Intel ER8100STUS - Express 8100 Router, Reference Manual

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Reference.bok : Chapter08 58 Wed Apr 22 10:58:08 1998
58
IP Concept
IP Routing
Mapping methods
NAT can use one of the following mapping methods:
Static Mapping
Static Mapping can be used to translate an internal network to an equal sized ex-
ternal network (for example a class B network to a class B network). Static Map-
ping, can also be used to translate individual internal IP addresses to external IP
addresses.
Static Mapping is described in
“Static Mapping”, p. 60 .
Dynamic Mapping
Dynamic Mapping can be used to translate between IP networks of different siz-
es, that is, a large internal network can be translated to smaller external network
or vice versa (for example a class B internal network could be translated to class
C external network addresses).
Dynamic Mapping is described in
“Dynamic Mapping”, p. 60 .
How NAT works
NAT uses a Network Address Translation table which contains internal and cor-
responding external IP addresses. Each entry in the table can be either a network
address, a subnet address or an individual IP address.
For packets from the router, addresses matching entries in the internal list are
translated to the associated external addresses. For packets from the remote rout-
er, addresses matching entries in the external list are translated to the associated
internal addresses.
Note
The internal and external addresses are NOT source and destination IP
addresses, but are addresses which are translated if they pass through
the router.
When an IP packet is translated by NAT, both the source and destination IP ad-
dresses can be translated by different entries in the table, as shown in the follow-
ing diagram.
For packets from the internal network, address A for either the source or destina-
tion IP address is translated to address B, and address C is translated to address D.
For packets from the external network, address B for either the source or desti-
nation address is translated to address A, and IP address D is translated to address
C.
Internal
Address
IP packet from internal net
1351
Source
Source
IP packet from remote network
Destination
Destination
A A
A
C
C
C
B
B
B
D
D
D
External
Address