42
6 F 3 B 0 3 6 2
c. Notes on IP addresses
The following IP addresses cannot be used with the Ethernet Port, both for the local port and for
remote port addresses.
•Addresses starting with 127 (7FH).
•Addresses in which the network address is all zeros or all ones.
•Addresses in which the host address is all zeros or all ones.
d. Broadcast addresses
The only broadcast address that can be used with the Ethernet Port is 255.255.255.255
(FF.FF.FF.FF).
The broadcast address 0.0.0.0 used on some UNIX systems (4.2 BSD) cannot be used.
UNIX is an operating system used on engineering workstations (EWS) and other computers. There are
several versions of UNIX, including ones developed by AT&T Bell Laboratories Inc., and ones
developed by the University of California at Berkeley. In particular, 4.2 BSD is a version developed at
Berkeley.
(2) Subnets and the subnet mask
One technique for managing a network with a large number of ports is to divide that network into
multiple subnets. A subnet operates by taking some number of bits in the host address of the IP
address for each class, and using those bits as a subnetwork address. This allows the installation to
operate the network internally as multiple subnetworks, while the network is recognized externally
as a single network.
The subnet mask is used to determine how many bits of the host address to allocate as the subnet
address. In the subnet mask, the network address field and the subnetwork address field are set to 1
and the remaining field (the subnet host address field) is set to 0.
Example: When a subnetwork address field of 12 bits is used in a class A IP address.
1111 1111 1111 1111 1111 0000 0000 0000 (FFFFF000)
When a subnet mask is used, all ports on the subnetwork must use the same subnet mask.
In the Ethernet Port, the subnet mask cannot be set by the user. The subnet masks (with the
network address field filled with ones) for each class are generated automatically as shown in table
4.7 from the specified IP address.
Table 4.7 Relationship between the Local IP Address
and the Generated Subnet Mask on the Ethernet Port
Local IP Address Subnet mask
class A
class B
class C
255.0.0.0 (FF000000)
255.255.0.0 (FFFF0000)
255.255.255.0 (FFFFFF00)
Host address
field
Subnetwork
address field
Usage Recommendations
1. The PU235N/245N does not allow the user to set the subnet mask. Therefore the Ethernet Port
cannot be used in systems that operate a subnetwork.
Network
address field