english: They found that there are some obvious features of the commonly used VPN protocols, such as IPSec, L2TP/TPSec and PPTP, which often use specific ports.
non_english: Ze ontdekten een aantal opvallende kenmerken van de meest gebruikte VPN-protocols, zoals IPSec, L2TP/TPSec an PPTP, die vaak specifieke poorten gebruikten.