You must select one of the following modes of operation when you enable the PIX Firewall as an Easy VPN Remote device:
Client modeIn this mode, VPN connections are initiated by traffic, so resources are only used on demand. In client mode, the PIX Firewall applies Network Address Translation (NAT) to all IP addresses of clients connected to the inside (higher security) interface of the PIX Firewall. To use this mode, you must also enable the DHCP server on the inside interface, as described in " Using the PIX Firewall DHCP Server."
Network extension modeIn this mode, VPN connections are kept open even when not required for transmitting traffic. This option does not apply NAT to any IP addresses of clients on the inside (higher security) interface of the PIX Firewall.
In network extension mode, the IP addresses of clients on the inside interface are received without change at the Easy VPN Server. If these addresses are registered with the Network Information Center (NIC), they may be forwarded to the public Internet without further processing. Otherwise, they may be translated by the Easy VPN Server or forwarded to a private network without translation.
http://www.cisco.com/en/US/products/sw/secursw/ps2120/products_configuration_guide_chapter09186a00800eb72d.html
