The reason why it failed is that firewalls, proxy servers and NATs on the network were incompatible with either L2TP or PPTP. You might have an experience that you stayed in the hotel room and tried to connect to your company's network by remote access VPN with either L2TP or PPTP but failed. In many cases, a user tries to establish a VPN connection by either L2TP or PPTP on the network which is with firewalls, proxy servers and NATs, but he will fail. But such extensions of legacy VPN protocols still have a problem of compatibles. Recently some venders of VPN products with IPsec, L2TP and PPTP tried to invent the extend method to pass through these wall devices, and some of VPN products are implemented with that extensions. Therefore generally firewalls, proxies and NATs are unable to pass these legacy VPN packets. These packets are special forms of IP packets. For example, IPsec and L2TP use ESP (Encapsulating Security Payload) packets, and PPTP uses GRE (Generic Routing Encapsulation) packets. There protocols were developed in the era before NATs were widely spread. Tunnels of legacy VPN protocols, such as IPsec, L2TP and PPTP, cannot often be established through firewalls, proxy servers and NATs. Not only for purpose of security, but also companies use firewalls, proxies and NATs in order to share the precious IP addresses with many computer users in the office.

Generally, in company networks of nowadays, there are firewalls to isolate between the inside network and outside for ensuring security.

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