s/(([0-2]?\d{1,2}\.){3})([0-2]?\d{1,2})/###.###.###.$3/g;

##</code><code>##

$ perl -le 'foreach my $c ( q{123.45.67.89 123.45.67.98}, qw/ 0.0.0.0 10.1.2.3 172.001.002.003 192.168.128.1 255.255.255.255 / ) { my $d = $c; $d =~ s/(([0-2]?\d{1,2}\.){3})([0-2]?\d{1,2})/###.###.###.$3/g; print $c, q{ -> }, $d; }'

##</code><code>##

123.45.67.89 123.45.67.98 -> ###.###.###.89 ###.###.###.98
0.0.0.0 -> ###.###.###.0
10.1.2.3 -> ###.###.###.3
172.001.002.003 -> ###.###.###.003
192.168.128.1 -> ###.###.###.1
255.255.255.255 -> ###.###.###.255