How to Connect Ethernet I/O Modules with RadioLinx Ethernet Modems – 8500770

Connecting Acromag Series 900EN I/O Modules to an existing network with RadioLinx Ethernet Modems

Background – IP AddressesHow to Connect Acromag Ethernet I/O Modules Via RadioLinx Ethernet Modems

Any TCP/IP packet contains two addresses: a unique fixed MAC address, and a unique IP address.

To communicate on the Internet, computers must have unique identifiers in order to differentiate one computer from another. This is accomplished through the assignment of an IP address. An IP address is a 32-bit identifier comprised of 4 octets (8-bits), with each octet a number from 0 to 255 and separated by a decimal point. For example, Acromag Series 900EN I/O modules connect using a default IP address of

Every host on a network must have at least one unique IP address. A personal computer with more than one network interface card (NIC) will have a unique IP address for every NIC card installed, as each NIC card may function as a node on a network. Additionally, all hosts on the same physical network must have the same network prefix.

IP Address Classes

IP addresses are divided into separate classes. These classes are pre-allocated to organizations that request them in address blocks. The three classes used for host groups are Class A, Class B, and Class C:

  1. Class A: first octet identifies the network address; the remaining octets the host or node address.
  2. Class B: first two octets identify the network; the last two the host.
  3. Class C: first three octets identify the network; and fourth octet the host.

As such, the total number of hosts that may be allowed on a network is determined by its Class designation.


A network is further divided into subnets. For example, a subnet is a network on which you are currently located. That network can be made larger by adding more subnets.

Subnet division is accomplished using subnet masks. The default subnet mask for a Class A network would be For Class B it would be For a Class C network, a default subnet mask is That is, the subnet mask flags the bits of the IP address that belong to the network address (first part), with the remaining bits corresponding to the host portion (last part). Thus, the unique subnet to which any IP address belongs to is recovered by performing a bit-wise AND operation between the IP address and the subnet mask. As such, a default IP address of with a subnet mask of tells us that this Class C sub-network address is and the host address is at node 100 or

In assigning a node address, the first (.0) and last nodes (.255 for our example) are typically reserved and should not be used. The first node address is reserved for the network server ( for our example). The last node address is usually reserved for a network bridge or switch ( for our example). Other nodes may also be reserved for servers and other devices on the network. For example, the node 10 address is commonly used for a DNS server. The use of reserved node addresses may degrade performance.



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