ANSWER:

You want to setup a VPN server on the company LAN, which is also accessible from the Internet. External machines would use a VPN client to connect to the VPN server, and the VPN server would forward packets to other machines on the company LAN. The only catch is that you need the response packets from the LAN machines to go to the VPN server — this can either be done by configuring the router, or configuring all the machines (the former being easier). This could all be done with OpenVPN.

(LAN machines) 10.0.0.0/24 <-switch-> 10.0.0.2/24 (VPN server) 192.168.0.1/24 <-encrypted tunnel-> 192.168.0.10/24 (VPN client)

In this scenario the LAN machines would see packets coming from 192.168.0.10.

by mgorven from http://superuser.com/questions/417809