On a machine running Ubuntu Feisty, running "aptitude update" or "apt-get
update" times out while trying to connect to security.ubuntu.com:

Err http://security.ubuntu.com feisty-security Release.gpg
  Could not connect to security.ubuntu.com:80 (82.211.81.138), connection
timed out

However, if I run "host security.ubuntu.com" I get:

security.ubuntu.com has address 91.189.88.31
security.ubuntu.com has address 91.189.88.37

Somehow the box seems to be holding onto an old IP address for
security.ubuntu.com and won't let it go. It's been doing this for several days
now. I managed to get the updates by plugging the numeric IP address into my
sources.list, but when I switch back to the dns name the timeouts resume, even
after a reboot. I'm not behind a proxy or anything like that.

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

-Steve