Nat Functionality - Bay Networks NauticaRS Reference Manual

NAT Functionality

117237-E Rev.00
NAT functions at the interface between the paths on the router and
the router engine. This allows NAT to use the path's IP address
facilitating the scenario shown in
this level to allow NAT to manipulate the higher level protocols.
The protocols used with NAT in a Nautica router are ICMP (UDP,
TCP), SMTP, POP3, HTTP, FTP, TFTP, Telnet, RLogin, NNTP
and X11.
Most of these protocols make use of UDP or TCP protocols,
which have a port field in their headers. This field is used by NAT
to select the internal addresses when a packet is received from an
external source. ICMP functions differently from UDP and TCP
and is handled separately by the NAT protocol.
To translate addresses, the router needs a table of internal
addresses with the associated external port number and path.
Some of the table entries are configured manually, for well known
port numbers (page
when a connection is made from the internal network outside.
The following is an example of a table entry for a Web server,
Internal
IP Address Port
10.0.0.1 HTTP
where the Web server's address is 10.0.0.1 and the path to the
remote router is RemotePPP.
When an external packet is received by the path, with a
destination address equal to the NAT address for the path, the
table is searched for a match on the path and port number. The
destination address and port number in the IP header and higher
protocols is replaced with the internal information from the table
before the packet is passed onwards.
Introduction to Protocols and Routing
Figure
9-20), and some are automatically created
Path
RemotePPP
1-8. It is also placed at
External
Port
HTTP
1-37