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My office laptop has a VPN client installed. At home, I start this client, choose one host out of given many hosts and connect using an RSA token. From this point onwards, my browser is able to access all the office sites, as if I was in office and I can ssh to my office machines.

I am trying to get some basic understanding of how this thing works. I have heard of keywords like tunelling, but not sure how and where these are coming into picture.

(1) After getting on VPN, does my IP address change? or is it still same as what is provided by by ISP?

(2) Fine, even when I start a vpn client software, how does it tell firefox to do things any differently such that it allows me to access office intra sites? i.e. how does this client change settings used by firefox...?

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up vote 1 down vote accepted

A VPN is an additional network interface. So your IP address doesn't change, but you have an additional IP address.

Firefox, and other programs, don't need to do anything special. They'll always use the network device "closest" to the destination. If the IP address assigned to the VPN is in the same subnet as the machines at the office, the VPN will be used automatically. If they're in different subnets, then the VPN software just adds a route directing all traffic bound for the office subnet(s) to take the VPN.

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But what does the flow look like? How does the vpn client interfere with outgoing traffic flow from browser..? Does it run as a priviledged deamon process, which has power to modify internet traffic going from the machine? – p2pnode May 10 '12 at 23:43
Some part of it runs as a driver, presenting a network interface to the host, (roughly the same as if it had an Ethernet link to the remote network). – David Schwartz May 11 '12 at 0:56
@p2pnode: a VPN does not "interefere" with outgoing traffic flow. The OS uses a routing table which lists what network interfaces to use based on the destination subnet of a packet. A VPN does modify the routing table, so thus that is why people usually are puzzled by the "interference." – surfasb May 11 '12 at 5:15

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