This seems to work as well, though obviously not a 1 liner..

#!/usr/bin/perl
use Modern::Perl qw(2014);
while (<DATA>) {
    my @octets = split('\.',$_);
    chomp @octets;
    for (0..2) {
        $octets[$_] =~ s/\d/x/g;
    }
    say join(".", @octets);
}


__DATA__
1.2.3.4
192.168.0.1
255.255.255.128
23.65.98.101
[download]

Outputs:

x.x.x.4
xxx.xxx.x.1
xxx.xxx.xxx.128
xx.xx.xx.101
[download]

I suppose for further obfuscation one could just stick 3 x's in each of the first three octets and append the last octet of the IP to that string, thus hiding the format...

...just a thought...