无法在R中安装包分层

我试图在R中安装软件包分层，但出现以下错误：

install.packages('stratification')
Installing package into ‘C:/Users/ramaaj-cont/Documents/R/win-library/3.2’
(as ‘lib’ is unspecified)
trying URL        'http://cran.rstudio.com/bin/windows/contrib/3.2/stratification_2.2-5.zip'

Warning in install.packages :
  InternetOpenUrl failed: 'The server name or address could not be resolved'
Error in download.file(url, destfile, method, mode = "wb", ...) : 
  cannot open URL 
'http://cran.rstudio.com/bin/windows/contrib/3.2/stratification_2.2-5.zip'
Warning in install.packages :
  download of package ‘stratification’ failed

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有人能告诉我是什么问题吗？

防火墙问题？代理问题？你是怎么发现的？我可以访问其他网站，所以。。有什么问题吗？手动下载包的zip文件并安装？这是一个已知问题：我现在下载并解压缩了包。如何安装？