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The IT department of our company have blocked FB. but if we keep on hitting try again button again and again after 6-7 attempts it opens. Why is this so?

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you could alway see the method they are using on the other side :-) serverfault.com/questions/50443/… I think it might be one of the multiple IPs are finnaly reached. you could test if they use DNS block or IP block by testing some numbers compnetworking.about.com/od/traceipaddresses/f/… – Psycogeek Jan 20 at 4:53
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2 Answers

up vote 3 down vote accepted

Firewalls generally work on IP addresses (this is best practice). Facebook uses a content distribution network and is spread over hundreds of different servers.

It is likely that they have blocked some of the IP addresses related to facebook, but not all, and each time you try, there is a chance facebook.com will resolve to one of the IP addresses that isn't blocked.

Recommend that they use a proxy server to control access to different sites, as they can more effectively block at the URL level with one.

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"You are". Each time you ask your browser to open facebook.com it will attempt to resolve it to an IP address. If this request goes to DNS it can come back with a different IP time, or multiple IP addresses. Try it for yourself here: kloth.net/services/nslookup.php – Paul Jan 20 at 4:59
"Confused". Yes, you could identify an IP address that works, and use it in your c:\windows\system32\drivers\etc\hosts file. This isn't guaranteed however, as facebook pulls in content from different urls, some of which may still resolve to blocked IP addresses. So you might find some things don't load correctly. – Paul Jan 20 at 5:17
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up vote 1 down vote

They've blocked Faceless by locking out the IP address blocks used by the service. These are served out round-robin. You've managed to iterate through to a block that resolves through DNS and that isn't being blocked by the firewall.

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