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We have five PC in our network, with internet connection(i.e same ISP). But some sites are not opening in our network. Like some sites which works outside our network, don't open. I have reset the router, switch but not working. Is it a problem of Router or with the ISP.

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Can you access the problematic sites by IP address? –  Ƭᴇcʜιᴇ007 Feb 21 '11 at 13:54
    
yes! i can, but cant by domain name. –  Harish Kurup Feb 21 '11 at 13:54
    
Some DNS servers block certain IP addresses. Some let you control this, like OpenDNS, others -- like your ISP's -- probably give you no control. –  martineau Feb 21 '11 at 14:08
    
@martineau what the solution for that...i have run ipconfig /flushdns, then also not working.. –  Harish Kurup Feb 21 '11 at 14:13
    
Switch to OpenDNS. How to set it up depends on your OS, although it's sometimes possible to put it into your router's config. The OpenDNS website has good information and instructions (and it's free). –  martineau Feb 21 '11 at 20:21

2 Answers 2

Now that we know you can get at them via IP it's (probably) a DNS resolution issue.

If the router is providing local DNS resolution try setting it to use another (free) DNS service like OpenDNS.com.

Perhaps contact your ISP to find out why those names are not resolving properly.

Hope that helps...

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ok @techie007, i will try that and let you know. –  Harish Kurup Feb 21 '11 at 14:38
    
Contact the site's ISP also. I've helped two companies with the same problem and it ended up the site's ISP had blacklisted the companies' IPs. Former malware incidents but both companies on some ISPs' blacklist. –  surfasb Feb 21 '11 at 16:27

If it's not a DNS issue, it could be MTU.

On one client, try setting its MTU to 1400 or even 1000 and see if the problem goes away. If it does, then try to find the highest MTU setting you can use without having the problem of sites not opening.

If your router does NAT, see if your NAT has an MSS-clamping feature. If it's not enabled, enable it. If your router has a firewall, make sure it is set to allow ICMP messages (especially ICMP "destination unreachable -- fragmentation required but 'don't fragment' bit set" messages) to pass through.

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