Echelon i.LON 600 LonWorks/IP Server - Manual (Page 95)

Section 3: Appendixes
DHCP
Devices in an IP network have assigned addresses such as 192.168.1.100. For small
networks, manually configuring each device’s IP address is fairly simple and not very
time consuming. As the number of computers on your network grows, however,
assigning each computer on the network its own IP address can be cumbersome. To solve
this problem, a system called Dynamic Host Configuration Protocol (DHCP) was created
to automatically assign network computers an IP address. Most computers use DHCP.
With DHCP, instead of using a pre-defined address, computers broadcast a message on
the local network asking the DHCP server to assign them an address. The DHCP server
stores a list of the assigned addresses and makes sure that no two requestors are given the
same address. This greatly simplifies the job of the network administrator, but in the case
of web servers (or i.LON 600s), can create some difficulties.
Go back to Figure 41 . The addresses assigned to these computers (192.168.1.100 and
192.168.1.101) were most likely assigned manually, but could have been assigned
automatically using DHCP. Many NAT gateways, in addition to providing NAT
functionality, also provide built-in DHCP servers.
DHCP Servers
DHCP servers are configured to assign a range of valid Internet addresses. With a simple
NAT gateway, like the one used in
Figure 40 , the range is often 192.168.1.2 to
192.168.1.254. As an example, the Linksys Model BEFSR81 NAT gateway assigns the
first PC to request an address 192.168.1.100, the second PC to request an address
192.168.1.101, the next gets 192.168.1.103, and so on. The address a PC is assigned is
determined by the order in which the PCs are powered up on a network. PCs 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 PC (or i.LON 600). This usually is not an
issue for a home PC that is used to browse the Internet because the PC is always the
initiator of the web page request. However, if you want a PC to act as a web server, it
must have a permanent address so other PCs can access it. The same is true for an i.LON
600 participating in a L
ON W ORKS /IP channel.
The solution is to avoid using DHCP for devices whose addresses must be known by
external users. This includes FTP servers, time servers, web servers, database servers,
and i.LON 600s.
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 Linksys 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 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
i.LON 600 in a L
ON W ORKS /IP channel, you would enable static port mapping on the
i.LON 600 User’s Guide 87