Norton Antivirus 2011 LAN update

I have Norton Antivirus 2011 running on six machines on a LAN network. I currently have six machines downloading the same thing six times on my internet connection, and a small 10Mb update becomes 60Mb. That is significant since I have a 1Gig cap per month on the internet connection. The internet connection is shared via a hub. How can I have one machine download updates and the other five get updates from that machine.