> Any thoughts on flushing the old ip address if indeed that's the case?

If DNS information is persisting after a reboot I can think of three
possible causes:

#1: You have a static mapping for security.ubuntu.com in your /etc/hosts file.

#2: Your DNS server is holding cached records. This could be proper,
as an update to DNS may have been required but the TTL on the previous
information hasn't yet expired, or the DNS server could have it's own
static map or needs a kick in the seat.

#3: security.ubuntu.com may be in a round robin DNS configuration for
load balancing and one of the servers in the round robin is down.

#1 is easy enough to fix. #2 is fixable by flushing the cache of your
DNS server. Restarting the DNS service on that machine will have the
same effect. Your router may be acting as a DNS server for your
network. If it's your ISP...good luck with  their phone support.If
it's #3 there isn't anything you can do, the Ubuntu folks would have
to fix that.


-- 
Andrew S. Zbikowski | http://andy.zibnet.us
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