If you look into the sources.list of our latest release, you will find a new scheme in your debian list. It looks like http://http.debian.net/debian unstable main. This is the scheme of a new Debian Mirror Redirector that has quite a few advantages and works fine so far. It does not only try to choose the fastest mirror for you, it also helps with load balancing and allows parallel downloads. Besides that it is ready for the upcoming IPv6.
If you have an older release installed, do not hesitate to switch your debian.list to the new scheme. If you watch apt-get update after the change for a few runs,, you should see different mirrors being used.
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