It may consume a good bit of memory, but why worry about it until you *know* that it's a problem? If you know it is, then put the data in a database and let it do the heavy lifting for you. And just because I'm bored:

#!/usr/bin/perl -w
use strict;
use warnings;

my %IPs;
while(<DATA>) {
   if (m#^(\d+)([\s\d/]+)IP\s+([^\s]+)\s+([^\s]+)\s+([^\s]+)#) {
       push @{$IPs{$1}}, sprintf("%-10s %-24s %s", $1 . $2, $3, $4);
   }
}

print "KEY   \tIP         NAME                     STATUS\n"
.     "------\t---------- ------------------------ --------------\n";
for my $IP (sort keys %IPs) {
   print sprintf("%-6s\t",$IP), join("\n\t", sort @{$IPs{$IP}}), "\n";
}

__DATA__
2345      IP   NAME.com         online IPADDRESS1
2345/1243 IP Name-interface.com inactive IPADDRESS2
2345/3213 IP NAME-interfce2.com online IPADDRESS1
2345/1212 IP Name-interface3.com online IPADDRESS3
4321      IP   CNAME.com         dead IPADDRESS_1
4321/1643 IP CName-interface.com online IPADDRESS_1
4321/3673 IP CNAME-interfce2.com online IPADDRESS_2
[download]

gives us

KEY     IP         NAME                     STATUS
------  ---------- ------------------------ --------------
2345    2345       NAME.com                 online
        2345/1212  Name-interface3.com      online
        2345/1243  Name-interface.com       inactive
        2345/3213  NAME-interfce2.com       online
4321    4321       CNAME.com                dead
        4321/1643  CName-interface.com      online
        4321/3673  CNAME-interfce2.com      online
[download]