User's Manual

Chapter 10 Network Address Translation (NAT)
B222s User’s Guide
96
Port Forwarding
A port forwarding set is a list of inside (behind NAT on t he LAN) servers, for exam ple, web or FTP,
that you can m ake visible to t he out side w orld even though NAT m akes your whole inside network
appear as a single com put er to t he out side wor ld.
Finding Out More
See
Section 10.5 on page 100 for advanced technical inform ation on NAT.
10.2 The Port Forwarding Screen
Use t he Port For w arding screen to forward incom ing service requests t o t he server( s) on your
local net work.
You m ay ent er a single port num ber or a range of port num bers t o be forwarded, and t he local I P
address of the desired server. The port num ber identifies a service; for exam ple, web service is on
port 80 and FTP on port 21. I n som e cases, such as for unknown ser vices or where one server can
support m ore t han one service ( for exam ple bot h FTP and web service) , it m ight be bett er to
specify a range of port num bers. You can allocate a server I P address t hat corresponds t o a port or
a range of port s.
The m ost oft en used port num bers and services are shown in
Appendix E on page 249. Please refer
to RFC 1700 for furt her inform ation about port num bers.
Note: Many resident ial broadband I SP account s do not allow you t o run any server
processes ( such as a Web or FTP server) from your locat ion. Your I SP m ay
periodically check for servers and m ay suspend your account if it discovers any
act ive services at your locat ion. I f you ar e unsure, r efer t o your I SP.
Configuring Servers Behind Port Forwarding (Example)
Let's say you want t o assign port s 21- 25 t o one FTP, Telnet and SMTP server (A in t he exam ple) ,
port 80 t o another ( B in t he exam ple) and assign a default server I P addr ess of 10.0.0.35 to a t hird
( C in t he exam ple). You assign the LAN I P addresses and t he I SP assigns t he WAN I P address. The
NAT net work appears as a single host on t he I nt er net .
Figure 52 Mult iple Servers Behind NAT Exam ple
A=10.0.0.33
D=10.0.0.36
C=10.0.0.35
B=10.0.0.34
WAN
LAN
10.0.0.1
IP Address assigned by ISP