#!/usr/bin/perl

use strict;
use warnings;

use Socket qw( inet_aton inet_ntoa );

sub fr_dotted { unpack 'N', inet_aton @_ ? $_[0] : $_ }
sub to_dotted { inet_ntoa pack 'N', @_ ? $_[0] : $_ }


my $next = fr_dotted('100.1.3.41');
my $max  = fr_dotted('100.1.3.50');

while (<>) {
   chomp;
   my ($host) = split(' ');
   
   die("Exceed max\n") if $next > $max;
   my $addr = $next++;

   print($host, ' ', to_dotted($addr), "\n");
}
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renum.pl infile > outfile

  -or-

perl -i.bak renum.pl file
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Untested.