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Troubleshooting Q&A - March 10, 2005

Why Your Router May Be Blocking Web Sites

Could your wireless router really be keeping you from connecting to some Web sites and preventing you from sending and receiving e-mail? It could. The problem may lie in something called Maximum Transmission Units. Plus, how to setup TCP/IP using Mac OS X.

By Ron Pacchiano

Q. I was hoping you might be able to help me with a sporadic problem I've been experiencing. I have a PC running Windows XP Home Edition connected to a DSL line. Until recently, it had never given me any problems. My birthday had just passed and my uncle bought me a D-Link DI-624 wireless router. He helped me set it up and everything worked fine. As time went on, however, I started to notice that I was having difficulty connecting to certain Web sites or sending and receiving e-mails. These were sites and addresses that have been using for some time now, so I couldn't understand the why I was having a problem.

The really strange part was that I had no problem accessing these sites from other computers. So just to satisfy my increasing curiosity, I tired removing the D-Link router from my PC and plugged my DSL line directly into my PC. To my surprise, all of the sites that had been giving me trouble were now working flawlessly. Somehow the router was the cause of my problem. Thinking it might be defective, I had my uncle exchange it for me. After reinstalling it, those sites became inaccessible again. Do you have any idea what might be causing this to happen and what I need to do to

A. That is very strange. I've heard of this type of problem before, but never seen it first hand. I wish you would have included the links to some of those sites that had given you the problem so that I could have tested this first before advising you.

What I'm thinking is that your MTU or Maximum Transmission Unit 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 Internetīs 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 canīt 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 is 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 users have found that changing their MTU setting to 576 improves performance while others have not noticed any improvement at all. The minimum value that an MTU can be set to is 68. In more recent Windows systems, the operating system is able to sense whether your connection should use 1,500 or 576 and select 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. As I mentioned, it would have been nice if you included the links to the sites you were having difficulty with, but this phenomena has been reported by users who have had problems sending or receiving e-mail, or connecting to secure sites such as eBay, banking sites and Hotmail. So I would 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:

To change the MTU rate follow the steps below:

  • Open your browser and enter the IP address of your router (typically 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, to change this enter the number in the MTU field and click the Apply button to save your settings.
  • If changing the MTU does not resolve the problem, continue changing the MTU 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, I would suggest contacting D-Link technical support for assistance at. I hope this helps.

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

Q. I'm a long time PC user, but I've always been curious about Macs. Recently ,the company I worked for filed for Chapter 11 and allowed me to keep one of its iMac computers. Unfortunately, I know nothing about these systems or, more to the point, Mac OS X. I'd like to get this system up and running on my home network, but haven't got the slightest idea how to go about doing it. Could you tell me how to setup TCP/IP using Mac OS X? Thanks.

A. Normally, I would never touch a question regarding Mac OS X simply because like you, I don't know much about it. However, this is one of the things I've actually done. So, in this case I'll make an exception. To configure TCP/IP from your Mac just follow these steps:

  • From the Apple Menu click on System Preferences.
  • Under System Preferences, click on the Network icon.
  • Verify that the Show field in Network is set to Built-In Ethernet. If it is not, click on the drop down menu and select Built-In Ethernet.
  • Now click on the TCP/IP tab. You should note that if you have a DSL connection using PPPoE you may first have to click on the PPPoE tab and uncheck Connect Using PPPoE before going to the TCP/IP tab.
  • Now click the drop down menu in the Configure field and select Using DHCP. Leave the DHCP Client ID, Domain Name Servers, and Search Domains all blank.
  • Finally click on Apply Now.

You should now be able to obtain an IP address from your router and get online. If you do not see an IP address, reboot your computer and try again. Good luck!

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