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.
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...
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.