Surprisingly to me, on 10 million iterations, the regex is faster:
split: 1.749s
regex: 1.497s

But they aren't doing the same thing. Mine is grabbing the first 3 dot delimited fields. Yours is removing everything after the last dot. Assuming well formed-ness of the IP, the results will be the same. But I like my odds when the IP isn't well formed. (You haven't seen an IP yet with 5 octets? I have, and they're one short typo away...)