$ perl -nle 's/([0-9]?\d\d?|2[0-4]\d|25[0-5])\.([0-9]?\d\d?|2[0-4]\d|25[0-5])\.([0-9]?\d\d?|2[0-4]\d|25[0-5])\.([0-9]?\d\d?|2[0-4]\d|25[0-5])/xxx.xxx.xxx.$4/; print' filename

##</code><code>##

$ perl -nle 's/\d+\./xxx./g; print' filename