webpage request

what happens when we type a url like “www.google.com” in web browser

To understand in better  way :

Suppose you know the address of  three Networks :—-

                                               1.     Your college network—-

Network address—68.80.2.0/24

                                                                 

 A  college  router :——

                                                                     IP address ——  68.85.2.1

                                                                     MAC address—-00.22.6B.45:1F:1B

 

And  your laptop from which  you request for web page is connected to  switch which is  in turn connected to college router—-

 

           IP address—    68.85.2.101 (ip address not initially given by dhcp server in step – 5)

 

           MAC address—00:16:D3:23:68:8A

 

2. A  internet service provider(ISP) to which college router is connected  to the router of ISP

 

            ISP   Network address:–   68.80.0.0/13

 

DNS server is situated here –

 

             IP address  —68.87.71.226

 

3. A google network

 

             Network Address – 64.233.160.0/19

 

Google web server IP –  64.233.169.105l

 

You boots up your laptop and then connects it to Ethernet cable connected  to the college Ethernet switch.

Which in turn is connected to college’s router.. router is connected to an ISP which is providing DNS service

for college .we assume DHCP server is running with  in router.

 

The interesting thing is  your laptop can’t do anything(open web page) without ip address..

 

First  action taken by your laptop is to run DHCP protocol to obtain :—-

 

IP address

 

Address of first hop  router.

 

Address of DNS server

 

Main steps to be followed :

 

DHCP Request———->

 

DNS Query And ARP ————–>

 

INTRA DOMAIN ROUTING  to DNS SERVER ———–>

 

 

Web client server interaction :—— > ( TCP AND HTTP)

 

DHCP Request———->

 

 

 

1 .     The OS of your laptop creates a DCHP request message and puts this message with in a UDP segment  with

DES PORT -67 (DHCP server)

 

Source port – 68(DCHP client).

 

The UDP segment is placed in Ip datagram     destination add – 255.255.255.255

 

you do”t have ip add                                             Source add–  0.0.0.0

 

2. IP Datagram is then placed  with in Ethernet frame :—

 

DES  MAC add—FF.FF.FF.FF.FF.FF (broadcast add)

 

Your laptop          MAC  Source add–  00:16:D3:23:68:8A

 

3.The broadcast Ethernet frame containing DHCP request is the first frame sent by your laptop to the Ethernet

switch.  Switch broadcast the incoming frame on all outgoing ports including port connected to  router..

 

4. The router receives that frame contains DHCP request  —

 

Ethernet demultiplexed to IP datagram

 

IP    datagram demultiplexed    to  UDP segment

 

From UDP segment  DHCP message is extracted

 

5.DHCP  server allocate ip address from  pool of  68.80.2.0/24  to your

 

Laptop.    Now  DHCP server creates a ACK message containing

 

IP Address —-

 

IP address of DNS server—

 

IP address of default gateway router—–

 

This dhcp message put in to UDP segment put in IP datagram

 

which is put inside Ethernet frame..

 

it has    destination add — 00:16:D3:23:68:8A

 

source         add–  00.22.6B.45:1F:1B

 

6. the Ethernet  frame is sent by router to the switch  which is also connected to your laptop .. switch forward

it towards your lap top.

 

7. your laptop receive this Ethernet frame then  extracts the ip datagram from it ..     

 

Then extracts udp segment from ip datagram

 

Extract  DHCP ACK message from UDP segment..

 

Install all addresses  given by dhcp message and send datagram with destination address outside of its subnet

68.85.2.0/24

 

DNS Query And ARP ————–>

 

8. The os of your laptop thus creates a dns query message putting www.google.com in dns message.   Dns

message is then placed in udp segment with Destination port   –   53(DNS server)

 

                         Destination ip add – 68.87.71.226 (DNS server)

 

                         Source  IP add –  your laptop

 

                        Destination MAC add –  ( address of router interface obtain by ARP)

 

9.  your  laptop then places the datagram into Ethernet frame this frame will be sent to  gateway router we

know  its ip address but do”t know its MAC address….  In order to find  MAC address    —

 

Use ARP protocol

 

10.  it creates ARP query

 

Destination ip add –  68.85.2.1

 

                      Destination mac add – FF.FF.FF.FF.FF.FF

 

Send it to switch which deliver it to all connected devices, including router.

 

11. Gate way router receive message containing ARP query message on its interface to college network..and its

ip add matches with ip add of interface of router interface.  Gateway router prepare ARP reply

 

Which contain MAC add –  00.22.6B.45:1F:1B

 

With   ip add – 68.85.2.1

 

Places the ARP reply message in Ethernet frame

 

Destination add of your laptop-   00.16.D3.23:68:8A

 

Send the frame to switch which deliver to your laptop.

 

12.  your lap top receives the frame      ARP reply and extract MAC address of router.

 

13. Now  your lap top can send Ethernet frame  containing  DNS query to  router  now packet has

 

Destination ip add —      68.87.71.226

 

Destination MAC add —   00.22.6B.45:1F:1B

 

INTRA DOMAIN ROUTING  to DNS SERVER ———–>

 

 

 

14.   the router receives packet of dns query and read it and check destination ip address of datagram in

forwarding table and sends the packet to the link connected to the router of ISP..

 

15. Router of ISP examines destination address and determines outgoing interface on which to forward the

datagram towards dns serve  from its forwarding table.

 

It is done by (RIP ,OSPF ,BGP) protocol   used from  router to dns  called inter domain protocol.

 

16 . DNS server receives the  ip datagram containing dns query .

 

It extracts  message and look up the name www.google.com

 

In its database and find the DNS resource record that has ip address of

 

Google web server 64.233.169.105. it do”t have it will ask to TLD dns server , root dns server,authorataive dns

server.

 

It forms DNS reply message containing this host name to ip address mapping. And places this in UDP segment

and then in IP datagram address to your laptop—-

 

17. your laptop finally has the ip address of google.com and ready to connect.

 

Web client server interaction :——   TCP AND HTTP

 

 

 

18. To send HTTP  Get request client first opens TCP socket to web server TCP SYN segment ( step one in  3 –

WAY HANDSHAKE)  destination port HTTP ( 80) ..

 

19. web serve responds with TCP SYN ACK(step – 2 in 3 – way handshake).

 

20 TCP connection is established.

 

21. HTTP request sent into tcp socket.

 

22. IP datagram containing HTTP request routed to www.google.com

 

23. Web server responds with HTTP reply(containing web page or images)

 

24. IP datagram containing HTTP reply routed back to client.

 

After this Request goes to  your Proxy server …

for more info visit http://igoro.com/archive/what-really-happens-when-you-navigate-to-a-url/

Thanks for reading this post

written by kapil kumar

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