Parameter assignment/addressing
3.2 Device name and IP address
PROFINET with STEP 7 V15
Function Manual, 12/2017, A5E03444486-AH
51
Relation between IP address and default subnet mask
An agreement exists relating to the assignment of IP address ranges and so-called "Default
subnet masks". The first decimal number (from the left) in the IP address determines the
structure of the default subnet mask with respect to the number of "1" values (binary) as
follows:
128 to 191 10xxxxxx.xxxxxxxx... B 255.255.0.0
Note
Range of values for the first decimal point
A value between 224 and 255 is also possible for the first decimal number of the IP address
(address class D
etc.). However, this is not recommended because there is no address
You can use the subnet mask to add further structures and form "private" subnets for a
subnet that is assigned one of the address classes A, B or C. This is done by setting other,
less significant bits of the subnet mask to "1". For each bit set to "1", the number of "private"
networks doubles and the number of devices they contain is halved. Externally, the network
continues to function as an individual network.
Example:
You have a subnet of address class B (for example, IP address 129.80.xxx.xxx) and change
the default subnet mask as follows:
Default subnet mask 255.255.0.0 11111111.11111111.00000000.
Subnet mask 255.255.128.0 11111111.11111111.10000000.
Result:
All devices with addresses from 129.80.001.000 to 129.80.127.254 are located in a subnet,
all devices with addresses from 129.80.128.000 to 129.80.255.254 in a different subnet.
Reading out an IP address in the user program
You can read out the IP address of a PROFINET device in the user program of a
S7-1500 CPU. You can find information in this FAQ
(https://support.industry.siemens.com/cs/ww/en/view/82947835
).
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