explore the neighborhood by looking at the PTR for the whole /24

Looking up the PTR resource records / reverses of the /24 neighbourhood of an IP address reveals lots of connections.
To do so I use two bash functions. One function for printing all the reverses and one for creating an html file with pointers to more information.

I named the first function ptr24 and it looks like.
g0:~$cat .bashrc | head -14
function ptr24 {
CNET=`echo $1|awk -F"." '{print $1"."$2"."$3}'`
echo "PTR lookup for $CNET.0/24"
for i in `seq 0 255`;do
	
	CUR=`dig +short -x $CNET.$i`;
	if [ -n "$CUR" ]; then
		echo -n "$CNET.$i -> ";
        	echo $CUR;
	fi
	
done
}


So, if for example we want to explore the neighbourhood of the gnu.org web server we could that.
g0:~$dig +short gnu.org
140.186.70.148
g0:~$ptr24 140.186.70.148
PTR lookup for 140.186.70.0/24
140.186.70.1 -> ge-core1.qcy.gnu.org.
140.186.70.10 -> fencepost.gnu.org.
140.186.70.11 -> leviathan.gnu.org.
140.186.70.13 -> mail.fsf.org.
140.186.70.14 -> gnusenet.gnu.org.
140.186.70.15 -> fencepost-ssh.gnu.org.
140.186.70.17 -> lists.gnu.org.
140.186.70.20 -> ftp.gnu.org.
140.186.70.21 -> alpha.gnu.org.
140.186.70.22 -> ftp-upload.gnu.org.
140.186.70.23 -> webmail.fsf.org.
140.186.70.25 -> livestream.fsf.org.
140.186.70.26 -> sandbox.gnewsense.org.
...
140.186.70.154 -> shop-dev.fsf.org.
140.186.70.155 -> testtaranis.gnu.org.
140.186.70.156 -> gplv3.fsf.org.
140.186.70.157 -> audio-video-dev.gnu.org.
140.186.70.253 -> ge-sw1.qcy.gnu.org.
g0:~$

I named the second function ptr24htm and looks like this.
g0:~$cat .bashrc |head -28|tail -15

function ptr24htm {
CNET=`echo $1|awk -F"." '{print $1"."$2"."$3}'`
echo "PTR lookup for $CNET.0/24<br />"
for i in `seq 0 255`;do

CUR=`dig +short -x $CNET.$i`;
if [ -n "$CUR" ]; then
    echo -n "$CNET.$i -> ";
    echo "<a href=http://ipduh.com/dns/?$CUR>$CUR</a><br />";
fi

done
}


To produce a ptr.html with the information related to gnu.org we can do the following.
g0:~$ptr24htm `dig +short gnu.org` > ptr.html
g0:~$

The ptr.html looks like
PTR lookup for 140.186.70.0/24
140.186.70.1 -> ge-core1.qcy.gnu.org.
140.186.70.10 -> fencepost.gnu.org.
140.186.70.11 -> leviathan.gnu.org.
140.186.70.13 -> mail.fsf.org.
140.186.70.14 -> gnusenet.gnu.org.
140.186.70.15 -> fencepost-ssh.gnu.org.
140.186.70.17 -> lists.gnu.org.
140.186.70.20 -> ftp.gnu.org.
140.186.70.21 -> alpha.gnu.org.
140.186.70.22 -> ftp-upload.gnu.org.
140.186.70.23 -> webmail.fsf.org.
140.186.70.25 -> livestream.fsf.org.
140.186.70.26 -> sandbox.gnewsense.org.
140.186.70.30 -> svnweb.fsf.org.
140.186.70.31 -> spamhaus-rsync.fsf.org.
140.186.70.32 -> defectivebydesign.org.
140.186.70.33 -> galactica.fsf.org.
140.186.70.34 -> catalyst.fsf.org.
140.186.70.35 -> archive.gnewsense.org.
140.186.70.36 -> littlenemo.fsf.org.
140.186.70.37 -> labyrinth.fsf.org.
140.186.70.38 -> sycophant.fsf.org.
140.186.70.39 -> zaphod.gnu.org.
140.186.70.40 -> agilus.fsf.org.
140.186.70.41 -> bluemchen.kde.org.
140.186.70.42 -> agia.fsf.org.
140.186.70.43 -> debbugs.gnu.org.
140.186.70.44 -> config.fsf.org.
140.186.70.45 -> jamsession.fsf.org.
140.186.70.46 -> groups.fsf.org.
140.186.70.47 -> UNUSED-47.gnu.org.
140.186.70.48 -> tor.fsf.org.
140.186.70.49 -> archive.fsf.org.
140.186.70.50 -> smtp.member.fsf.org.
140.186.70.51 -> colonialone.fsf.org.
140.186.70.52 -> mirror.fsf.org.
140.186.70.53 -> sunjammer.sugarlabs.org.
140.186.70.54 -> linux-libre.fsfla.org.
140.186.70.56 -> news.swpat.org.
140.186.70.58 -> zope.fsf.org.
140.186.70.59 -> www-old.fsf.org.
140.186.70.60 -> my.fsf.org.
140.186.70.61 -> ldap.fsf.org.
140.186.70.62 -> code.autonomo.us.
140.186.70.63 -> www-dev.fsf.org.
140.186.70.64 -> my-dev.fsf.org.
140.186.70.65 -> cloud9.fsf.org.
140.186.70.66 -> blag.fsf.org.
140.186.70.67 -> windows7sins.org.
140.186.70.69 -> ftp-dev.gnu.org.
140.186.70.70 -> savannah.gnu.org.
140.186.70.71 -> savannah.nongnu.org.
140.186.70.72 -> vcs.savannah.gnu.org.
140.186.70.73 -> download.savannah.gnu.org.
140.186.70.74 -> mgt.savannah.gnu.org.
140.186.70.75 -> internal.savannah.gnu.org.
140.186.70.76 -> vpn.savannah.gnu.org.
140.186.70.80 -> groups-dev.fsf.org.
140.186.70.81 -> cas.fsf.org.
140.186.70.82 -> jabber.fsf.org.
140.186.70.83 -> brains.fsf.org.
140.186.70.84 -> balance.fsf.org.
140.186.70.85 -> eccles.gnewsense.org.
140.186.70.86 -> bloodnok.gnewsense.org.
140.186.70.87 -> seagoon.gnewsense.org.
140.186.70.88 -> config.gnewsense.org.
140.186.70.89 -> elpa.gnu.org.
140.186.70.90 -> heinlein.fsf.org.
140.186.70.91 -> mycroft.fsf.org.
140.186.70.92 -> eggs.gnu.org.
140.186.70.93 -> columbia.fsf.org.
140.186.70.94 -> directoryng-dev.fsf.org.
140.186.70.95 -> resolver1.fsf.org.
140.186.70.96 -> crm.fsf.org.
140.186.70.97 -> vinge.fsf.org.
140.186.70.98 -> nonce.fsf.org.
140.186.70.99 -> id-dev.fsf.org.
140.186.70.100 -> treehouse.sugarlabs.org.
140.186.70.101 -> UNUSED101.sugarlabs.org.
140.186.70.102 -> lightwave.sugarlabs.org.
140.186.70.103 -> UNUSED103.sugarlabs.org.
140.186.70.104 -> dextrose.sugarlabs.org.
140.186.70.105 -> UNUSED105.sugarlabs.org.
140.186.70.106 -> pootle.sugarlabs.org.
140.186.70.107 -> usr.sugarlabs.org.
140.186.70.108 -> UNUSED108.sugarlabs.org.
140.186.70.109 -> template-lucid.sugarlabs.org.
140.186.70.110 -> identity.sugarlabs.org.
140.186.70.111 -> UNUSED111.sugarlabs.org.
140.186.70.112 -> zatoichi.sugarlabs.org.
140.186.70.113 -> openlesson.sugarlabs.org.
140.186.70.114 -> UNUSED114.sugarlabs.org.
140.186.70.115 -> buildslave-ubuntu-lucid-64bit.sugarlabs.org.
140.186.70.116 -> UNUSED116.sugarlabs.org.
140.186.70.117 -> UNUSED117.sugarlabs.org.
140.186.70.118 -> UNUSED118.sugarlabs.org.
140.186.70.119 -> UNUSED119.sugarlabs.org.
140.186.70.120 -> UNUSED120.sugarlabs.org.
140.186.70.121 -> booki.treehouse.su.
140.186.70.122 -> anno.treehouse.su.
140.186.70.123 -> aslo-web.sugarlabs.org.
140.186.70.124 -> status.treehouse.su.
140.186.70.125 -> rt.sugarlabs.org.
140.186.70.126 -> schooltool.sugarlabs.org.
140.186.70.127 -> mapspress.sugarlabs.org.
140.186.70.128 -> monitoring.treehouse.su.
140.186.70.129 -> idea.sugarlabs.org.
140.186.70.130 -> dirac.fsf.org.
140.186.70.131 -> www.fsf.org.
140.186.70.132 -> lists.fsf.org.
140.186.70.133 -> testlucid.gnu.org.
140.186.70.134 -> resolver2.fsf.org.
140.186.70.135 -> seeder.gnu.org.
140.186.70.136 -> edit.fsf.org.
140.186.70.137 -> crm-dev.fsf.org.
140.186.70.138 -> logger.fsf.org.
140.186.70.139 -> dbd-dev.fsf.org.
140.186.70.140 -> social.gnu.org.
140.186.70.141 -> wiki-dev.swpat.org.
140.186.70.142 -> news-dev.swpat.org.
140.186.70.143 -> wiki.swpat.org.
140.186.70.144 -> bluemchen2.kde.org.
140.186.70.145 -> termite.fsf.org.
140.186.70.146 -> airhorn.fsf.org.
140.186.70.147 -> directory-dev.fsf.org.
140.186.70.148 -> wildebeest.gnu.org.
140.186.70.149 -> goodbye.gnu.org.
140.186.70.150 -> directory-p.fsf.org.
140.186.70.151 -> www.nongnu.org.
140.186.70.153 -> bitcoin.fsf.org.
140.186.70.154 -> shop-dev.fsf.org.
140.186.70.155 -> testtaranis.gnu.org.
140.186.70.156 -> gplv3.fsf.org.
140.186.70.157 -> audio-video-dev.gnu.org.
140.186.70.253 -> ge-sw1.qcy.gnu.org.