diff --git a/.gitignore b/.gitignore
index d4aa077..21c4153 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 test/isinetaddr
 test/iscidraddr
+test/isinetaddr6
diff --git a/Makefile b/Makefile
index 54fb911..c64f5e1 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 SRCDIR   = src
-SRCFILES = $(SRCDIR)/isinetaddr.c $(SRCDIR)/iscidraddr.c
+SRCFILES = $(SRCDIR)/isinetaddr.c $(SRCDIR)/iscidraddr.c $(SRCDIR)/isinetaddr6.c
 INCDIR   = include
 TESTDIR  = test
 
@@ -7,6 +7,10 @@ CC = cc
 CFLAGS = -fstack-protector-all -I$(INCDIR) -Wall -Wextra -pedantic
 
 test:
+	@make test4
+	@make test6
+
+test4:
 	@$(CC) $(CFLAGS) $(SRCFILES) $(TESTDIR)/isinetaddr_test.c -o $(TESTDIR)/isinetaddr
 	@echo -n test/isinetaddr: ''
 	@$(TESTDIR)/isinetaddr
@@ -14,4 +18,9 @@ test:
 	@echo -n test/iscidraddr: ''
 	@$(TESTDIR)/iscidraddr
 
+test6:
+	@$(CC) $(CFLAGS) $(SRCFILES) $(TESTDIR)/isinetaddr6_test.c -o $(TESTDIR)/isinetaddr6
+	@echo -n test/isinetaddr6: ''
+	@$(TESTDIR)/isinetaddr6
+
 .PHONY: test
diff --git a/include/isinetaddr.h b/include/isinetaddr.h
index 80e7696..798b6d8 100644
--- a/include/isinetaddr.h
+++ b/include/isinetaddr.h
@@ -1,3 +1,4 @@
 #pragma once
 int isinetaddr(const char *str);
 int iscidraddr(const char *str);
+int isinetaddr6(const char *str);
diff --git a/src/isinetaddr6.c b/src/isinetaddr6.c
new file mode 100644
index 0000000..7ef79a2
--- /dev/null
+++ b/src/isinetaddr6.c
@@ -0,0 +1,90 @@
+#include <ctype.h>
+#include <string.h>
+#include <stdlib.h>
+#include <isinetaddr.h>
+
+static const int  MAX_DIGITLEN  = 4;
+static const int  MAX_HEXTETS   = 8;
+static const int  MAX_HEXDIGITS = 32;
+static const int  MAX_STRLEN    = 40;
+static const char SEP = ':';
+
+static int has_consecutive_chars(const char *str, char c, int n);
+static char* expand(const char *str, size_t strlen, char *new_str, size_t headlen);
+
+int
+isinetaddr6(const char *str)
+{
+  int hextets = 1, digitlen = 0, hexdigits = 0;
+  size_t len = (str == NULL ? 0 : strnlen(str, MAX_STRLEN));
+
+  if (len == 0) {
+    return 0;
+  } else if (strncasecmp(str, "::ffff", 6) == 0) {
+    return isinetaddr(&str[7]);
+  }
+  for (size_t i = 0; i < len; i++) {
+    if (has_consecutive_chars(&str[i], SEP, 3)) {
+      return 0;
+    } else if (has_consecutive_chars(&str[i], SEP, 2)) {
+      char new_str[MAX_STRLEN];
+      return isinetaddr6(expand(str, len, new_str, i));
+    } else if (str[i] == SEP) {
+      if (digitlen < MAX_DIGITLEN) {
+        return 0;
+      } else {
+        digitlen = 0;
+        hextets++;
+      }
+    } else if (isxdigit(str[i])) {
+      if (digitlen == MAX_DIGITLEN) {
+        return 0;
+      } else {
+        digitlen++;
+        hexdigits++;
+      }
+    } else {
+      return 0;
+    }
+  }
+  if (hextets == MAX_HEXTETS) {
+    return hexdigits <= MAX_HEXDIGITS && digitlen == MAX_DIGITLEN;
+  } else {
+    return 0;
+  }
+}
+
+static int
+has_consecutive_chars(const char *str, char c, int n)
+{
+  for (int i = 0; i < n; i++) {
+    if (*str != c) {
+      return 0;
+    }
+    str++;
+  }
+  return 1;
+}
+
+static char*
+expand(const char *str, size_t strlen, char *new_str, size_t headlen)
+{
+  char *ptr = new_str;
+  size_t taillen = (strlen - headlen) - 2;
+  size_t bodylen = MAX_STRLEN - taillen;
+  size_t i = headlen + 2;
+  size_t j = headlen;
+
+  while (i++ < strlen) {
+    if (has_consecutive_chars(&str[i], SEP, 2)) {
+      return NULL;
+    }
+  }
+  memcpy(ptr, &str[0], headlen);
+  ptr += headlen;
+  while (++j < bodylen) {
+    *ptr++ = j % 5 == 0 ? ':' : '0';
+  }
+  memcpy(ptr, &str[headlen + 2], taillen);
+  return new_str;
+}
diff --git a/test/isinetaddr6_test.c b/test/isinetaddr6_test.c
new file mode 100644
index 0000000..7e4fe11
--- /dev/null
+++ b/test/isinetaddr6_test.c
@@ -0,0 +1,72 @@
+#include <isinetaddr.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+const char *valid[] = {
+  /* valid IPv6 (single colon)*/
+  "0000:0000:0000:0000:0000:0000:0000:0001",
+  "2001:0db8:85a3:0000:0000:8a2e:0370:7334",
+  "fe80:0000:0000:0000:0202:b3ff:fe1e:8329",
+  "abcd:abcd:abcd:abcd:abcd:abcd:abcd:abcd",
+  "1234:5678:9abc:def0:face:face:b00c:0ffe",
+  "2001:19f0:5401:0000:0000:ffff:1e61:face",
+  "dead:beef:cafe:babe:affe:8a2e:0370:7334",
+
+  /* valid IPv6 (double colon) */
+  "fe80::c001:a0ff:fe12:3456",
+  "2001:0db8::1",
+  "2001:abcd:ef01:2345::",
+  "1234::5678",
+  "0000:00::1111",
+  "000::1111:1111",
+  "000::1111:1111:1111",
+  "0000:00::1111",
+  "0000:0000:0000:0000:0000:0000:0000:00::",
+  "2001:0db8:85a3:0000:0000::8a2e:0370:7334",
+  "2001::5",
+  "::ffff:192.168.2.1",
+  "::FFFF:192.168.2.1",
+  "::",
+  "::1",
+};
+
+const char *invalid[] = {
+  /* invalid IPv6 (single colon) */
+  "0000:0000:0000:0000:0000:0000:0000:000Z",
+  "0000:0000:0000:0000:0000:0000:0000:0",
+  "0000:0000:0000:0000:0000:0000:0000:",
+
+  /* invalid IPv6 (double colon) */
+  ":::", "2001:::1", "2001:::1::",
+  "2001::1::", "::1::",
+  "2001:db8:85a3::8a2e:3700:7334",
+  "::ffff", "::ffff:", "::ffff:00",
+
+  /* edge cases */
+  NULL
+};
+
+int
+main(void) {
+  size_t len;
+  /* IPv6: valid */
+  len = sizeof(valid) / sizeof(valid[0]);
+  for (size_t i = 0; i < len; i++) {
+    if (isinetaddr6(valid[i]) != 1) {
+      fprintf(stderr, "assertion failed: '%s' should be valid\n", valid[i]);
+      abort();
+    }
+  }
+  /* IPv6: invalid */
+  len = sizeof(invalid) / sizeof(invalid[0]);
+  for (size_t i = 0; i < len; i++) {
+    if (isinetaddr6(invalid[i]) != 0) {
+      fprintf(stderr, "assertion failed: '%s' should NOT be valid\n", invalid[i]);
+      abort();
+    }
+  }
+  /* Done */
+  printf("OK\n");
+  return EXIT_SUCCESS;
+}