[Question] IPv4: Does the last IP in any subnet imply multicast?

Hey folks,

I'm currently implementing an OSINT tool that tries to parse all IANA-assigned IP ranges, including IPv4 ones. Therefore I'm gonna refer to the related RFCs 791 and 1878 here in advance.

I've got a question related to whether or not addresses are unicast/multicast by default, and implied by their prefix notation.

For example, in a 192.169.0.0/24 network, the last IP 192.169.0.255 would be the multicast address for the D-subnet. In regards to RFC 791, this is dependent of the subnet bitmask (later: prefix length).

For example, /24 means that 255.255.255.0 is the bitmask that decides whether or not a switch or router will send the data packet to an uplink port or keep it within the same network.

Now the question is regarding multicast. As per the old RFC 791, an IP like 192.169.255.255/16 would imply that it's a multicast, not for the D-subnet, but for the C-subnet as well. Later specifications seem to imply that multicast addresses don't necessarily depend on /8, /16, /24 or similar notations as the prefix is variable and applied as a bitmask.

This means that for example, an not-dividable-by-8 bitmask could lead to different IP ranges, depending on the bitmask, and therefore leading to a different IP address that represents the multicast address for the specific subnet. For example, a network like 10.0.64.0/18 network would have a multicast address for the C-subnet that isn't necessarily a 255 as implied by the older RFC 791.

Now my question into the open is kind of this:

• Is the last IP of a declared subnet with a variable prefix always the multicast address for a specific subnet?

• If not: Are only the specifically mentioned IPv4 addresses assigned by IANA a multicast address, and every other address should be regarded a unicast address by default?

Thanks in advice :)