sort -t"." -k1,1n -k2,2n -k3,3n -k4,4n ip.lst

#cat t.txt 
 172.30.100.103
 172.30.161.239
 172.31.155.5
 172.31.155.5
 172.30.100.103
 172.30.100.103
 172.30.100.103
 172.31.176.59
 172.31.176.59
 172.31.176.59
 172.31.176.59
 172.30.100.103
 172.30.161.239
 172.30.161.239
 172.30.161.239

#cat t.txt| sort | uniq -c | sort -n -r  
   5  172.30.100.103
   4  172.31.176.59
   4  172.30.161.239
   2  172.31.155.5

sort:对IP部分进行排序。
uniq -c:打印每一重复行出现的次数。(并去掉重复行)
sort -n -r:按照重复行出现的次序倒序排列。