3Com 3C840 manual Port Address, Translation PAT, Static NAT, PAT Example

C-4APPENDIX C: ADDRESS TRANSLATION TUTORIAL

Static NAT	Static NAT operates like Dynamic NAT except you may add entries to the
	address translation table and indicate specific IP addresses from the pool
	to map to specific private LAN IP addresses. This allows controlled access
	by the outside world.
Port Address	PAT is used when several privately addressed workstations share a single
Translation (PAT)	public address. PAT uses the TCP and UDP port numbers to map multiple
	private addresses to the single public address. For normal applications
	such as web browsing and FTP transfers, PAT can be configured by just
	enabling the feature. When accesses are originated from the private
	addressed LAN, a mapping is established between the source port
	number and the source private address. When the response is received on
	the public addressed WAN port, the destination port is mapped back to
	the private address.
	Static PAT port mappings or the PAT default address need to be
	configured when an application will initiate a TCP or UDP connection
	from the public network. If a publicly accessible server resides on a
	privately addressed LAN, static ports can be defined for the applications
	they are running.
	For example, TCP port 80 for a Web Server and TCP port 21 for a FTP
	server can be statically assigned. The PAT default address can be used
	with, or instead of, static port assignments, and is set to the private
	address of a workstation on the local LAN.
	If an incoming IP data packet is received on a WAN port and there is no
	existing dynamic or static port mapping, the packet will be translated
	using the PAT default address.
PAT Example	Your remote site administrator or ISP provides one address for the
	OfficeConnect Remote 840's Local WAN IP address (In the example
	above, 10.0.0.1). You make up a private IP network for the LAN and
	assign an IP address from that network to each workstation and to the
	OfficeConnect Remote 840 (LAN IP address).
	RIP (if enabled) must be set to “listen only” so the OfficeConnect Remote
	840 will learn routing information from the WAN but will not broadcast
	the private network information.