ClearAll[CanonicalizeCIDR]
CanonicalizeCIDR[str_String] := Module[{i, ip, chop, keep, change},
  If[StringMatchQ[str, "*.*.*.*/*"],
   i = StringSplit[str, "." | "/"];
   i = Interpreter["Integer"] /@ i;
   If[MatchQ[i, {_Integer, _Integer, _Integer, _Integer, _Integer}],
    If[AllTrue[i, Between[{0, 255}]],
     {ip, {chop}} = TakeDrop[i, 4];
     ip = Catenate[IntegerDigits[ip, 2, 8]];
     {keep, change} = TakeDrop[ip, chop];
     change = ConstantArray[0, Length[change]];
     ip = Partition[Join[keep, change], 8];
     ip = ToString[FromDigits[#, 2]] & /@ ip;
     StringRiffle[ip, "."] <> "/" <> ToString[chop]
     ,
     Failure["Invalid range of numbers", <|"Input" -> str|>]
     ]
    ,
    Failure["Invalid format", <|"Input" -> str|>]
    ]
   ]
  ]
CanonicalizeCIDR["87.70.141.1/22"]
CanonicalizeCIDR["36.18.154.103/12"]
CanonicalizeCIDR["62.62.197.11/29"]
CanonicalizeCIDR["67.137.119.181/4"]
CanonicalizeCIDR["161.214.74.21/24"]
CanonicalizeCIDR["184.232.176.184/18"]