I understand, that you need a hop- and section-wise comparison, so the following might get you started. However, the naive - text based - approach below will fail when the order of hops/IP's changes - e.g. by means of topology changes or selection of an alternative route having more/less intermediate hops. IF that is also of concern to you, a real network model (nodes and edges (graphs)) would be better suited than a plain text comparison. HTH

                          IP#1    IP#2/2b   IP#3     IP#4   Section  D
+iff
                          ============================================
+==================
OK   (alternate route):   HOP1 --- HOP2  ----------- HOP3   (#1)
                          HOP1 --- HOP2b ----------- HOP3   (#2)  --> 
+@hop2: IP#2-->IP#2b

EEK! (non-equidistant):   HOP1 --- HOP2  ----------- HOP3   (#1)
     a) shorter route     HOP1 --------------------- HOP2   (#2a) --> 
+@hop2: IP#2-->IP#4 (err?)
     b) longer  route     HOP1 --- HOP2 --- HOP3 --- HOP4   (#2b) --> 
+@hop3: IP#4-->IP#3 (err?)
[download]

use strict;
use warnings;

sub prettyip {
  my $ip = shift;
  $ip =~ s/ (\d+) / sprintf("%03d",$1) /smgex;
  return $ip;
}

my %last_seen_ip_from_hop;   # last IP seen for key=HOP
my $section      = 1;        # section within the file
my $previous_hop = 0;        # previous HOP / new section event

while (my $line = <DATA>) {
  if ($line =~ /^(\d+),(\S+)/) { # extract HOP and IP
    my ($hop, $ip)    = ($1, $2);
    my $last_ip_seen  = $last_seen_ip_from_hop{$hop};
    my ($changemark_pre, $changemark_pos)  = ("", "");

    # detect a new section (A/B/C)
    $section++, print "\n" if $previous_hop > $hop; # new file/section
    $previous_hop = $hop;

    # notify if a change occured for a given hop since last seen
    if (defined $last_ip_seen and $ip ne $last_ip_seen) {
      $changemark_pre = 'changed to';
      $changemark_pos = '(was: ' . prettyip($last_ip_seen) . ')';
    }
    $last_seen_ip_from_hop{$hop} = $ip; # init or update current HOP/I
+P

    printf "sect.%2d / hop %2d: %15s %15s %s\n",
      $section, $hop, $changemark_pre, prettyip($ip), $changemark_pos;
  }
}


__DATA__
13,4.69.137.70
14,4.69.134.70
15,4.69.134.113
16,4.69.135.185
17,4.69.134.246
18,4.68.18.75
19,4.59.0.10
20,124.211.34.129
21,203.181.100.61
22,118.155.197.140
23,124.211.10.66
24,163.139.130.138
25,163.139.124.57
26,202.215.179.1
27,202.215.179.11

13,4.69.137.74
14,4.69.134.70
15,4.69.134.113
16,4.69.135.185
17,4.69.134.246
18,4.68.18.11
19,4.59.0.10
20,124.211.34.121
21,203.181.100.61
22,118.155.197.140
23,124.211.10.66
24,163.139.130.138
25,163.139.124.57
26,202.215.179.1
27,202.215.179.11

13,4.69.137.70
14,4.69.134.78
15,4.69.134.125
16,4.69.135.185
17,4.69.134.250
18,4.68.18.139
19,4.59.0.10
20,124.211.34.121
21,203.181.100.189
22,118.155.197.140
23,124.211.10.66
24,163.139.130.138
25,163.139.124.57
26,202.215.179.1
27,202.215.179.11
[download]

Output:

sect. 1 / hop 13:                 004.069.137.070
sect. 1 / hop 14:                 004.069.134.070
sect. 1 / hop 15:                 004.069.134.113
sect. 1 / hop 16:                 004.069.135.185
sect. 1 / hop 17:                 004.069.134.246
sect. 1 / hop 18:                 004.068.018.075
sect. 1 / hop 19:                 004.059.000.010
sect. 1 / hop 20:                 124.211.034.129
sect. 1 / hop 21:                 203.181.100.061
sect. 1 / hop 22:                 118.155.197.140
sect. 1 / hop 23:                 124.211.010.066
sect. 1 / hop 24:                 163.139.130.138
sect. 1 / hop 25:                 163.139.124.057
sect. 1 / hop 26:                 202.215.179.001
sect. 1 / hop 27:                 202.215.179.011

sect. 2 / hop 13:      changed to 004.069.137.074 (was: 004.069.137.07
+0)
sect. 2 / hop 14:                 004.069.134.070
sect. 2 / hop 15:                 004.069.134.113
sect. 2 / hop 16:                 004.069.135.185
sect. 2 / hop 17:                 004.069.134.246
sect. 2 / hop 18:      changed to 004.068.018.011 (was: 004.068.018.07
+5)
sect. 2 / hop 19:                 004.059.000.010
sect. 2 / hop 20:      changed to 124.211.034.121 (was: 124.211.034.12
+9)
sect. 2 / hop 21:                 203.181.100.061
sect. 2 / hop 22:                 118.155.197.140
sect. 2 / hop 23:                 124.211.010.066
sect. 2 / hop 24:                 163.139.130.138
sect. 2 / hop 25:                 163.139.124.057
sect. 2 / hop 26:                 202.215.179.001
sect. 2 / hop 27:                 202.215.179.011

sect. 3 / hop 13:      changed to 004.069.137.070 (was: 004.069.137.07
+4)
sect. 3 / hop 14:      changed to 004.069.134.078 (was: 004.069.134.07
+0)
sect. 3 / hop 15:      changed to 004.069.134.125 (was: 004.069.134.11
+3)
sect. 3 / hop 16:                 004.069.135.185
sect. 3 / hop 17:      changed to 004.069.134.250 (was: 004.069.134.24
+6)
sect. 3 / hop 18:      changed to 004.068.018.139 (was: 004.068.018.01
+1)
sect. 3 / hop 19:                 004.059.000.010
sect. 3 / hop 20:                 124.211.034.121
sect. 3 / hop 21:      changed to 203.181.100.189 (was: 203.181.100.06
+1)
sect. 3 / hop 22:                 118.155.197.140
sect. 3 / hop 23:                 124.211.010.066
sect. 3 / hop 24:                 163.139.130.138
sect. 3 / hop 25:                 163.139.124.057
sect. 3 / hop 26:                 202.215.179.001
sect. 3 / hop 27:                 202.215.179.011
[download]