Echelon IP-852 Channel - Manual (Page 47)

IP-852 Channel User’s Guide 39
However, if you are in control of your DHCP server, you may be able to configure your
DHCP server to always assign your SmartServer or i.LON dev ices the same IP address.
This is called making a static reservation, and is supported by most DHCP servers.
Using DHCP with static reservations is acceptable and is similar to using static IP
addresses. If you decide to use this technique, each SmartServer and i.LON device can
be instructed to acquire its IP address from the DHCP server by enabling the
Automatically Obtain IP Address on the TCP/IP Web page for your SmartServer or
i.LON device. See your device’s documentation for how to do this.
DHCP Servers
DHCP servers are configured to assign a range of valid Internet addresses. With a
simple NAT gateway, like the one shown in Figure 4.4, the range is often 192.168.1.2 to
192.168.1.254. As an example, the NAT gateway assigns the first computer to request
an address 192.168.1.100, the second computer to request an address 192.168.1.101, the
next gets 192.168.1.103, and so on. The address a computer is assigned is determined by
the order in which the computers are powered up on a network. Computers request an
address each time they are powered. This means that by using DHCP you run the risk of
losing a previously assigned address for a given computer (or IP-852 device). This
usually is not an issue for a home computer that is used to browse the Internet because
the computer is always the initiator of the Web page request. However, if you want a
computer to act as a Web server, it must have a permanent address so other computers
can access it. The same is true for an IP-852 device participating in an IP-852 channel.
The solution is to avoid using DHCP without static IP address reservations for devices
whose addresses must be known by external users. This includes FTP servers, time
servers, Web servers, database servers, and IP-852 devices.
When a computer does not use DHCP and is assigned an address manually, it has a
static IP address. It is possible to have a network that defines a range of addresses that
will be allocated dynamically by the DHCP server, and a range that will be managed
manually. In the NAT gateway mentioned above, 192.168.1.2 to 192.168.1.99 are
managed manually.
ISP Address Allocation
Cable or DSL service in the United States costs about $40 - $50 per month for a single
dynamically allocated IP address. Depending on your telephone or cable provider, you
may be able to purchase a business account that provides one or more static IP addresses
at a higher cost.
In the example, if the address provided to your home by the ISP is static, you only need
to setup static port mapping and inform outside users to go to 131.23.203.17:80 or
131.23.203.17:81 to view your Web pages. Similarly, if you wanted to include your home
IP-852 device in an IP-852 channel, you would enable static port mapping on the NAT
gateway and enter 131.23.203.17:1628 in the IP-852 Configuration Server. The packets
on the channel would flow unimpeded.
You will run into problems with your network if your ISP does not offer static addresses.
Even if static port mapping is enabled on your NAT gateway, you may not be able to
access computers within your home because the house IP address (provided by the ISP)
may change unpredictably. This is a common problem. As a result, you should use a
static IP address for both your NAT gateway and the SmartServer or i.LON
device in your local network.