$ perl -E 'my @x = split /\./, "10.1.1.0"; $x[3] % 2 ? ++$x[3] : --$x[3]; say join ".", @x'
10.1.1.-1
$ perl -E 'my @x = split /\./, "10.1.1.255"; $x[3] % 2 ? ++$x[3] : --$x[3]; say join ".", @x'
10.1.1.256