# [Tex/LaTex] add urls in reference

urls

i want to add urls in my reference

i am using the following code in my .tex file

however, i get the following output in my console

! Undefined control sequence.
{http://en.wikipedia.org/wiki/Bu...

?
! Missing $inserted. <inserted text>$
logic_layer}, 23 12 2011.
?
! Extra }, or forgotten \$.
, 23 12 2011.
?

! LaTeX Error: Command \item invalid in math mode.

whereas my .pdf file entry against this code is this.