Siemens SIMATIC ET 200AL, System Manual

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Parameter assignment/addressing
3.2 Device name and IP address
PROFINET with STEP 7
Function Manual, 05/2021, A5E03444486-AL
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Example of the subnet mask
Subnet mask: 255.255.0.0 (decimal) = 11111111.11111111.00000000.00000000 (binary)
IP address: 192.168.0.2 (decimal) = 11000000.10101000.00000000.00000010 (binary)
Meaning: The first 2 bytes of the IP address determine the network - i.e., 192.168. The last
two bytes address the device, i.e. 0.2.
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:
IP address (decimal) IP address (binary) Address class Default subnet mask
0 to 126 0xxxxxxx.xxxxxxxx.... A 255.0.0.0
128 to 191 10xxxxxx.xxxxxxxx... B 255.255.0.0
192 to 223 110xxxxx.xxxxxxxx... C 255.255.255.0
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 check
for these values.
Masking other subnets
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:
Masks Decimal Binary
Default subnet mask 255.255.0.0 11111111.11111111.00000000
.00000000
Subnet mask 255.255.128.0 11111111.11111111.10000000
.00000000
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.