This may be a problem with many other broadband users, but I have noticed and faced this with my BSNL broadband connection. The problem was that, certain sites were not opening via the broadband.I tried with another internet connection and these sites were opening without any problem. So the problem was not with my system. Neither it was the result of some virus attack.
It is the MTU(Maximum transmission unit) and modifying it in the modem settings should solve the problem. This turned out to be a half-solution for my problem. To completely settle the issue, I had to change the MTU in modem as well as in windows settings. I shall explain how to achieve this if you also face such an issue.
dslreports.com says the following:
The MTU setting controls the maximum ethernet packet size your PC will send (you did know the Internet works in packets, didn’t you?). Why a limit? Because although larger packets can be constructed and sent, your ISP and Internet backbone routers and equipment will chop up (fragment) any packets larger than their limit. These parts are then reassembled by the target equipment before reading. This fragmentation and reassembly is not optimal.
Unless otherwise set, Windows defaults MTU to 1500, or a lower value of 576 for external networks
So the problem is that, this MTU 1500 may not be the correct one for your network. How to find the correct MTU then? Do a trial and error experiment.
Again, from dlsreports:
Windows 2000/XP users:
Go to Start/ Programs/ Accessories/ Command Prompt and type the following:
ping -f -l 1472 www.dslreports.com
(That is a dash lower case “L,” not a dash “1.” Also note the spaces in between the sections.)
ping -s 1472 www.dslreports.com
OS X users:
ping -D -s 1472 www.dslreports.com
Linux and OS X commands are case sensitive.
Press Enter. Then reduce 1472 by 10 until you no longer get the “packet needs to be fragmented” error message. Then increase by 1 until you are 1 less away from getting the “packet need to be fragmented” message again.
Add 28 more to this (since you specified ping packet size, not including IP/ICMP header of 28 bytes), and this is your MaxMTU.
Note:If you can ping through with the number at 1472, you are done! Stop right there. Add 28 and your MaxMTU is 1500.
For PPPoE, your MaxMTU should be no more than 1492 to allow space for the 8 byte PPPoE “wrapper,” but again, experiment to find the optimal value. For PPPoE, the stakes are high: if you get your MTU wrong, you may not just be sub-optimal, things like UPLOADING or web pages may stall or not work at all!
Once you have found the maxMTU, go to your modem settings. In the case of BSNL ADSL modem, open your browser, type http://192.168.1.1/, put the username and password (which is, by default, admin, admin).
In the interface setup (or an option similar to it) Change TCP MTU Option value to the maxMTU you have found above. Save the settings, and restart the modem.
Download the TCPOptimizer from SpeedGuide.net.
Click on “Choose settings: custom”
Enter the maxMTU you have found, and click on Apply changes
You may have to probably restart windows to see the effect of the change
In debian based systems (like Ubuntu), enter the following command in the terminal and press enter
sudo ifconfig eth0 mtu 1492
You may be prompted to enter the root password, enter it.
In the above, maxMTU is assumed to be 1492. Change it to the one you have found by trial and err.r
In non-debian linux systems, you have to simply login as root in the terminal
and use the command
ifconfig eth0 mtu 1492
I hope above solution will be useful to many BSNL broadband users, as there are many new ADSL modems coming into the market with MTU configured 1500