Why Your Router May Be Blocking Web Sites

I think your router’s Maximum Transmission Unit (MTU) is too large for the Web sites or e-mail servers you’re trying to communicate with. The MTU is the largest size packet or frame, specified in octets (eight-bit bytes), that can be sent in a packet or frame-based network such as the Internet. The Internet4s Transmission Control Protocol (TCP) uses the MTU to determine the maximum size of each packet in any transmission. Too large an MTU size may mean retransmissions if the packet encounters a router that can’t handle that large a packet. Too small an MTU size means relatively more header overhead and more acknowledgements that have to be sent and handled. Most computer operating systems provide a default MTU value that’s suitable for most users.

For example, in Windows 95, the default MTU was 1,500 octets, partly because this is the Ethernet standard MTU. The Internet de facto standard MTU is 576, but many ISPs often suggest using 1,500. If you frequently access Web sites that encounter routers with an MTU size of 576, you may want to change to that size. Apparently some people have found that changing the MTU setting to 576 improves performance while others haven’t noticed any improvement at all. The minimum value for an MTU is 68. In more recent Windows systems, the operating system can sense whether your connection should use 1,500 or 576, and it selects the appropriate MTU for the connection.

If, however, you are experiencing problems, you might want to try manually adjusting your router’s MTU settings and see if it makes a difference. People who have had problems sending or receiving e-mail, or connecting to secure sites such as eBay, banking sites and Hotmail, have reported this phenomenon. So I suggest you try decreasing your current MTU settings in increments of ten (e.g., 1,492, 1,482, 1,472 and so on). To make this modification on your router simply follow these steps:

  • Open your browser, enter your router’s IP address (typically in the address bar and click OK.
  • Enter your username (admin) and password (blank by default). Click OK to enter the Web configuration page for the device.
  • Click on the Home tab and go to the WAN tab.
  • The default MTU is 1,500. Enter the new value in the MTU field and click the Apply button to save your settings.
  • If changing the MTU size does not resolve the problem, continue decreasing the MTU value in increments of ten.

If you do not have an MTU option in your router, you might need to upgrade your firmware. If you still have problems, contact D-Link technical support for assistance at. I hope this helps.

And FYI: AOL DSL+ users must set their MTU for 1,400


Posted on August 2, 2009, in Web Dev and tagged . Bookmark the permalink. Leave a comment.

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