使用ubuntu 16.04 更新的时候（sudo apt-get update） 报错
Ign:149 http://mirrors.ustc.edu.cn/ubuntu xenial-backports/main DEP-11 64x64 Icons
Reading package lists... Done
W: The repository 'http://mirrors.ustc.edu.cn/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://mirrors.ustc.edu.cn/ubuntu xenial-security Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://mirrors.ustc.edu.cn/ubuntu xenial-updates Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://mirrors.ustc.edu.cn/ubuntu xenial-proposed Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://mirrors.ustc.edu.cn/ubuntu xenial-backports Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://mirrors.ustc.edu.cn/ubuntu/dists ... ce/Sources Connection failed [IP: 202.141.160.110 80]
E: Failed to fetch http://mirrors.ustc.edu.cn/ubuntu/dists ... ce/Sources Connection failed [IP: 202.141.160.110 80]
E: Failed to fetch http://mirrors.ustc.edu.cn/ubuntu/dists ... ce/Sources Connection failed [IP: 202.141.160.110 80]
E: Failed to fetch http://mirrors.ustc.edu.cn/ubuntu/dists ... ce/Sources Connection failed [IP: 202.141.160.110 80]
E: Failed to fetch http://mirrors.ustc.edu.cn/ubuntu/dists ... ce/Sources Connection failed [IP: 202.141.160.110 80]

能够Telnet 成功：202.141.160.110 80
页面也能打开：http://mirrors.ustc.edu.cn/ubuntu/
我是在virtualbox中安装的ubuntu
为什么会出现这种情况呢？有哪位朋友直到怎么解决这个问题？换了几个源都不行。。。

这些实验是在虚拟系统中还是宿主系统中执行的？
确认虚拟机网络连接畅通。

虚拟机网络是没问题的，上网和telnet都正常

考虑网络环境问题，例如公司/学校网络中的特殊设定。