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$ perl -E 'my @x = split /\./, "10.1.1.5"; $x[3] % 2 ? --$x[3] : ++$x[
+3]; say join ".", @x'
10.1.1.4
$ perl -E 'my @x = split /\./, "10.1.1.6"; $x[3] % 2 ? --$x[3] : ++$x[
+3]; say join ".", @x'
10.1.1.7

$ perl -E 'my @x = split /\./, "10.1.1.5"; $x[3] % 2 ? ++$x[3] : --$x[
+3]; say join ".", @x'
10.1.1.6
$ perl -E 'my @x = split /\./, "10.1.1.6"; $x[3] % 2 ? ++$x[3] : --$x[
+3]; say join ".", @x'
10.1.1.5

$ 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