The external interface of the Proxy Server can be configured to forward requests for internal resources based on the URL contained in the request. For example, if the external interface of the proxy server is registered in the DNS as proxy.acme.net, you might also want to register another host name to the same IP address, such as ftp.acme.net.
When the request arrives at the external network interface of the proxy server, it will see the request for ftp.acme.net and forward it an internal server that you have specified to answer requests for ftp.acme.net.

VPN solutions support remote access^[1] and private data communications over public networks as a cheaper alternative to leased lines. VPN clients communicate with VPN servers utilizing a number of specialized protocols. VPN consists of private network access over public connections and can provide secure but more cost-effective LAN-WAN communication.
A VPN supplies network connectivity over a possibly long physical distance. In this respect, a VPN is a form of Wide Area Network (WAN) and VPNs enable

1. file sharing,
2. video conferencing and
3. similar network services.

Virtual private networks generally do not provide any new functionality that isn't already offered through alternative mechanisms, but a VPN implements those services more efficiently in most cases.
A key feature of a VPN is its ability to work over both private networks as well as public networks like the Internet. Using a method called tunneling, a VPN can use the same hardware infrastructure as existing Internet or intranet links. VPN technologies include various security mechanisms to protect the virtual, private connections.
Specifically, a VPN supports at least three different modes of use:

1. Internet remote access client connections
2. LAN-to-LAN internetworking
3. Controlled access within an intranet

[1] Remote access is the ability to access a computer or a network remotely through a network connection. Remote access enables users to access the systems they need when they are not physically able to connect directly. Hence, users access systems remotely by using a telecommunications or internet connection.