IPv4 addresses ending in .0 and .255 are completely valid for ordinary use. This applies to addresses within all of the the RFC1918 address ranges.
TCP/IP subnets used to be defined to be of a particular class. A class C subnet would have 253 addresses available for use, with .0 reserved for the network address, and .255 reserved for the broadcast address. The 192.168.0.0/16 private network was defined to be 256 class C blocks, so none of the .0 or .255 addresses were available for normal use.
This changed with the advent of Classless Inter-Domain Routing (CIDR) in RFC 1519. It became possible to define arbitrary subnets, such as 192.168.4.0/22.
In the subnet 192.168.4.0/22, the host ID has 10 bits rather than 8, and the network mask is 255.255.252.0. The network address is 192.168.4.0. The broadcast address, with the host bits all set to 1, is 192.168.7.255. However, inside the 1024 addresses in the subnet’s range are other addresses that end in .255, and they are not broadcast addresses: 192.168.4.255, 192.168.5.255, 192.168.6.255. Similarly, there are addresses ending in .0 that are not the network address.
The reverse can happen as well. You could define a subnet 192.168.4.0/26, and the broadcast address on that subnet would be 192.168.4.63. That address would then not be valid for regular use, even though it doesn’t end in .0 or .255.
Even if your network is set up so that the routers divide it into /24 blocks, the runZero Explorer has no way to know that, so it scans every IP address. If a response is determined to be a response to a broadcast, it gets ignored. Otherwise, if the explorer gets a response, it records and processes it, even if the address ends in .0 or .255.
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