This is achieved by allowing the user to tunnel through the internet or another public network using a protocol such as microsoft point - to - point tunneling protocol pptp available with the microsoft windows nt version 4 . 0 or microsoft windows 2000 operating system , or layer two tunneling protocol l2tp available with the windows 2000 operating system 其實(shí)現(xiàn)方法是允許用戶使用如microsoft windows nt 4 . 0或microsoft windows 2000操作系統(tǒng)中的microsoft點(diǎn)對(duì)點(diǎn)隧道協(xié)議( pptp )或者windows 2000操作系統(tǒng)中的第二層隧道協(xié)議( l2tp ) ,將internet或另一公用網(wǎng)絡(luò)作為通道。