Networking Basics
Q.What is Network?
Ans:
A network is simply a collection of computers or other hardware devices
that are connected together, either physically or logically, using special
hardware and software, to allow them to exchange information and cooperate.
Networking is the term that describes the processes involved in designing,
implementing, upgrading, managing and otherwise working with networks and
network technologies.
Q.What are The Advantages (Benefits) and Disadvantages (Costs) of Networking?
Ans:
Most of the benefits of
networking can be divided into two generic categories: connectivity and sharing.
Networks allow computers, and hence their users, to be connected together. They
also allow for the easy sharing of information and resources, and cooperation
between the devices in other ways. Since modern business depends so much on the
intelligent flow and management of information, this tells you a lot about why
networking is so valuable.
Here, in no particular order, are some of the specific advantages generally associated with networking:
Here, in no particular order, are some of the specific advantages generally associated with networking:
a)
Connectivity and Communication
b) Data Sharing
c) Hardware Sharing
d) Internet Access
e) Internet Access Sharing
f) Data Security and Management
g) Performance Enhancement and Balancing
h) Entertainment.
b) Data Sharing
c) Hardware Sharing
d) Internet Access
e) Internet Access Sharing
f) Data Security and Management
g) Performance Enhancement and Balancing
h) Entertainment.
The Disadvantages (Costs) of Networking
Here are a few of the items that balance
against the advantages of networking
a)
Network Hardware, Software and Setup Costs
b) Hardware and Software Management and Administration Costs
c) Undesirable Sharing
d) Illegal or Undesirable Behavior
f) Data Security Concerns:
b) Hardware and Software Management and Administration Costs
c) Undesirable Sharing
d) Illegal or Undesirable Behavior
f) Data Security Concerns:
Q.Explain Circuit Switching and Packet Switching
Networks ?.
Ans:
Circuit Switching :- In this
networking method, a connection called a circuit is set up between two devices,
which is used for the whole communication. Information about the nature of the
circuit is maintained by the network. The circuit may either be a fixed one that
is always present, or it may be a circuit that is created on an as-needed basis.
Even if many potential paths through intermediate devices may exist between the
two devices communicating.
The classic example of a circuit-switched
network is the telephone system. When you call someone and they answer, you
establish a circuit connection and can pass data between you. The next time you
call, you get a new circuit, which may (probably will) use different hardware
than the first circuit did, depending on what's available at that time in the
network.
Packet Switching:- In this network type,
no specific path is used for data transfer. Instead, the data is chopped up into
small pieces called packets and sent over the network. The packets can be
routed, combined or fragmented, as required to get them to their eventual
destination. On the receiving end, the process is reversed—the data is read from
the packets and re-assembled into the form of the original data. A
packet-switched network is more analogous to the postal system than it is to the
telephone system (though the comparison isn't perfect.)
Q.Explain Common Names For Messages ?
Ans: The most common terms that are used for
messages are the following:
Packet: This term is considered by many to
most correctly refer to a message sent by protocols operating at the network
layer of the OSI Reference Model. So, you will commonly see people refer to “IP
packets”. However, this term is commonly also used to refer generically to any
type of message.
Datagram: This term is basically
synonymous with “packet” and is also used to refer to network layer
technologies. It is also often used to refer to a message that is sent at a
higher level of the OSI Reference Model.
Frame: This term is most commonly
associated with messages that travel at low levels of the OSI Reference Model.
In particular, it is most commonly seen used in reference to data link layer
messages. It is occasionally also used to refer to physical layer messages, when
message formatting is performed by a layer one technology. A frame gets its name
from the fact that it is created by taking higher-level packets or datagrams and
“framing” them with additional header information needed at the lower
level.
Cell: Frames and packets, in general, can
be of variable length, depending on their contents; in contrast, a cell is most
often a message that is fixed in size. For example, the fixed-length, 53-byte
messages sent in Asynchronous Transfer Mode (ATM) are called cells. Like frames,
cells usually are used by technologies operating at the lower layers of the OSI
model.
Protocol Data Unit (PDU) and Service Data
Unit (SDU): These are the formal terms used in the OSI Reference to describe
protocol messages. A PDU at layer N is a message sent between protocols at layer
N. It consists of layer N header information and an encapsulated message from
layer N+1, which is called both the layer N SDU and the layer N+1
PDU.
Q.Explain Headers, Payloads and Footers
?
Ans:
Messages are the structures used
to send information over networks.Every protocol uses a special formatting
method that determines the structure of the messages it employs. Obviously, a
message that is intended to connect a Web server and a Web browser is going to
be quite different from one that connects two Ethernet cards at a low level.
In generic terms, each message contains
the following three basic elements.
Header: Information that is placed before
the actual data. The header normally contains a small number of bytes of control
information, which is used to communicate important facts about the data that
the message contains and how it is to be interpreted and used. It serves as the
communication and control link between protocol elements on different
devices.
Data: The actual data to be transmitted,
often called the payload of the message (metaphorically borrowing a term from
the space industry!) Most messages contain some data of one form or another, but
some actually contain none: they are used only for control and communication
purposes. For example, these may be used to set up or terminate a logical
connection before data is sent.
Footer: Information that is placed after
the data. There is no real difference between the header and the footer, as both
generally contain control fields. The term trailer is also sometimes used.
Q.Explain Unicast, Broadcast and Multicast Messages and Message Addressing Methods ?
Ans:
In a networking technology that
uses messages to send data, there are a number of tasks that must be undertaken
in order to successfully transmit the data from one place to another. One is
simply the addressing of the message—putting an address on it so that the system
knows where it is supposed to go. Another is transmitting the message, which is
of course sending it to its intended recipient.
There are several different ways of
addressing and transmitting a message over a network, One way in which messages
are differentiated is in how they are addressed, and to how many recipients.
Which method is used depends on what the function of the message is, and also on
whether or not the sender knows specifically whom they are trying to contact, or
only generally.
Message Transmission Methods:- To help explain these different methods, I
will use a real-world analogy. Consider a social function with 300 people that
is being held in a large hall. These people are mingling and are having
different conversations. There are different kinds of messages that may need to
be sent in this setting, much as is the case with networks.
Bearing this analogy in mind, consider
these three kinds of message transmissions.
Unicast Messages: These are messages that
are sent from one device to another device; they are not intended for others. If
you have a friend at this social event, this is the equivalent of pulling him or
her aside for a private conversation. Of course, there is still the possibility
of someone else at the event overhearing your conversation—or even eavesdropping
on it. The same is true in networking as well—addressing a message to a
particular computer doesn't guarantee that others won't also read it, just that
they normally will not do so.
Broadcast Messages: As the name suggests,
these messages are sent to every device on a network. They are used when a piece
of information actually needs communicating to everyone on the network, or used
when the sending station needs to send to just one recipient, but doesn't know
its address.
For example, suppose a new arrival at the
social gathering saw a blue sedan with New Hampshire plates in the parking lot
that had its lights left on. He of course does not know whose car this is. The
best way to communicate this information is to broadcast it by having the host
make an announcement that will be heard by all, including the vehicle’s owner.
In networks, broadcast messages are used for a variety of purposes, including
finding the locations of particular stations or the devices that manage
different services.
Multicast Messages: These are a compromise
between the previous two types: they are sent to a group of stations that meet a
particular set of criteria. These stations are usually related to each other in
some way, such as serving a common function, or being set up into a particular
multicast group. (Note that one can also consider broadcast messages to be a
special case of multicast, where the group is “everyone”.
Message Addressing Methods : Since the transmission methods above differ based on how many and which devices receive the transmission, they are tied directly to the methods used for addressing:
Unicast Addressing: Unicast delivery
requires that a message be addressed to a specific recipient. This is the most
common type of messaging, so this addressing capability is present in almost all
protocols.
Broadcast Addressing: Broadcasts are
normally implemented via a special address that is reserved for that function.
Whenever devices see a message sent to that address, they all interprets it as
meaning “this message goes to everyone”.
Multicast Addressing: Multicasts are the
most complex type of message because they require a means of identifying a set
of specific devices to receive a message. It is often necessary to create
several such groups, which may or may not partially overlap in their membership.
Some mechanism is needed to manage which devices are in which groups.
Q.Explain Connection-Oriented and Connectionless
Protocols ?
Ans:
Protocols are divided into two
categories based on their use of connections:
Connection-Oriented Protocols: These
protocols require that a logical connection be established between two devices
before transferring data. This is generally accomplished by following a specific
set of rules that specify how a connection should be initiated, negotiated,
managed and eventually terminated. Usually one device begins by sending a
request to open a connection, and the other responds. They pass control
information to determine if and how the connection should be set up. If this is
successful, data is sent between the devices. When they are finished, the
connection is broken.eg.TCP/IP
Connectionless Protocols: These protocols
do not establish a connection between devices. As soon as a device has data to
send to another, it just sends it.eg.UDB
Q.What Is the Difference Between Bits and Bytes?
Ans: The terms bit and byte are common in
computer networking. Both terms refer to digital data transmitted over a network
connection. For example, bits and bytes both may represent network addresses or
port numbers.
A bit is a single numeric value, either
'1' or '0', that encodes a single unit of digital information. A byte is a
sequence of bits; usually eight bits equal one byte.
For example, in Internet Protocol (IP)
networking, IP addresses contain 32 bits or 4 bytes. The bits encode the network
address so that it can be shared on the network. The bytes divide the bits into
groups.
The IP address 192.168.0.1, for instance,
is encoded with the following bits and bytes:
11000000 10101000 00000000 00000001
Bits are grouped into bytes to, generally
speaking, increase the efficiency of computer hardware, including network
equipment, disks and memory.
Q.What is Packets and how packets travel?.
Normally a packets are divided into three main parts:
Q.What is Packets and how packets travel?.
Ans: The transfer of data from one place
to another place in the form of digital signals and this data is broken into
small data segment before being transmitted from one computer to another
computer. These small data segments are
called packets. Each packets contains the information about its source and
destination (sender's and receiver's information). A normal packets size between
1000 to 1500 bytes. The packets send to
its destination using best possible path and this makes a network more
efficient.The data transmission protocols perform the data sequencing, data
routing, flow control and error controls functions for the efficient and error
free transmission of data. A protocol set the long message size into suitable packets size for error free communication and find the
most suitable path between the sender and receiver before sending. At the end
function of protocol to detect and recover errors for successful data communication between both
ends.
Normally a packets are divided into three main parts:
Payload: It is the part of the packet that
contains the body of data or actual part of data.. This is the actual data that
the packet is delivering to the destination. If a packet is fixed-length, then
the payload may be padded with blank
information to make it the right size.
Trailer: he trailer, sometimes called the
footer, typically contains a couple of bits that tell the receiving device that
it has reached the end of the packet. It may also have some type of error
checking,in other words it is used to
check the errors in packet at destination. The most common error checking used
in packets is Cyclic Redundancy Check (CRC). CRC is pretty neat. Here is how it
works in certain
computer networks: It takes the sum of all
the 1s in the payload and adds them together. The result is stored as a
hexadecimal value in the trailer. The receiving device adds up the 1s in the
payload and compares the result to the
value stored in the trailer. If the values match, the packet is good. But if the
values do not match, the receiving device sends a request to the originating
device to resend the packet.
Header: It contains the information about
type of data, the sender and receiver of data, and the actual sequence of data
so that at destination multiple packets can be assembled in proper
order.
e.g. : As an example, let's look at
how an e-mail message might get broken into packets. Let's say that you send an
e-mail to a friend. The e-mail is about 3,500 bits (3.5 kilobits) in size. The
network you send it over uses fixed-length packets of 1,024 bits (1 kilobit).
The header of each packet is 96 bits long and the trailer is 32 bits long,
leaving 896 bits for the payload. To break the 3,500 bits of message into
packets, you will need four packets (divide 3,500 by 896). Three packets will
contain 896 bits of payload and the fourth will have 812 bits. Here is what one
of the four packets would contain:
Each packet's header will contain the
proper protocols, the originating address (the IP address of your computer), the
destination address (the IP address of the computer where you are sending the
e-mail)
and the packet number (1, 2, 3 or 4 since
there are 4 packets). Routers in the network will look at the destination
address in the header and compare it to their lookup table to find out where to
send the
packet. Once the packet arrives at its
destination, your friend's computer will strip the header and trailer off each
packet and reassemble the e-mail based on the numbered sequence of the
packets.
Q.What
are Basic Communication Modes of Operation ?
Ans:Let's begin with a look at the three basic
modes of operation that can exist for any network connection, communications
channel, or interface.
Simplex
Operation :- In simplex operation, a network cable or communications channel can
only send information in one direction; it's a “one-way street”. This may seem
counter-intuitive: what's the point of communications that only travel in one
direction? In fact, there are at least two different places where simplex
operation is encountered in modern networking.
The first is when two distinct channels
are used for communication: one transmits from A to B and the other from B to A.
This is surprisingly common, even though not always obvious. For example, most
if not all fiber optic communication is simplex, using one strand to send data
in each direction. But this may not be obvious if the pair of fiber strands are
combined into one cable.
Simplex operation is also used in special
types of technologies, especially ones that are asymmetric. For example, one
type of satellite Internet access sends data over the satellite only for
downloads, while a regular dial-up modem is used for upload to the service
provider. In this case, both the satellite link and the dial-up connection are
operating in a simplex mode.
Half-Duplex Operation:- Technologies that
employ half-duplex operation are capable of sending information in both
directions between two nodes, but only one direction or the other can be
utilized at a time. This is a fairly common mode of operation when there is only
a single network medium (cable, radio frequency and so forth) between
devices.
While this term is often used to describe
the behavior of a pair of devices, it can more generally refer to any number of
connected devices that take turns transmitting. For example, in conventional
Ethernet networks, any device can transmit, but only one may do so at a time.
For this reason, regular (unswitched) Ethernet networks are often said to be
“half-duplex”, even though it may seem strange to describe a LAN that
way.
Full-Duplex Operation :- In
full-duplex operation, a connection between two devices is capable of sending
data in both directions simultaneously. Full-duplex channels can be constructed
either as a pair of simplex links (as described above) or using one channel
designed to permit bidirectional simultaneous transmissions. A full-duplex link
can only connect two devices, so many such links are required if multiple
devices are to be connected together.
Note that the term “full-duplex” is
somewhat redundant; “duplex” would suffice, but everyone still says
“full-duplex” (likely, to differentiate this mode from
half-duplex).
Q.Comparing Half-Duplex and Full-Duplex Operation?
Ans: Out Of these three options,
full-duplex is obviously the one that yields the highest performance.
Full-duplex operation doubles the theoretical bandwidth of the connection. If a
link normally runs at 1 Mbps but can work in full-duplex mode, it really has 2
Mbps of bandwidth (1 Mbps in each direction). Remember the key word
“theoretical” however—you do not really get double the performance in real life,
because communications usually do not involve sending lots of data in both
directions at once. However, you certainly get better throughput than in a
half-duplex mode.
In some cases, the mode of operation is a
function of the technology and cannot be changed. In others, however,
full-duplex mode is a matter of the correct hardware settings, and also whether
the software supports full-duplex operation or not. Thus, getting higher
performance in this area is sometimes simply a matter of ensuring proper
configuration.
Full-duplex operation has been pretty much
taken for granted in communications for years. The more interesting development
has been the rise in significance of full-duplex operation for local area
networking. Traditionally, LANs have always used half-duplex operation on a
shared access medium. As the use of switches has increased, allowing dedicated
bandwidth to each computer, full-duplex operation has become very popular.
Full-duplex operation in Ethernet not only allows the simultaneous transmission
of data in both directions, it also eliminates contention for the formerly
shared access medium—no more collisions. The combination of these two effects
improves performance, sometimes substantially.
Q.What is IP Addresses ?
Ans:To keep all of these machines straight, each
machine on the Internet is assigned a unique address called an IP address. IP
stands for Internet protocol, and these addresses are 32-bit numbers, normally expressed as four "octets" in a "dotted decimal
number." A typical IP address looks like this:
216.27.61.137
The four numbers in an IP address are called octets
because they can have values between 0 and 255, which is 28 possibilities per
octet.
Every machine on the Internet has a unique IP
address. A server has a static IP address that does not change very often. A
home machine that is dialing up through a modem often has an IP address that
is assigned by the ISP when the machine
dials in. That IP address is unique for that session -- it may be different the
next time the machine dials in. This way, an ISP only needs one IP address for
each modem it supports, rather than for each customer.
On a UNIX machine, type nslookup at the command
prompt, along with a machine name, like www.eduinkerala.com -- e.g. "nslookup
www.eduinkerala.com" -- to
display the IP address of the machine, and you can use
the command hostname to learn the name of your machine. (For more information on
IP addresses, see IANA.)
As far as the Internet's machines
are concerned, an IP address is all you need to talk to a server. For example,
in your browser, you can type the URL http://182.18.135.36 and arrive at the machine
that contains the Web server for www.eduinkerala.com. On some servers, the
IP address alone is not sufficient, but on most large servers it
is.
Q: What Is a Default Gateway?.
Ans: In computer networking, a default gateway is the device that passes traffic from the local subnet to devices on other subnets. The default gateway often connects a local network to the Internet, although internal gateways for local networks also exist.
Internet default gateways are typically one of two types:
On home or small business networks with abroadband router to share the Internet connection, the home router serves as the default gateway.
On home or small business networks without a router, such as for residences with dialup Internet access, a router at the Internet Service Provider location serves as the default gateway.
Default network gateways can also be configured using an ordinary computer instead of a router. These gateways use two network adapters, one connected to the local subnet and one to the outside network. Either routers or gateway computers can be used to network local subnets such as those in larger businesses.
Q: What Is a Default Gateway?.
Ans: In computer networking, a default gateway is the device that passes traffic from the local subnet to devices on other subnets. The default gateway often connects a local network to the Internet, although internal gateways for local networks also exist.
Internet default gateways are typically one of two types:
On home or small business networks with abroadband router to share the Internet connection, the home router serves as the default gateway.
On home or small business networks without a router, such as for residences with dialup Internet access, a router at the Internet Service Provider location serves as the default gateway.
Default network gateways can also be configured using an ordinary computer instead of a router. These gateways use two network adapters, one connected to the local subnet and one to the outside network. Either routers or gateway computers can be used to network local subnets such as those in larger businesses.
Q.What is Subnet
Mask?
Ans: An IP address has two
components, the network address and the host address. A subnet mask separates
the IP address into the network and host addresses
(<network><host>). Subnetting further divides the host part of an IP
address into a subnet and host address
(<network><subnet><host>). It is called a subnet mask because
it is used to identify network address of an IP address by perfoming bitwise AND
operation on the netmask.
A Subnet mask is a 32-bit number that tells the IP
belongs to which subnet. Subnet Mask is made by setting network bits to all "1"s
and setting host bits to all "0"s. Within a given network, two host addresses
are reserved for special purpose. The "0" address is assigned a network address
and "255" is assigned to a broadcast address, and they cannot be assigned to a
host.Examples of commonly used netmasks for classed networks are 8-bits (Class A), 16-bits (Class B) and 24-bits (Class C), and classless networks are as follows:
Applying a subnet mask to an IP address separates network address from host address. The network bits are represented by the 1's in the mask, and the host bits are represented by 0's. Performing a bitwise logical AND operation on the IP address with the subnet mask produces the network address. For example, applying the Class C subnet mask to our IP address 216.3.128.12 produces the following network address: IP: 1101 1000 . 0000 0011 . 1000 0000 . 0000 1100 (216.003.128.012)
Mask: 1111 1111 . 1111 1111 . 1111 1111 . 0000 0000 (255.255.255.000)
---------------------------------------------
1101 1000 . 0000 0011 . 1000 0000 . 0000 0000 (216.003.128.000)
Subnetting Network 216 . 3 . 128 . (0000 0000) (1st half assigned to the web host)
216 . 3 . 128 . (1000 0000) (2nd half assigned to the customers)
The web host will have the subnet mask of 216.3.128.128
(/25). Now, we'll further divide the 2nd half into eight block of 16 IP
addresses. 216 . 3 . 128 . (1000 0000) Customer 1 -- Gets 16 IPs (14 usable)
216 . 3 . 128 . (1001 0000) Customer 2 -- Gets 16 IPs (14 usable)
216 . 3 . 128 . (1010 0000) Customer 3 -- Gets 16 IPs (14 usable)
216 . 3 . 128 . (1011 0000) Customer 4 -- Gets 16 IPs (14 usable)
216 . 3 . 128 . (1100 0000) Customer 5 -- Gets 16 IPs (14 usable)
216 . 3 . 128 . (1101 0000) Customer 6 -- Gets 16 IPs (14 usable)
216 . 3 . 128 . (1110 0000) Customer 7 -- Gets 16 IPs (14 usable)
216 . 3 . 128 . (1111 0000) Customer 8 -- Gets 16 IPs (14 usable)
-----------------------------
255 . 255 . 255 . (1111 0000) (Subnet mask of 255.255.255.240)
CIDR - Classless Inter Domain Routing With CIDR, a network of IP addresses is allocated in 1-bit increments as opposed to 8-bits in classful network. The use of a CIDR notated address can easily represent classful addresses (Class A = /8, Class B = /16, and Class C = /24). The number next to the slash (i.e. /8) represents the number of bits assigned to the network address. The example shown above can be illustrated with CIDR as follows: 216.3.128.12, with subnet mask of 255.255.255.128 is written as
216.3.128.12/25
Similarly, the 8 customers with the block of 16 IP addresses can be
written as:
216.3.128.129/28, 216.3.128.130/28, and etc.
With an introduction of CIDR addressing scheme, IP
addresses are more efficiently allocated to ISPs and customers; and hence there
is less risk of IP addresses running out anytime soon. For detailed
specification on CIDR, please review RFC 1519. With
introduction of additional gaming, medical, applicance and telecom devices
requiring static IP addresses in addition to more than 6.5 billion (July 2006
est.) world population, the IPv4 addresses with CIDR addressing scheme will
eventually run out. To solve shortage of IPv4 addresses, the IPv6 (128-bit)
address scheme was introduced in 1993.Q.Explain Open Systems Interconnection (OSI) model? Ans: The OSI Reference Model provides the basis for understanding how technologies like Ethernet and HomePNA have some important similarities; it explains how a PC can communicate using any of several different sets of protocols, even simultaneously; it is an important part of understanding the differences between interconnection devices such as repeaters, hubs, bridges, switches and routers; and it also explains how many WAN technologies interoperate. The OSI Reference Model is comprised of seven conceptual layers, each assigned a “ranking” number from one to seven. The layer number represents the position of the layer in the model as a whole, and indicates how “close” the layer is to the actual hardware used to implement a network. The first and lowest layer is the physical layer, which is where low-level signaling and hardware are implemented. The seventh and highest layer is the application layer, which deals with high-level applications employed by users: both end users and the operating system software. You can see that as we proceed from the first layer to the seventh, we move up the layer stack and in so doing, increase our level of abstraction. This means that the higher a layer is in the stack, the more it deals with logical concepts and software, and the less it deals with the hardware of a network and the “nuts and bolts” of making it work. The first layer is the most concrete, as it deals with the actual hardware of networks, and the specific methods of sending bits from one device to another. It is the domain of hardware engineers and signaling experts. The second layer is a bit more abstract but still deals with signaling and hardware. As you proceed through the third, fourth and subsequent layers, the technologies at those layers become increasingly abstract. By the time you reach the seventh layer, you are no longer dealing with hardware or even operating system concepts very much; you are in the realm of the user and high-level programs that rely on lower levels to do the “heavy lifting” for them. The OSI Reference Model does not formally assign any relationship between groups of adjacent layers. However, to help explain how the layers work, it is common to categorize them into two layer groupings: Lower Layers (Layers 1, 2, 3 and 4): The lower layers of the model—physical, data link, network and transport—are primarily concerned with the formatting, encoding and transmission of data over the network. They don't care that much about what the data is or what it is being used for, just about moving it around. They are implemented in both hardware and software, with the transition from hardware to software occurring as you proceed up from layer 1 to layer 4. Upper Layers (Layers 5, 6 and 7): The higher layers of the model—session, presentation and application—are the ones that are concerned primarily with interacting with the user, and implementing the applications that run over the network. The protocols that run at higher layers are less concerned with the low-level details of how data gets sent from one place to another; they rely on the lower layers to provide delivery of data. These layers are almost always implemented as software running on a computer or other hardware device. The OSI Reference Model divides networking functions into a stack of seven layers, numbered 1 through 7 from the bottom up. To help illustrate the differing levels of abstraction between layers near the top and those on the bottom, they are sometimes divided into two layer groupings—the lower layers and the upper layers. Of course, not everyone agrees on exactly how the division should be accomplished. In particular, the transport layer is sometimes considered an upper layer and sometimes a lower layer. |
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7) .Application Layer : The application layer provider different
services to the application. Example of services provided by this layer are file
transfer, electronic messaging e-mail, virtual terminal access and network
management.
6) Presentation Layer : The Presentation layer is responsible for protocol conversion, date encryption/decryption, Expanding graphics command and the date compression.
This layer makes the communications between two host possible.
5) Session Layer : This layer is responsible for establishing the process-to-process communication between the host in the network. This layer is responsible for establishing and ending the sessions across the network. The interactive login is an example of services provided by this layer in which the connective are re-connected in care of any interruption.
4) Transport Layer : This layer is responsible for end-to-end delivers of messages between the networked hosts. It first divides the streams of data into chunks or packets before transmission and then the receiving computer re-assembles the packets. It also guarantee error free data delivery without loss or duplications.
3) Network Layer : This layer is responsible for translating the logical network address and names into their physical address ( MAC address). This layer is also responsible for addressing, determining routes for sending and managing network problems such as packet switching, data congestion and routines.
2) Data Link Layer : Data link layer is responsible for controlling the error between adjacent nodes and transfer the frames to other computer via physical layer. Data link layer is used by hubs and switches for their operation.
1) Physical Layer : Physical Layer is responsible for transmitting row bit stream over the physical cable. The physical layer defines the hardware items such as cables, cards, voltages etc.
Q.How to remember the layer of OSI model?
Ans:The easiest way to remember the different layers of OSI Model is to use the mnemonic "All people seem To need Data Processing":
6) Presentation Layer : The Presentation layer is responsible for protocol conversion, date encryption/decryption, Expanding graphics command and the date compression.
This layer makes the communications between two host possible.
5) Session Layer : This layer is responsible for establishing the process-to-process communication between the host in the network. This layer is responsible for establishing and ending the sessions across the network. The interactive login is an example of services provided by this layer in which the connective are re-connected in care of any interruption.
4) Transport Layer : This layer is responsible for end-to-end delivers of messages between the networked hosts. It first divides the streams of data into chunks or packets before transmission and then the receiving computer re-assembles the packets. It also guarantee error free data delivery without loss or duplications.
3) Network Layer : This layer is responsible for translating the logical network address and names into their physical address ( MAC address). This layer is also responsible for addressing, determining routes for sending and managing network problems such as packet switching, data congestion and routines.
2) Data Link Layer : Data link layer is responsible for controlling the error between adjacent nodes and transfer the frames to other computer via physical layer. Data link layer is used by hubs and switches for their operation.
1) Physical Layer : Physical Layer is responsible for transmitting row bit stream over the physical cable. The physical layer defines the hardware items such as cables, cards, voltages etc.
Q.How to remember the layer of OSI model?
Ans:The easiest way to remember the different layers of OSI Model is to use the mnemonic "All people seem To need Data Processing":
| Layer | Name | |
| Mnemonic | ||
| 7 | Application | All |
| 6 | Presentation | People |
| 5 | Session | Seem |
| 4 | Transport | To |
| 3 | Network | Need |
| 2 | Data Link | Data |
| 1 | Physical | Processing |
Q.How Trace Route Works: TTLs? Ans: "Traceroute'' is a network debugging utility that attempts to trace the path a packet takes through the network - its route. A key word here is ``attempts'' - by no means does traceroute work in all cases TTL stands for Time To Live. When a TCP packet is sent, its TTL is set, which is the number of routers (hops) it can pass through before the packet is discarded. As the packet passes through a router the TTL is decremented until, when the TTL reaches zero, the packet is destroyed and an ICMP "time exceeded" message is returned. The return message's TTL is set by the terminating router when it creates the packet, and decremented normally. Trace Route works by setting the TTL for a packet to 1, sending it towards the requested destination host, and listening for the reply. When the initiating machine receives a "time exceeded" response, it examines the packet to determine where the packet came from - this identifies the machine one hop away. Then the tracing machine generates a new packet with TTL 2, and uses the response to determine the machine 2 hops away, and so on. Unfortunately not all TCP stacks behave correctly. Some TCP stacks set the TTL for the ICMP "time exceeded" message to that of the message being killed. So if the TTL is 0, the packet will be killed by the next machine to which it is passed. This can have two effects on a trace. If the computer is an intermediate machine in the trace, the entry will remain blank. No information is returned to the machine conducting the trace because the "time exceeded" message never makes it back. If the machine you are doing a trace to has this bug in its TCP stack, return packets won't reach the originating machine unless the TTL is high enough to cover the round trip. So Trace Route will show a number of failed connections equal to n (the number of hops to the destination machine) minus 1. Q.How Ping Works?
Ans:"Ping" (Packet INternet Groper) is without a doubt
the best-known network administration tool. It is one of the simplest tools,
because all it does is send packets to check if a remote machine is responding
and, by extension, if it is accessible over the network.
PING is an application that makes use of the Internet Control Message Protocol (ICMP) protocol, and allows the user to test the reachability of another host:
Given a host A and a host B. The host B is
reachable from host A, if (IP) packets send by host A arrive at host B and
can/are being processed by host B.
If a host is unreachable, there can be either a problem with the communication path (i.e. there is no communication path) or the other host may be down. Further diagnostics is needed, one can use for instance TRACEROUTE for this purpose. Ping relies on the ICMP protocol, which is used to diagnose transmission conditions. For this reason, it uses two types of protocol messages (out of the 18 offered by ICMP): Type 0, which corresponds to an "echo request" command, sent by the source machine; Type 8, which corresponds to an "echo reply" command, sent by the target machine. At regular intervals (by default, every second), the source machine (the one running the ping command) sends an "echo request" to the target machine. When the "echo reply" packet is received, the source machine displays a line containing certain information. If the reply is not received, a line saying "request timed out" will be shown.
The Internet Ping program works much like a sonar
echo-location, sending a small packet of information containing an ICMP
ECHO_REQUEST to a specified computer, which then sends an ECHO_REPLY packet in
return. The IP address 127.0.0.1 is set by convention to always indicate your
own computer. Therefore, a ping to that address will always ping yourself and
the delay should be very short. This provides the most basic test of your local
communications.
The PING application uses ICMP messages to test the reachability, such ICMP messages are encapsulated in IP packets. The PING application uses two ICMP messages: the ICMP echo request message, and the ICMP echo reply message.
When a PING message is received by the receiving
host, it sends back an echo reply message in which the identifier, sequence
number and optional data field are left unchanged. The identifier field in the
ICMP echo reply message is used to allow
identification of the PING process running at the sending host. By the sequence
number field the sending host can keep track of for which ICMP echo request
messages a reply massage has been received. The optional data field is used to
store the time at which the ICMP echo request message has been send. On receipt
of the associated reply message the sending host can determine the time that was
needed for the packet to traval through the Internet to the target host and back
again: i.e. the round trip time (RTT).
Q.Explain DIG command with Example ?
Ans: dig is a command-line tool for querying
DNS name servers for information about host addresses, mail exchanges, name
servers, and related information.
$ dig www.eduinkerala.com
The opening section of dig’s output
tells us a little about itself (version 9.2.3) and the global options that are
set (in this case, printcmd). This part of the output can be quelled by using
the +nocmd option, but only if it’s the very first argument on the command line
(even preceding the host you’re querying).
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 41369 ;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 13, ADDITIONAL: 4 Here, dig tells us some technical details about the answer received from the DNS server. This section of the output can be toggled using the +[no]comments option—but beware that disabling the comments also turns off many section headers.
;; QUESTION
SECTION:
;www.eduinkerala.com. IN A
In the question section, dig reminds us of
our query. The default query is for an Internet address (A). You can turn this
output on or off using the +[no]question option.
;; ANSWER SECTION:www.eduinkerala.com. 14400 IN CNAME eduinkerala.com. eduinkerala.com. 14386 IN A 182.18.135.36 Finally, we get our answer: the address of www.eduinkerala.com is 182.18.135.36. I don’t know why you’d ever want to turn off the answer, but you can toggle this section of the output using the +[no]answer option.
a.Get the address(es) for
eduinkerala.com
; <<>> DiG
9.3.6-P1-RedHat-9.3.6-16.P1.el5_7.1 <<>> www.eduinkerala.com A +noall
+answer
;; global options: printcmdwww.eduinkerala.com. 13215 IN CNAME eduinkerala.com. eduinkerala.com. 13201 IN A 182.18.135.36
b.Get a list of eduinkerala.com mail
servers.
; <<>> DiG
9.3.6-P1-RedHat-9.3.6-16.P1.el5_7.1 <<>> www.eduinkerala.com MX +noall
+answer
;; global options: printcmdwww.eduinkerala.com. 13168 IN CNAME eduinkerala.com. eduinkerala.com. 14400 IN MX 0 mx.eduinkerala.com. eduinkerala.com. 14400 IN MX 0 eduinkerala.com. c. get a list of DNS servers authoritative for eduinkerala.com.
; <<>> DiG
9.3.6-P1-RedHat-9.3.6-16.P1.el5_7.1 <<>> www.eduinkerala.com NS +noall
+answer
;; global options: printcmdwww.eduinkerala.com. 13141 IN CNAME eduinkerala.com. eduinkerala.com. 85142 IN NS ns2.quick2host.co.in. eduinkerala.com. 85142 IN NS ns1.quick2host.co.in.
d.Get ANY
; <<>> DiG
9.3.6-P1-RedHat-9.3.6-16.P1.el5_7.1 <<>> www.eduinkerala.com ANY
+noall +answer
;; global options: printcmdwww.eduinkerala.com. 13098 IN CNAME eduinkerala.com.
e.
More obscurely, for the present anyway, you can also poll for a host’s
IPv6 address using the AAAA option.
#dig www.eduinkerala.com AAAA
+short
eduinkerala.com.
f.
Get a short answer
g.Get long answer.
According to its man page, the +multiline
option will give you an answer with “the SOA records in a verbose multi-line
format with human-readable comments.” In general, the answers retrieved using
the +multiline option will appear more like BIND config files than they will
without it.
#dig +nocmd eduinkerala.com any +multiline
+noall +answer
eduinkerala.com. 84932 IN NS ns1.quick2host.co.in. eduinkerala.com. 84932 IN NS ns2.quick2host.co.in. eduinkerala.com. 14164 IN MX 0 mx.eduinkerala.com. eduinkerala.com. 14164 IN MX 0 eduinkerala.com. eduinkerala.com. 12917 IN A 182.18.135.36
h.Query a different
nameserver.
#dig @ns1.quick2host.co.in. www.eduinkerala.com
If you want to look up a large number of
hostnames, you can put them in a file (one name per line) and use the -f option
to query each one in turn.
#dig -f
/path/to/host-list.txt
# the same, with more focused
output
dig -f /path/to/host-list.txt +noall +answer i.Perhaps you’re a devotee of trace route and like to watch how to get from point A to point B. You can do a similar thing with dig’s +trace option.
#dig eduinkerala.com +trace
j.Use the -x option to lookup the main hostname
associated with an IP address.
#dig -x 182.18.135.36
+short
ns1.quick2host.co.in. |
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