NAT can be used to perform several
functions. This section describes in detail the operation of the
following NAT functions:
- Translating inside local
addresses - This function establishes a mapping between
inside local and global addresses.
- Overloading inside global
addresses - You can conserve addresses in the inside global
address pool by allowing source ports in TCP connections or User
Datagram Protocol (UDP) conversations to be translated. When
different inside local addresses map to the same inside global
address, the TCP or UDP port numbers of each inside host are
used to distinguish between them.
- TCP load distribution - A dynamic
form of destination translation can be configured for some
outside-to-inside traffic. When a mapping scheme is established,
destination addresses that match an access list are replaced
with an address from a rotary pool. Allocation is done on a
round-robin basis, and is done only when a new connection is
opened from the outside to the inside. All non-TCP traffic is
passed untranslated (unless other translations are in effect).
- Handling overlapping networks -
NAT can be used to resolve addressing issues that arise when
inside addresses overlap with addresses in the outside network.
This can occur when two companies merge, both with duplicate
addresses in the networks. It can also occur if you switch ISPs
and the address you were assigned by your former ISP is
reassigned to another client.
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