But I suspect perl could do this even quicker.

Yup, under one second on a really really old laptop to both generate a file of ips and then read and compare then

$ perl -le " for( 1 .. 10 ){ printf qq{%d.%d.%d.%d\n}, $_ , $_, $_, $_
+ for 1..255; } warn time-$^T; " > 2
0.265625 at -e line 1.

$ perl -lne " $tri = substr $_, 0, 3; if( $tri > 255 ){ die }   END{wa
+rn  time-$^T; } "  2
0.359375 at -e line 1, <> line 2550.
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