我正在尝试在R中安装包分层，但收到以下错误：
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
有人可以告诉我这是什么问题吗？