Allocating Ip Addresses; Subnet Masks; Cidr - Omron CJ1W-EIP21 Operation Manuals

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Allocating IP Addresses

You must assign IP addresses nodes so that each IP address is assigned only once in the network or
between several networks.
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Subnet Masks

Operation and management of a network can become very difficult if too many nodes are connected on
a single network. In such a case it can be helpful to configure the system so that a single network is
divided up into several subnetworks. Internally the network can be treated as a number of subnetworks,
but from the outside it acts as a single network and uses only a single network ID. To establish subnet-
works, the host ID in the IP address is divided into a subnet ID and a host ID by using a setting called
the subnet mask. The subnet mask indicates which part of the host ID is to be used as the subnet ID.
All bits in the subnet mask that correspond to the bits in the IP address used either as the network ID or
subnet ID are set to "1," and the remaining bits, which correspond to the bits in the IP address actually
used for the host ID, are set to "0."
The following example shows the subnet mask for an 8-bit subnet ID used in class-B IP addresses.
Class B
Subnet mask
Set the same subnet mask for all of the nodes on that subnetwork. The EtherNet/IP Unit supports
CIDR (Classless Inter-Domain Routing). The subnet mask can be set to 192.0.0.0 to
255.255.255.252. If subnetworks are not used, set the following subnet mask values for IP address
classes A to C.
Class A
Class B
Class C
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CIDR

CIDR, or classless interdomain routing, is used to assign IP addresses that do not use classes. IP
addresses that use classes are separated into blocks according to network IDs and host IDs, resulting
in inefficient usage of IP address space. CIDR does not use classes, so IP address space can be
divided as required to more efficiently use IP address space. For example, using a subnet mask setting
with CIDR enables building a horizontally distributed network exceeding 254 nodes even if a class C
address block (e.g., 192, 168...) is used.
192.0.0.0 to 255.255.255.252
CJ-series EtherNet/IP Units Operation Manual for NJ-series CPU Unit (W495)
Bit 31
1 0 Network number (14 bits)
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Network ID
Class
255.0.0.0
255.255.0.0
255.255.255.0
Subnet Mask Range
4 Determining IP Addresses
15
Host number (16 bits)
1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
Subnet ID Host ID
Subnet mask
0
FF FF FF 00
4
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