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6.2 TCP/IP Primer on IP Addresses
Obtaining IP addresses to interact over an existing, operating, network can involve a num-
ber of complications, and must usually be done with cooperation from your ISP and/or
network systems administrator. For this reason, it is suggested that the user begin instead
by using a direct connection between a PC and the RCM3700 using an Ethernet crossover
cable or a simple arrangement with a hub. (A crossover cable should not be confused with
regular straight through cables.)
In order to set up this direct connection, you will have to use a PC without networking, or
disconnect a PC from the corporate network, or install a second Ethernet adapter and set
up a separate private network attached to the second Ethernet adapter. Disconnecting your
PC from the corporate network may be easy or nearly impossible, depending on how it is
set up. If your PC boots from the network or is dependent on the network for some or all
of its disks, then it probably should not be disconnected. If a second Ethernet adapter is
used, be aware that Windows TCP/IP will send messages to one adapter or the other,
depending on the IP address and the binding order in Microsoft products. Thus you should
have different ranges of IP addresses on your private network from those used on the cor-
porate network. If both networks service the same IP address, then Windows may send a
packet intended for your private network to the corporate network. A similar situation will
take place if you use a dial-up line to send a packet to the Internet. Windows may try to
send it via the local Ethernet network if it is also valid for that network.
The following IP addresses are set aside for local networks and are not allowed on the
Internet: 10.0.0.0 to 10.255.255.255, 172.16.0.0 to 172.31.255.255, and 192.168.0.0 to
192.168.255.255.
The RCM3700 uses a 10/100-compatible Ethernet connection with a 10Base-T interface,
which is the most common scheme. The RJ-45 connectors are similar to U.S. style tele-
phone connectors, except they are larger and have 8 contacts.
An alternative to the direct connection using a crossover cable is a direct connection using
a hub. The hub relays packets received on any port to all of the ports on the hub. Hubs are
low in cost and are readily available. The RCM3700 uses 10 Mbps Ethernet, so the hub or
Ethernet adapter must be either a 10 Mbps unit or a 10/100 unit that adapts to 10 Mbps.
In a corporate setting where the Internet is brought in via a high-speed line, there are typi-
cally machines between the outside Internet and the internal network. These machines
include a combination of proxy servers and firewalls that filter and multiplex Internet traf-
fic. In the configuration below, the RCM3700 could be given a fixed address so any of the
computers on the local network would be able to contact it. It may be possible to configure
the firewall or proxy server to allow hosts on the Internet to directly contact the controller,
but it would probably be easier to place the controller directly on the external network out-
side of the firewall. This avoids some configuration complications by sacrificing some
security.
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