Error during update

An error occurd during the update. This is usually some sort of network problem, please check your network connection and retry.

Failed to fetch http://mirror3.ubuntu.com/dists/feisty-seveas/Release.gpg Could not resolve 'mirror3.ubuntu.com'
Failed to fetch http://mirror3.ubuntu.com/dists/feisty-seveas/extras/source/Sources.gz Could not resolve 'mirror3.ubuntu.com'
Failed to fetch http://mirror3.ubuntu.com/dists/feisty-seveas/freenx/source/Sources.gz Could not resolve 'mirror3.ubuntu.com'
Failed to fetch http://medibuntu.sos-sts.com/repo/dists/feisty/free/binary-i386/Packages.gz 302 Found
Failed to fetch http://medibuntu.sos-sts.com/repo/dists/feisty/non-free/binary-i386/Packages.gz 302 Found
Failed to fetch http://medibuntu.sos-sts.com/repo/dists/feisty/free/source/Sources.gz 302 Found
Failed to fetch http://medibuntu.sos-sts.com/repo/dists/feisty/non-free/source/Sources.gz 302 Found