Ports and port mapping – Echelon IP-852 Channel User Manual
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Using NAT, DHCP, DNS on an IP-852 Channel
Home Network
192.168.1.100
192.168.1.101
192.168.1.1
NAT
Gateway
LAN Side
WAN Side
131.28.203.17
ISP Internet
WAN Side Address is
Assigned by ISP
Figure 4.3 NAT Gateway Structure
Ports and Port Mapping
A fully qualified URL consists of an IP address and a port. The URL
www.echelon.com:80 is a fully qualified URL. Port 80 is recognized as the default port
for Web servers worldwide. Browsers typically append a URL with port 80 so you do not
have to enter the full URL when accessing a Web site.
Ports allow a single computer to run multiple services. For example, www.echelon.com
can run both a Web server and an FTP server. It could also run a time server and other
applications as well. Each service may be assigned different ports. For example, Web
browsers use port 80 as the default when they access www.echelon.com and maps the
address to an IP address:port such as 205.229.51.8:80. When accessing an FTP client,
FTP clients typically use port 21 so ftp://www.echelon.com will map to an IP address:port
such as 205.229.51.8:21. Both the browser and the FTP client may simultaneously
access www.echelon.com because the requests are differentiated by port.
Most businesses use port 80 for their public Web site so customers have easy access to
their Web sites. However, if you wanted to host a less public site, you could assign it a
non-standard port number. For example, you could use www.mycompany.com to attract
a wide audience to your business, or you could assign your URL a non-standard port
(www.mycompany.com:81) to “hide” your Web site from the general public. Changing
ports does not provide security to your Web site, so other methods of security must be
used for servers that contain sensitive information. Another reason to use non-standard
ports is to allow access from the Internet to one of your home computers.
The Internet Assigned Numbers Authority (IANA) lists common or “well known” ports as
well as registered and dynamic ports.
for more information.
Port mapping can be used to connect two computers behind a NAT gateway that access
the Internet through a single IP address. The NAT gateway forwards packets received
from the Internet to the correct computer using different port settings. To ensure that
packets are forwarded to the proper computer, the NAT gateway must be setup to
perform static port mapping.