Cisco ISA500 Series Administration Manual page 258

VPN
Configuring the Site-to-Site VPN
Cisco ISA500 Series Integrated Security Appliance Administrator Guide
one site to access the hosts at the other site, Network Address Translation
(NAT) is used on the routers to change both the source and destination
addresses to different subnets.
Figure 11 Networking example that simulates two merging companies with
the same IP addressing scheme
In this example, when the host 172.16.1.2 at Site A accesses the same IP-
addressed host at Site B, it connects to a 172.19.1.2 address rather than to
the actual 172.16.1.2 address. When the host at Site B to accesses Site A, it
connects to a 172.18.1.2 address. NAT on Router A translates any 172.16.x.x
address to look like the matching 172.18.x.x host entry. NAT on Router B
changes 172.16.x.x to look like 172.19.x.x.
This configuration only allows the two networks to communicate. It
NOTE
does not allow for Internet connectivity. You need additional paths to
the Internet for connectivity to locations other than the two sites; in
other words, you need to add another router or firewall on each side,
with multiple routes configured on the hosts.
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