or complex LAN. Some
switches are capable of routing based on IP addresses but are still called
switches as a marketing term. A switch normally has numerous ports, with the
intention being that most or all of the network is connected directly to the
switch, or another switch that is in turn connected to a switch.
Routers
Routers are networking devices that forward data packets between
networks using headers and forwarding tables to determine the best path to
forward the packets. A network
environment that consists of several interconnected networks employing
different network protocols and architectures requires a sophisticated device
to manage the flow of traffic between these diverse networks. Such a device,
sometimes referred to as an intermediate system, but more commonly called
a router, must be able to determine how to get incoming packets
(or datagrams) to the destination network by the most efficient route.
Routers gather information about the networks to which they are connected, and
can share this information with routers on other networks. The information
gathered is stored in the router's internal routing table, and includes
both the routing information itself and the current status of various network
links. Routers exchange this routing information using special routing
protocols.
A router is connected to at least
two networks, commonly two LANs or WANs or a LAN and its ISP's network. Routers
are located at gateways, the places where two or more networks connect, and are
the critical device that keeps data flowing between networks and keeps the
networks connected to the Internet. When data is sent between locations on one
network or from one network to a second network the data is always seen and
directed to the correct location by the router. The router accomplishes this by
using headers and forwarding tables to determine the best path for forwarding
the data packets, and they also use protocols such as ICMP to communicate with
each other and configure the best route between any two hosts. The Internet
itself is a global network connecting millions of computers and smaller
networks. There are various routing protocols which are helpful for
various different environments and will be discussed later.
Firewall
A firewall is a network security system designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both.
Network firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.
Hardware and Software Firewalls
Firewalls can be either hardware or software but the ideal firewall configuration will consist of both. In addition to limiting access to your computer and network, a firewall is also useful for allowing remote access to a private network through secure authentication certificates and logins.
Hardware firewalls can be purchased as a stand-alone product but are also typically found in broadband routers, and should be considered an important part of your system and network set-up. Most hardware firewalls will have a minimum of four network ports to connect other computers, but for larger networks, business networking firewall solutions are available.
Software firewalls are installed on your computer (like any software) and you can customize it; allowing you some control over its function and protection features. A software firewall will protect your computer from outside attempts to control or gain access your computer.
Common Firewall Filtering Techniques
Firewalls are used to protect both home and corporate networks. A typical firewall program or hardware device filters all information coming through the Internet to your network or computer system. There are several types of firewall techniques that will prevent potentially harmful information from getting through:
Packet Filter: Looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules. Packet filtering is fairly effective and transparent to users, but it is difficult to configure. In addition, it is susceptible to IP spoofing.
Application Gateway: Applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose a performance degradation.
Circuit-level Gateway: Applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking.
Proxy Server: Intercepts all messages entering and leaving the network. The proxy server effectively hides the true network addresses.
later we will discuss about ip subnet and many more...
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