ASP.NET,C#.NET,SQL
Server,.NET Framework interview questions
Here I
am posting the interview questions whatever i have faced in my interviews
I have searched for so many websites and gathered information
from my friends to answer the questions perfectly.
i think these questions are very helpful for the people who are
trying to get the job on .NET
The most common question for experience persons is
Why would you like to change the company?
1) I am looking for a more challenging career in a firm with a
larger employee base such as yours.
2) Keeping in mind my career goals, the time has come for me to
move onto the next rung of
the ladder and make a mark for myself. This can be achieved in a
company like this.
3) It is just a career move to enhance my knowledge in my own area
of interest.
After completion of this question only interview will go for
further questions
1) Procedure can return zero or n values whereas function can
return one value which is mandatory.
2) Procedures can have input, output parameters for it whereas
functions can have only input parameters.
3) Procedure allows select as well as DML statement in it whereas
function allows only select statement in it.
4) Functions can be called from procedure whereas procedures
cannot be called from function.
5) Exception can be handled by try-catch block in a procedure
whereas try-catch block cannot be used in a function.
6) We can go for transaction management in procedure whereas we
can't go in function.
7) Procedures cannot be utilized in a select statement whereas
function can be embedded in a select statement.
Difference between Abstract and Interface
Abstract Class:
-Abstract class provides a set of rules to implement next class
-Rules will be provided through abstract methods
-Abstract method does not contain any definition
-While inheriting abstract class all abstract methods must be
override
-If a class contains at least one abstract method then it must be
declared as an “Abstract Class”
-Abstract classes cannot be instantiated (i.e. we cannot create
objects), but a reference can be created
-Reference depends on child class object’s memory
-Abstract classes are also called as “Partial abstract classes”
-Partial abstract class may contain functions with body and
functions without body
-If a class contains all functions without body then it is called
as “Fully Abstract Class” (Interface)
Interface:
-If a class contains all abstract methods then that class is known
as “Interface”
-Interfaces support like multiple inheritance
-In interface all methods r public abstract by default
-Interfaces r implementable
-Interfaces can be instantiated, but a reference cannot be created
Index types in SQL Server
Clustered Index
Only 1 allowed per table physically rearranges the data in the
table to confirm to the index constraints for use on columns that are frequently
searched for ranges of data for use on columns with low selectivity.
Non-Clustered Index
Up to 249 allowed per table creates a separate list of key values
with pointers to the location of the data in the data pages For use on columns
that are searched for single values
A clustered index is a special type of index that reorders the way
records in the table are physically stored. Therefore table can have only one
clustered index. The leaf nodes of a clustered index contain the data pages. A
non-clustered index is a special type of index in which the logical order of
the index does not match the physical stored order of the rows on disk. The
leaf node of a non-clustered index does not consist of the data pages. Instead,
the leaf nodes contain index rows.
Included Column Index (New in SQL Server 2005)
In SQL Server 2005, the functionality of non-clustered indexes is
extended by adding non-key columns to the leaf level of the non-clustered
index. Non-key columns can help to create cover indexes. By including non-key
columns, you can create non-clustered indexes that cover more queries. The
Database Engine does not consider non-key columns when calculating the number
of index key columns or index key size. Non-key columns can be included in
non-clustered index to avoid exceeding the current index size limitations of a
maximum of 16 key columns and a maximum index key size of 900 bytes. Another
advantage is that using non-key column in index we can have index data types
not allowed as index key columns generally.
In following example column Filename is varchar(400), which will
increase the size of the index key bigger than it is allowed. If we still want
to include in our cover index to gain performance we can do it by using the
Keyword INCLUDE.
USE AdventureWorks
GO
CREATE INDEX IX_Document_Title
ON Production.Document (Title, Revision)
INCLUDE (FileName)
Non-key columns can be included only in non-clustered indexes.
Columns can’t be defined in both the key column and they INCLUDE list. Column
names can’t be repeated in the INCLUDE list. Non-key columns can be dropped
from a table only after the non-key index is dropped first. For Included Column
Index to exist there must be at least one key column defined with a maximum of
16 key columns and 1023 included columns.
Avoid adding unnecessary columns. Adding too many index columns,
key or non-key as they will affect negatively on performance. Fewer index rows
will fit on a page. This could create I/O increases and reduced cache
efficiency. More disk space will be required to store the index. Index
maintenance may increase the time that it takes to perform modifications,
inserts, updates, or deletes, to the underlying table or indexed view.
Another example to test:
Create following Index on Database AdventureWorks in SQL SERVER
2005
USE AdventureWorks
GO
CREATE NONCLUSTERED INDEX IX_Address_PostalCode
ON Person.Address (PostalCode)
INCLUDE (AddressLine1, AddressLine2, City, StateProvinceID)
GO
Test the performance of following query before and after creating
Index. The performance improvement is significant.
SELECT AddressLine1, AddressLine2, City, StateProvinceID, PostalCode
FROM Person.Address
WHERE PostalCode BETWEEN '98000'
AND '99999';
GO
Interview questions
What are differences between Array list and Hash table?
Ans: 1) Hash table store data as name, value pair. While in array only
value is store.
2) To access value from hash table, you need to pass name. While
in array, to access value, you need to pass index number.
3) you can store different type of data in hash table, say int,
string etc. while in array you can store only similar type of data.
What are differences between system.stringbuilder and
system.string?
The main difference is system.string is immutable and
system.stringbuilder is a mutable. Append keyword is used in string builder but
not in system.string.
Immutable means once we created we cannot modified. Suppose if we
want give new value to old value simply it will discarded the old value and it
will create new instance in memory to hold the new value.
What are the differences between Application object and session
object?
Ans: The session object is used to maintain the session of each user.
If one user enter in to the application then they get session id if he leaves
from the application then the session id is deleted. If they again enter in to
the application they get different session id.
But for application object the id is maintained for whole
application.
What are the different types of indexes?
Ans: Two types of indexes are there one is clustered index and
non-clustered index
How many types of memories are there in .net?
Ans: Two types of memories are there in .net stack memory and heap
memory
Is it possible to set the session out time manually?
Ans: Yes we can set the session out time manually in web.config.
What are differences between function and stored procedure?
Ans:
1) Function returns only one value but procedure returns one or more
than one value.
2) Function can be utilized in select statements but that is not
possible in procedure.
3) Procedure can have an input and output parameters but function has
only input parameters only.
4) Exceptions can be handled by try catch block in procedures but
that is not possible in function.
What are the differences between Abstract and interface?
Ans: 1) Abstract cannot be instantiated but we can inherit.
Interface it cannot be inherit it can be instantiate
2) Interface contain only declarations no definitions. Abstract
contain declarations and definitions.
3) The class which contains only abstract methods is interface
class. A class which contains abstract method is called abstract class
4) Public is default access specifier for interface we don’t have
a chance to declare other specifiers. In abstract we have chance to declare
with any access specifier
Can you Explain Page lifecycle in .net?
Can you Explain .NET architecture in .net?
What is the difference between primary key and unique key with not
null?
Ans: There is no difference between primary key and unique key with not
null.
What is boxing and unboxing concepts in .net?
Ans: Boxing is a process of converting value type into reference type
Unboxing is a process of converting reference type to value type.
What are the differences between value type and reference type?
Ans: Value type contain variable and reference type are not containing value directly in its memory.
Memory is allocated in managed heap in reference type and in value
type memory allocated in stack. Reference type ex-class value type-struct,
enumeration
Is it possible to host the website from desktop?
Ans: Yes
Why we go for page rendering in Asp.Net Page life cycle?
Ans: Browser understands an only html control that’s why in page
rendering we will convert the aspx controls into html controls.
Write a sample query for self join?
Ans: Select e1.ename, e2.empid from emp e1, emp e2 where
e1.empid=e2.mgrid;
Can we change the index of primary key on table?
Ans: No
How to change the name of the table or stored procedure in sql?
Ans: sp_rename oldtablename newtablename
For changing the column name
Sp_rename
‘tablename.[Oldcolumnname]’,’newcolumnname’,’Column’
Ex:sp_rename ‘tblemp.first’,’namechange’,’Column’
How to find out which index is defined on table?
Ans: sp_helpindex tablename
Can you write the program to find the length of string without
using library function?
Ans: for (int i=0; str[i]!=”\n”; i++)
{
Count++;
}
What is the difference between scope_identity() and
current_identity()?
Ans: Scope_identity and current _identity both are similar and it will
return the last identity value generated in the table.
Scope_Identity will return the identity value in table that is
currently in scope
What are difference between GET and POST Methods?
Ans:
GET Method ():
1) Data is appended to the URL.
2) Data is not secret.
3) It is a single call system
4) Maximum data that can be sent is 256.
5) Data transmission is faster
6) this is the default method for many browsers
POST Method ():
1) Data is not appended to the URL.
2) Data is Secret
3) it is a two call system.
4) There is no Limit on the amount of data. That is characters any
amount of data can be sent.
5) Data transmission is comparatively slow.
6) No default and should be explicitly specified.
What are difference between truncate and delete?
Ans: 1) Delete keep the lock over each row where Truncate keeps the
lock on table not on all the row.
2) Counter of the Identity column is reset in Truncate where it is
not reset in Delete.
3) Trigger is not fired in Truncate where as trigger is fired in
Delete.
4) In TRUNCATE we cannot rollback.
5) In DELETE we can rollback
What is the difference Grid View and between Data Grid (Windows)?
Ans:
1) GridView Control Enables you to add sorting, paging and editing
capabilities without writing any code.
2)GridView Control Automatically Supports paging by setting the
‘PagerSetting’ Property.The Page Setting Property supports four Modles
a. Numeric(by default)
b. Next Previous
c. NumericFirstLast
d. Next PreviousLast
3)It is Used in asp.net
4)GridView Supports RowUpdating and RowUpdated Events.
5)GidView is Capable of Pre-Operations and Post-Operations.
6)GridView Has EditTemplates for this control
7)It has AutoFormat
DataGrid(Windows)
1)DataGid Control raises single Event for operations
2)DataGird Supports the SortCommand Events that occur when a
column is Soted.
3)DataGrid Supports UpdataCommand Event that occurs when the
UpdateButton is clicked for an item in the grid.
4)DataGrid is used in Windows GUI Application.
5)It doesnot have EditTemplates for this control
6)It doesnot have AutoFormat
If I write System.exit (0); at the end of the try block, will the
finally block still execute?
Ans: No in this case the finally block will not execute because when
you say system.exit(0),the control immediately goes out of the program, and
thus finally never executes.
What are the different levels of State management in ASP.NET?
Ans:
State management is the process by which you maintain state and
page information over multiple requests for the same or different pages.
There are 2 types State Management:
1. Client – Side State Management
This stores information on the client's computer by embedding the
information into a Web page, a uniform resource locator (url), or a cookie. The
techniques available to store the state information at the client end are
listed down below:
a. View State – Asp.Net uses View State to track the values in the
Controls. You can add custom values to the view state. It is used by the
Asp.net page framework to automatically save the values of the page and of each
control just prior to rendering to the page. When the page is posted, one of
the first tasks performed by page processing is to restore view state.
b. Control State – If you create a custom control that requires
view state to work properly, you should use control state to ensure other
developers don’t break your control by disabling view state.
c. Hidden fields – Like view state, hidden fields store data in an
HTML form without displaying it in the user's browser. The data is available
only when the form is processed.
d. Cookies – Cookies store a value in the user's browser that the
browser sends with every page request to the same server. Cookies are the best
way to store state data that must be available for multiple Web pages on a web
site.
e. Query Strings - Query strings store values in the URL that are
visible to the user. Use query strings when you want a user to be able to
e-mail or instant message state data with a URL.
2. Server – Side State Management
a. Application State - Application State information is available
to all pages, regardless of which user requests a page.
b. Session State – Session State information is available to all
pages opened by a user during a single visit.
Both application state and session state information is lost when
the application restarts. To persist user data between application restarts,
you can store it using profile properties.
Abstract Class:
Abstract class is a class which can’t be instantiate. Class
should have “Abstract” key word with the name. In any one of the method
of class having abstract method with in it, then it should be define as
abstract class. The class which derived the abstract class should have
definition of the abstract method. These classes which derived the abstract
class and implement the abstract methods call concrete class.
Abstract class may have the definition of function or may
not. Below is the simple example of an abstract class
public abstract alass AbstractStudent
{
String Roll
{
get;
set;
}
String FirstName
{
get;
set;
}
String LastName
{
get;
set;
}
Public String GetStudentDetails()
{
//
Implementation of Method
}
public String SaveStudentDetails ()
{
//
Implementation of Method
}
public abstract String
CalculateWage();
}
So, the class having one abstract method so we need to mention the
class as "abstract" .
Difference between Abstract Class and Interface?
Abstract class is a class which can’t be instantiated and which can have methods with
definition as well as declaration also. This can be inherited.
As for Example:
public abstract class AbstractStudent
{
String Roll
{
get;
set;
}
String FirstName
{
get;
set;
}
String LastName
{
get;
set;
}
Public String GetStudentDetails()
{
//
Implementation of Method
}
public String SaveStudentDetails ()
{
//
Implementation of Method
}
public abstract String
CalculateWage();
}
Interface can only contain the methods declaration and can be
implemented in the class.
As for Example:
Public interface IStudnet
{
String Roll
{
get;
set;
}
String FirstName
{
get;
set;
}
String LastName
{
get;
set;
}
String GetStudentDetails();
String SaveStudentDetails ();
}
Below are the few main difference between Abstract Class and
Interface
a. In abstract class method can have definition
as well as declaration also. But Interface should have only definition.
b. All the Methods are Public as default
and don’t have any access Modifier Controls in interface, whereas for
abstract class we can have access modifier for methods.
c. Abstract class can have constructor or
destructor, whereas interface not.
d. Abstract class can’t be part of multiple
inheritance and we can implement multiple interface.
What do you mean by String objects are immutable?
String objects are immutable as its state cannot be modified
once created. Every time when we perform any operation like add, copy, replace,
and case conversion or when we pass a string object as a parameter to a method
a new object will be created.
Example:
String str = "ABC";
str.Replace("A","X");
Here Replace() method will not change data that
"str" contains, instead a new string object is created to hold
data "XBC" and the reference to this object is returned by
Replace() method.
So in order to point str to this object we need
to write below line.
str = str.Replace("A","X");
Now the new object is assigned to the variable str. earlier object
that was assigned to str will take care by garbage collector as this one
is no longer in used.
What is dll hell problem in .NET and how it will solve?
Ans: Dll hell, is kind of conflict that occurred previously, due to the
lack of version supportability of dll for (within) an application
.NET Framework provides operating system with a global assembly
cache. This cache is a repository for all the .net components that are shared
globally on a particular machine. When a .net component installed onto the
machine, the global assembly cache looks at its version, its public key and its
language information and creates a strong name for the component. The component
is then registered in the repository and indexed by its strong name, so there
is no confusion between the different versions of same component, or DLL
What is a Partial class?
Ans: Instead of defining an entire class, you can split the definition
into multiple classes by using partial class keyword. When the application
compiled, c# compiler will group all the partial classes together and treat
them as a single class. There are a couple of good reasons to use partial
classes. Programmers can work on different parts of classes without needing to
share same physical file
Ex:
Public partial class employee
{
Public void somefunction()
{
}
}
Public partial class employee
{
Public void function ()
{
}
}
What is difference between constants, read-only and, static?
Constants: The value can’t be changed
Read-only: The value will be initialized only once from the
constructor of the class.
Static: Value can be initialized once.
What is the cross page post backing?
Asp.Net 2.0 fixed this with built-in features that allowed us to
easily send information from one page to another.
Button control has property PostBackUrl that can
be set to URL of any page in our ASP.Net WebSite where we want to transfer all
form values to.
Along with that Asp.Net 2.0 Page class has a
property PreviousPage that allows us to get reference to the
Page object that initiated the postback (in other words to get the actual
reference to the Page object of the aspx page on which user clicked the Submit
button on a HTML form).
So for example lets create two sample pages in our Web
Application:
· SourcePage.aspx
· DestinationPage.aspx
In SoucePage in Html form we will put two TextBox controls (one
for First Name and one for Last Name) and one Button component and set
its PostBackUrl property to "~/DestinationPage.aspx".
SourcePage.aspx:
<form id="form1" runat="server">
<div>
First
Name: <asp:TextBox ID="FirstName" runat="server"></asp:TextBox><br />
Last
Name: <asp:TextBox ID="LastName" runat="server"></asp:TextBox><br /><br />
<asp:Button ID="Button1" runat="server" Text="Submit To Destination Page"PostBackUrl="~/CrossPagePostbacks/DestinationPage.aspx" />
</div>
</form>
When our user clicks the Submit button, all the values from the
HTML Form on SourcePage.aspx will be transfered to the DestinationPage.aspx and
we will also be able to get reference to the SourcePage.aspx in our
DestinationPage with the PreviousPage property like this:
So in our DestinationPage.aspx.cs code-behind we can easily access
two TextBox controls on SourcePage.aspx and show them in two label controls
like this:
protected void Page_Load(object sender, EventArgs e)
{
// first check if we had a
cross page postback
if ( (PreviousPage != null) &&
(PreviousPage.IsCrossPagePostBack) )
{
Page previousPage = PreviousPage;
TextBox firstName = (TextBox)previousPage.FindControl("FirstName");
TextBox lastName = (TextBox)previousPage.FindControl("LastName");
// we can now use the values
from TextBoxes and display them in two Label controls..
labelFirstName.Text = firstName.Text;
labelLastName.Text = lastName.Text;
}
}
You probably noticed that we first checked if PreviousPage property
of current page (DestinationPage.aspx) is NOT NULL, this is done to
avoid running our code in case that user opens our DestinationPage.aspx
directly, without running a cross page postback.
Also here we checked the another PreviousPage property
called IsCrossPagePostBack to see if we really had a CrossPagePostback.
(If Server.Transfer is used to redirect to this page, IsCrossPagePostBack property
will be set to FALSE.
TIP: We can be completely sure
that we have a real CrossPagePostback ONLY IF:
1. Page.PreviousPage is NOT NULL,
2. PreviousPage.IsCrossPagePostback is true
This important to check to avoid errors in code.
Now this is very useful and i'm sure you are eager to use this in
your next project. But wait, we are not over yet!
Finding the controls on PreviousPage with FindControl method and
type-casting them from object to their real type is
a little messy.
It feels like there must be a better solution for this!
And here it is: We can use the <%@ PreviousPageType
%> directive in the header of our DestinationPage.aspx like this
<%@ PreviousPageType VirtualPath="~/SourcePage.aspx" %>
to declare our previous page type, and then we can access Public
properties of the PreviousPage without typecasting.
Now all we need to do is to create some public properties on our
SourcePage.aspx.cs to expose data/Controls we want to the destionation page:
public partial class SourcePage : System.Web.UI.Page
{
public string FormFirstName
{
get { return FirstName.Text; }
}
public string FormLastName
{
get { return LastName.Text; }
}
}
And then we can change the Page_Load code in our
DestinationPage.aspx to much cleaner code like this:
protected void Page_Load(object sender, EventArgs e)
{
// first check if we had a
cross page postback
if ( (PreviousPage != null) &&
(PreviousPage.IsCrossPagePostBack) )
{
SourcePage prevPage = PreviousPage;
// we can now use the values
from textboxes and display them in two Label controls..
labelFirstName.Text = prevPage.FormFirstName;
labelLastName.Text = prevPage.FormLastName;
}
}
SourcePage type used in the code is
offcourse name of the partial class defined is SourcePage.aspx.cs that
inherits System.Web.UI.Page that is automatically created for us when we created new
WebForm in VisualStudio.
This code is much cleaner and easier to follow, there is no ugly
typecasting, just simple property values to use to retrieve the data from
previous page.
When should you use Abstract Class vs Interface while programming?
Ans: When we want that sub
class must implement all the methods of base class. In such a situation we will
implement the interface. In the other hand when we want only some
method of base class in our sub class then use base class as abstract class.
What is the difference between application exception and system
exception?
Ans: The difference between application exception and system exception
is that system exceptions are thrown by CLR and application exceptions are
thrown by applications.
What is the difference between authorization and authentication?
Ans: Authorization is a process of allowing or denying resources to
particular user or record
Declaration of authorization is
<authorization>
<allow users=”Suresh, Sanjay”/>
<deny users=”Ramana, Rakesh”>
</authorization>
Sometimes authorization allows the unauthorized persons at that
time we will use
<deny users=”?”/>
Authentication means
Authentication is a process where we identify the credentials of
user i.e. username, password and create an identity to mention user as an
authenticated.
What is the use of n-tier architecture and 3-tier architecture?
Check this article for 3-tier architecture 3 tier architecture example in asp.net
How to get the version of the assembly?
Ans: lbltxt.text=Assembly.
GetExecutingAssembly().GetName().Version.ToString();
What is the location of Global Assembly Cache on the system?
Ans: c:\Windows\assembly
What is the serialization?
Ans: Serialization is a process of converting object into a stream of
bites.
What is synchronization?
Ans: The mechanism needed to block one thread access to the data. If
the data is being accessed by another thread.
Synchronization can be accessed by using system.monitor class
A monitor class methods are enter, exit, pulse for this lock
statement is also used
Suppose if we need to synchronize some data at that time we need
to place that data in this block
Lock
{
}
Whatever the data has been placed into the lock block that data
has been blocked
What are the thread priority levels?
Ans: Thread priority levels are five types
0 - Zero level
1 - Below Normal
2 - Normal
3 - Above Normal
4 - Highest
By Default priority level is 2
What is the difference between .tostring(), Convert.tostring()?
Ans: The basic difference between them is “Convert” function handles
NULLS while
“.ToString()” does not it will throw a NULL reference exception
error. So as a good coding
practice using “convert” is always safe.
What is Collation?
Ans: Collation refers to a set of rules that determine how the data is
sorted and compared.
What is the difference between Primary key and unique key?
Ans: Primary key does not allow the null values but unique key allows
one null value.
Primary key will create clustered index on column but unique key
will create non-clustered index by default.
How many web.config files are there in 1 project?
Ans: There might be multiple web.config files for a single project
depending on the hierarchy of folders inside the root folder of the project, so
for each folder we can use one web.config file
What is the difference between throw and throw ex?
What is the difference between view state and hidden field?
Ans: viewstate is secured hidden field is insecure
Viewstate will store large amount of data but hidden filed will
store small amount of data.
What is the difference between binary serialization and xml
serialization?
What is the Difference between read only and constant variables?
Ans: Read only can assign the values at runtime only.
Constant will assign the values at compile time only.
We cannot modify the both variable values.
What is static keyword in .Net?
Ans: Static is same as constant variable but we can change the value of
static variable and we can access the variables without creating any instances
What is the use of business logic layer in 3-tier architecture in
.net?
Ans: Though a web site could talk to the data access layer directly, it
usually goes through another layer called the business layer. The business
layer is vital in that it validates the input conditions before calling a
method from the data layer. This ensures the data input is correct before
proceeding, and can often ensure that the outputs are correct as well. This
validation of input is called business rules, meaning the rules that the
business layer uses to make “judgments” about the data.
However, business rules don’t only apply to data validation;
these rules apply to any calculations or any other action that takes place in
the business layer. Normally, it’s best to put as much logic as possible in the
business layer, which makes this logic reusable across applications.
One of the best reasons for reusing logic is that applications
that start off small usually grow in functionality. For instance, a company
begins to develop a web site, and as they realize their business needs, they
later decide to add a smart client application and windows service to
supplement the web site. The business layer helps move logic to a central layer
for “maximum reusability.”
What happens when I enter a URL in my browser and click enter?
You type in the URL and hit go. The browser needs to translate
that URL www.somesite.com into an IP address so it knows what computer on the
internet to connect to (That URL is just there to make it easier for us humans
- kinda like speed-dial for phone numbers I guess). So your browser will see if
it already has the appropriate IP address cached away from previous visits to
the site. If not, it will make a DNS query to your DNS server (might be your
router or your ISP's DNS server) - seehttp://en.wikipedia.org/wiki/Domain_name… for more on DNS. Once your browser knows what
IP to use, it will connect to the appropriate webserver and ask for the page.
The webserver then returns the requested page and your browser renders it to
the screen.
The firewall will control connections to & from your computer.
For the most part it will just be controlling who can connect to your computer
and on what ports. For web browsing your firewall generally won't be doing a
whole lot.
Your router (see http://en.wikipedia.org/wiki/Router )
essentially guides your request through the network, helping the packets get
from computer to computer and potentially doing some NAT (seehttp://en.wikipedia.org/wiki/Network_add… )
to translate IP addresses along the way (so your internat LAN request can be
transitioned onto the wider internet and back).
IP Addresses (see http://en.wikipedia.org/wiki/IP_address )
are unique addresses for computers that basically allow computers to find each
other. Think of the IP address as a computer's well address or phone number,
you've got to know someone's phone number before you can call them and you've
got to know a computer's IP address before you can connect to it. Going back to
the start - that's what those URLS and DNS make possible, you don't know John Doe's
phone number so you look in the phone book; likewise your computer doesn't know
yahoo.com's IP address so it looks in DNS.
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