Content filtering would be the way to go.
For an interim solution, if you control your DNS servers, block it at the DNS level. 

From: [log in to unmask] [mailto:[log in to unmask]] On Behalf Of Trenton Ray
Sent: Thursday, October 04, 2012 4:29 AM
To: [log in to unmask]
Cc: [log in to unmask]
Subject: Re: Iptable rule required to block youtube

Have you looked into setting up a Squid proxy/filter? Much less of a headache than doing it at the iptables level. 

On 10/04/2012 08:26 AM, Michael Tiernan wrote:
On 10/4/12 3:27 AM, vivek chalotra wrote: 
And now i want to block youtube on my network.

It can be done with iptables however it's not for the faint of heart. I did some reading about it on a dd-wrt website and it wasn't something I found as an easy solution to a single problem such as this.

However, blocking by name string leaves open the ipaddress approach so you have to do both things and this isn't something easily maintained.

May I respectfully suggest that the problem isn't at the iptables level but at the user level?
A simple "You do it, you're cut off." rule is more effective and would move the responsibility from you and the system software to those managing the users.

-- 
  << MCT >>   Michael C Tiernan xmpp:[log in to unmask] +1 (617) 324-9173
  MIT - Laboratory for Nuclear Science - http://www.lns.mit.edu
  High Perf Research Computing Facility at The Bates Linear Accelerator
    Please avoid sending me MS-Word or MS-PowerPoint attachments.
    See http://www.gnu.org/philosophy/no-word-attachments.html