User's Manual

Configuring GRE, NAT, RIPSO, and BFE Services
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NAT Concepts and Terminology
Network Address Translation (NAT) offers a solution to two problems facing
companies that require Internet access:
• The diminishing number of available IP addresses for Internet hosts
• Private networks with unregistered addresses that cannot access the Internet
Using NAT, you can create a pool of registered IP network addresses that the
router maps to your unregistered local addresses. Where a company does not have
enough globally unique IP addresses for each host on its network, NAT can assign
a global IP address to hosts as needed. Similarly, a company using unregistered
addressing on its internal network can use NAT to translate those unregistered
addresses into registered addresses for making external connections.
Implementing NAT does not require widespread changes to a network’s hosts or
routers. You configure NAT on routers bordering the private and global networks.
Routers are configured with local and globally unique address ranges.
• IP addresses inside the local network (local addresses) are not globally unique
or are nonstandard. They are never advertised outside the local network.
• The globally unique addresses (global addresses) must be standard registered
addresses. Global addresses are advertised both within and outside the local
network.
NAT routers translate host addresses from inside private networks into
well-known addresses that can be used in the global network. On its return trip, a
packet using a NAT-assigned registered address destined for the internal network
is translated back into its original local address. NAT maintains a table of current
translations. Translations remain in the table until they become inactive and time
out, freeing up the registered address for use by other hosts.