diff --git a/lib/internal/util.js b/lib/internal/util.js
index e690d7f4323bcc..7a928adb71c043 100644
--- a/lib/internal/util.js
+++ b/lib/internal/util.js
@@ -8,6 +8,7 @@ const {
   Error,
   ErrorCaptureStackTrace,
   FunctionPrototypeCall,
+  NumberParseInt,
   ObjectDefineProperties,
   ObjectDefineProperty,
   ObjectFreeze,
@@ -33,7 +34,9 @@ const {
   SafeSet,
   SafeWeakMap,
   SafeWeakRef,
+  StringPrototypeIncludes,
   StringPrototypeReplace,
+  StringPrototypeSlice,
   StringPrototypeToLowerCase,
   StringPrototypeToUpperCase,
   Symbol,
@@ -795,6 +798,59 @@ function setupCoverageHooks(dir) {
   return coverageDirectory;
 }
 
+// Returns the number of ones in the binary representation of the decimal
+// number.
+function countBinaryOnes(n) {
+  // Count the number of bits set in parallel, which is faster than looping
+  n = n - ((n >>> 1) & 0x55555555);
+  n = (n & 0x33333333) + ((n >>> 2) & 0x33333333);
+  return ((n + (n >>> 4) & 0xF0F0F0F) * 0x1010101) >>> 24;
+}
+
+function getCIDR(address, netmask, family) {
+  let ones = 0;
+  let split = '.';
+  let range = 10;
+  let groupLength = 8;
+  let hasZeros = false;
+  let lastPos = 0;
+
+  if (family === 'IPv6') {
+    split = ':';
+    range = 16;
+    groupLength = 16;
+  }
+
+  for (let i = 0; i < netmask.length; i++) {
+    if (netmask[i] !== split) {
+      if (i + 1 < netmask.length) {
+        continue;
+      }
+      i++;
+    }
+    const part = StringPrototypeSlice(netmask, lastPos, i);
+    lastPos = i + 1;
+    if (part !== '') {
+      if (hasZeros) {
+        if (part !== '0') {
+          return null;
+        }
+      } else {
+        const binary = NumberParseInt(part, range);
+        const binaryOnes = countBinaryOnes(binary);
+        ones += binaryOnes;
+        if (binaryOnes !== groupLength) {
+          if (StringPrototypeIncludes(binary.toString(2), '01')) {
+            return null;
+          }
+          hasZeros = true;
+        }
+      }
+    }
+  }
+
+  return `${address}/${ones}`;
+}
 
 const handleTypes = ['TCP', 'TTY', 'UDP', 'FILE', 'PIPE', 'UNKNOWN'];
 function guessHandleType(fd) {
@@ -861,6 +917,7 @@ module.exports = {
   filterDuplicateStrings,
   filterOwnProperties,
   getConstructorOf,
+  getCIDR,
   getCWDURL,
   getInternalGlobal,
   getStructuredStack,
diff --git a/lib/os.js b/lib/os.js
index 4f8dda1531b5dc..c44147f0e1170d 100644
--- a/lib/os.js
+++ b/lib/os.js
@@ -24,7 +24,6 @@
 const {
   ArrayPrototypePush,
   Float64Array,
-  NumberParseInt,
   ObjectDefineProperties,
   StringPrototypeSlice,
   SymbolToPrimitive,
@@ -40,6 +39,7 @@ const {
   },
   hideStackFrames,
 } = require('internal/errors');
+const { getCIDR } = require('internal/util');
 const { validateInt32 } = require('internal/validators');
 
 const {
@@ -202,60 +202,6 @@ function endianness() {
 }
 endianness[SymbolToPrimitive] = () => kEndianness;
 
-// Returns the number of ones in the binary representation of the decimal
-// number.
-function countBinaryOnes(n) {
-  // Count the number of bits set in parallel, which is faster than looping
-  n = n - ((n >>> 1) & 0x55555555);
-  n = (n & 0x33333333) + ((n >>> 2) & 0x33333333);
-  return ((n + (n >>> 4) & 0xF0F0F0F) * 0x1010101) >>> 24;
-}
-
-function getCIDR(address, netmask, family) {
-  let ones = 0;
-  let split = '.';
-  let range = 10;
-  let groupLength = 8;
-  let hasZeros = false;
-  let lastPos = 0;
-
-  if (family === 'IPv6') {
-    split = ':';
-    range = 16;
-    groupLength = 16;
-  }
-
-  for (let i = 0; i < netmask.length; i++) {
-    if (netmask[i] !== split) {
-      if (i + 1 < netmask.length) {
-        continue;
-      }
-      i++;
-    }
-    const part = StringPrototypeSlice(netmask, lastPos, i);
-    lastPos = i + 1;
-    if (part !== '') {
-      if (hasZeros) {
-        if (part !== '0') {
-          return null;
-        }
-      } else {
-        const binary = NumberParseInt(part, range);
-        const binaryOnes = countBinaryOnes(binary);
-        ones += binaryOnes;
-        if (binaryOnes !== groupLength) {
-          if ((binary & 1) !== 0) {
-            return null;
-          }
-          hasZeros = true;
-        }
-      }
-    }
-  }
-
-  return `${address}/${ones}`;
-}
-
 /**
  * @returns {Record<string, Array<{
  *  address: string,
diff --git a/test/parallel/test-internal-util-getCIDR.js b/test/parallel/test-internal-util-getCIDR.js
new file mode 100644
index 00000000000000..f7dd3c1194b6a2
--- /dev/null
+++ b/test/parallel/test-internal-util-getCIDR.js
@@ -0,0 +1,23 @@
+// Flags: --expose-internals
+'use strict';
+require('../common');
+
+// These are tests that verify that the subnetmask is used
+// to create the correct CIDR address.
+// Tests that it returns null if the subnetmask is not in the correct format.
+// (ref: https://www.rfc-editor.org/rfc/rfc1878)
+
+const assert = require('node:assert');
+const { getCIDR } = require('internal/util');
+
+assert.strictEqual(getCIDR('127.0.0.1', '255.0.0.0', 'IPv4'), '127.0.0.1/8');
+assert.strictEqual(getCIDR('127.0.0.1', '255.255.0.0', 'IPv4'), '127.0.0.1/16');
+
+// 242 = 11110010(2)
+assert.strictEqual(getCIDR('127.0.0.1', '242.0.0.0', 'IPv4'), null);
+
+assert.strictEqual(getCIDR('::1', 'ffff:ffff:ffff:ffff::', 'IPv6'), '::1/64');
+assert.strictEqual(getCIDR('::1', 'ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff', 'IPv6'), '::1/128');
+
+// ff00:ffff = 11111111 00000000 : 11111111 11111111(2)
+assert.strictEqual(getCIDR('::1', 'ffff:ff00:ffff::', 'IPv6'), null);