BSNL has a Pan-India Presence. While it is forced by the central government to operate in remote locations, Indians are getting to enjoy the benefits of Internet thanks to the company. Well this article is not about extolling the virtues of the Internet Service Provider, but to remedy some of the strange problems that the service comes with.

Most of the Internet connections that BSNL provides come through fixed-line internet telephones. Though we can theoretically call it a Digital Subscriber Line (DSL), it is in fact a variation in the form of ADSL (Asynchronous Digital Subscriber Line). Interestingly, there are multiple iterations of ADSL like 1, 1.1, 2, 2+ etc. Though, the common user is usually not made aware of these.

Many of the ADSL users must have faced a weird issue of few sites not working or loading, while others are working at blazing speeds. For me popular sites like eBay, MSN & many others refused to work. In case they loaded, they took ridiculously long time to open up & that too devoid of formatting, with weird placement of tabs, forms etc. Since the ‘internet’ worked, I could not complain too. While many would blame the line or the modem, it’s not the case, albeit, the modem does tend to heat up a lot.

A little digging up offered a plausible solution.

The issue lies with incorrect setting of the MTU Value of the Modem. MTU or Maximum transmission Unit is the Maximum Size of each data packet which travels across the Internet. Larger sized MTU ensure bandwidth efficiency but they tend to choke interfaces. For the sake of understanding, it’s like pushing a large ball through a smaller pipe-line. Though a lot of material may pass at one time, the size of the ball will choke the pipeline. Hence it becomes necessary to break the large ball into smaller balls & then the pipeline will flow without a hindrance.

Now how to decide the most optimum MTU Size?

Since this guide is meant to help all BSNL users, I will mention the steps to identify the optimum MTU size for each user:

Start Command Prompt (Start -> Run -> Cmd)

At the DOS window that opens enter the following command AS IT IS. Do not alter:

ping -f -l 1492

If you have been facing problems with your net you should see an output as below:


The wordings to observe are: Packet needs to be fragmented but DF Set. This means the packet or MTU is too large to be sent in one shot.

Lower the size the packet in increments of (e.g. 1482, 1472, 1462, 1440, 1400) until you have a packet size that does not fragment & the command is able to ping successfully as below:


Now remember, many will have different values. For some it may work at 1400, for others at 1432 & so on. Once you have the value, let’s say 14XX Add 28 to that number (IP/ICMP headers) to get the optimal MTU setting. For example, if the largest packet size from ping tests is 1462, add 28 to 1462 to get a total of 1490 which is the optimal MTU setting and can be put in your router’s settings.

Let’s say the ideal value arrived through the above exercise is 1462, hence the number to be entered is 1490 as described below.

Open up Command Prompt if you have closed it & enter the following command:

netsh interface ipv4 set subinterface “Local Area Connection” mtu=1490 store=persistent

Note: The number is dependent on what you get in the test. In case you have received 1432, then the number to be entered in the string above will be 1432+28 = 1460. Though there is a provision of entering this MTU number via the Web-Interface of the ADSL Modem, it doesn’t seem to work.

To verify that the number has been set correctly, enter the string below in the Command Prompt: netsh interface ipv4 show subinterfaces


That’s it. All the sites should now work on your BSNL ADSL connection.

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