Siemens SIMATIC NET S7-300 Manual page 151

IP Addresses for IP Multicast
S Possible
Values
IP addresses from 224.0.0.0 to 239.255.255.255 can be used for IP multicast.
Since the IP addresses up to 224.0.0.255 are reserved for special purposes, we
recommend that you use IP addresses starting at 224.0.1.0 (default) for IP
multicast.
S Identification of the Multicast Group
A multicast group is not identified using the entire IP address; the first
addressed byte and the most significant bit of the second address byte are
ignored. This is an important feature that allows seemingly different IP
addresses to address the same multicast group.
Example:
The following IP addresses address the same multicast group.
IP address
224.0.1.0
225.0.1.0
226.128.1.0
S7-CPs for Industrial Ethernet Configuring and Commissioning
Release 01/2007
C79000-G8976-C182-07
5 Configuring Communication Connections
Resulting multicast address
0.1.0
0.1.0
0.1.0
1000 0000
A -151