It appears to be common practice to not use the first address in a subnet, that is the IP
192.168.0.0/24, or a more exotic example would be
ipcalc tool I frequently use follows the same practice:
$ ipcalc -n -b 172.20.20.64/29 Address: 172.20.20.64 Netmask: 255.255.255.248 = 29 Wildcard: 0.0.0.7 => Network: 172.20.20.64/29 HostMin: 172.20.20.65 HostMax: 172.20.20.70 Broadcast: 172.20.20.71 Hosts/Net: 6 Class B, Private Internet
But why is that
HostMin is not simply
64 in this case? The
64 address is a valid address, right? And whatever the answer, does the same apply to IPv6?
Perhaps slightly related: it also appears possible to use a TCP port 0 and an UDP port 0. Are these valid or used anywhere?