Intel ER8100STUS - Express 8100 Router, Reference Manual

Page: 71 / 150

Reference.bok : Chapter08 59 Wed Apr 22 10:58:08 1998
59
IP Concept
IP Routing
NAT and routing
For IP packets transmitted from the router, the packet is first routed to the appro-
priate LAN or WAN link using the destination address included in the packet
from the source then translated using the NAT table for that link.
For packets from the external network, the IP addresses are translated as they en-
ter the router. If they are then transmitted on another link on which NAT is en-
abled, the packet is first routed to the appropriate LAN or WAN link using the
destination address (which may already have been translated) then translated
again using the NAT table for the outgoing link.
NAT on the LAN link
It is possible to define NAT translation tables for the LAN link but it may be con-
fusing to think of the LAN as an external network. NAT on the LAN link works
in exactly the same way as for the WAN links.
NAT and IP filtering
When NAT is used in conjunction with IP filtering, for IP packets received by
the router, the packet addresses are first translated by NAT then the packets are
filtered by the IP Rx filters. For IP packets transmitted from the router, the pack-
ets are first filtered by the IP Tx filters then the packet addresses are translated
by NAT.
NAT and RIP updates
When NAT is used on a link, RIP updates received or sent via the link are dis-
carded if they do not match a translation entry in the NAT table. To pass all un-
matched RIP updates instead of discarding them, a static entry can be defined
with internal and external IP addresses of
0.0.0.0
and a network mask of
0.0.0.0
.