diff --git a/README.md b/README.md
index b5e5c037..16367ce5 100644
--- a/README.md
+++ b/README.md
@@ -4,29 +4,76 @@
 [![codecov](https://codecov.io/gh/arduino/ArduinoCore-API/branch/master/graph/badge.svg)](https://codecov.io/gh/arduino/ArduinoCore-API)
 [![Spell Check status](https://github.com/arduino/ArduinoCore-API/actions/workflows/spell-check.yml/badge.svg)](https://github.com/arduino/ArduinoCore-API/actions/workflows/spell-check.yml)
 
-This repository hosts the hardware independent layer of Arduino core.
+This repository hosts the hardware independent layer of Arduino core. In other words it contains the abstract definition of the Arduino core API, consisting of hardware-independent header files that are then included and implemented by the various platform-specific cores.
 
-All Arduino official cores are being ported to the new structure so they take advantage of this single repo.
+Having a single place where the Arduino API is defined means that there is no longer a String implementation within every Arduino core (a String module within ArduinoCore-avr, a String module within ArduinoCore-samd, a String module within ArduinoCore-megaavr …) but rather one String implementation within ArduinoCore-API which all other cores utilise. This has the pleasant side effects that bugs fixed or features added within the ArduinoCore-API String implementation are automatically propagated to all cores utilizing ArduinoCore-API.
 
-Including this repo in your existing Arduino core will allow the language to grow and include new features.
-For backwards compatibility, every revision of this repo will increase the `ARDUINO_API_VERSION` define.
+As of now, the following official cores are utilising ArduinoCore-API:
 
-Some cores have been ported to the new structure, for example:
-* megaAVR (https://github.com/arduino/ArduinoCore-megaAVR) 
-* nRF52-mbedos (https://github.com/arduino/ArduinoCore-nRF528x-mbedos)
-* classic AVR (https://github.com/arduino/ArduinoCore-avr/tree/api) 
-* SAMD (https://github.com/arduino/ArduinoCore-samd/tree/api)
+* [megaavr](https://github.com/arduino/ArduinoCore-megaAVR)
+* [mbed](https://github.com/arduino/ArduinoCore-mbed)
+* [samd](https://github.com/arduino/ArduinoCore-samd)
 
-These repositories **don't** contain the needed `api` subfolder; to "complete" the core you need to copy or symlink the `api` folder from this repo to the target's `cores/arduino` folder.
+There's an ongoing effort to port the others, while maintainers of third-party cores are strongly invited to follow the same route in order to stay up-to-date with the new language features. For backwards compatibility, every revision of this repo will increase the `ARDUINO_API_VERSION` define.
 
-### Porting tips
+## Documentation
 
-In the future, core APIs will be updated independently from the core, so all the compatible cores will seamlessly adopt new features.
-This requires support from all the IDEs, so in the meantime we suggest to release the core by copying a snapshot of this `api` folder.
+The Arduino API is documented in the official [language reference](https://www.arduino.cc/reference/en/), whose sources are located in [this repository](https://github.com/arduino/reference-en) and are open to contributions from the community.
 
-The most elegant and effective solution is to develop the core with `api` symlinked and produce the distributable archive by telling `tar` to follow symlinks. 
-Example command:
+## Support
+
+This repository is not directly usable by final users. If you need assistance with Arduino, see the [Help Center](https://support.arduino.cc/) and browse the [forum](https://forum.arduino.cc).
+
+## Development
+
+### Bugs & Issues
+
+If you want to report an issue with this core, you can submit it to the [issue tracker](https://github.com/arduino/ArduinoCore-API/issues) of this repository. Some rules apply:
+
+* If your issue is about a specific hardware platform, report it to its repository. This one is only about discussing the generic API.
+* Before posting, please check if the same problem has been already reported by someone else to avoid duplicates.
+* Remember to include as much detail as you can about your hardware set-up, code and steps for reproducing the issue. Make sure you're using an original Arduino board.
+
+### Contributions
+
+Contributions are always welcome! You can submit them directly to this repository as Pull Requests. Please provide a detailed description of the problem you're trying to solve. We also appreciate any help in testing issues and patches contributed by other users.
+
+### Unit testing
+
+This repository includes a test suite that covers most of the API and that is designed to run on generic hardware, thus not requiring a development board. We call this _host-based unit-testing_. In order to test the features that are only defined but not implemented in this repository, mock implementations are included.
+
+Please help us improve the coverage of the test suite!
+
+#### To build and run unit tests
+
+The unit tests are automatically built by GitHub as part of pull request checks (in `.github/workflows/unit-tests.yml`).
+
+To build and run locally:
+
+**Dependencies**
+
+* [CMake](https://cmake.org/)
+* [GCC](https://gcc.gnu.org/)
+
+On (Ubuntu) Linux run:
+
+```bash
+sudo apt-get install build-essential cmake
 ```
+
+From the project root:
+
+```bash
+cd test && mkdir build && cd build
+cmake ..
+make && bin/test-ArduinoCore-API
+```
+
+### Implementing ArduinoCore-API
+
+In order to compile a core which is implementing ArduinoCore-API you'll need to copy/symlink the `api` directory to the target's `cores/arduino` directory as part of your development and release workflow. The most elegant and effective solution is to develop your core with `api` symlinked and produce the distributable archive by telling `tar` to follow symlinks. Example:
+
+```bash
 tar --exclude='*.git*' -cjhvf $yourcore-$version.tar.bz2 $yourcore/
 ```
 
@@ -34,3 +81,12 @@ Documentation for how to integrate with a Arduino core (which is necessary if yo
 * [ArduinoCore-megaavr](https://github.com/arduino/ArduinoCore-megaavr#developing)
 * [ArduinoCore-mbed](https://github.com/arduino/ArduinoCore-mbed#clone-the-repository-in-sketchbookhardwarearduino-git)
 * [ArduinoCore-samd](https://github.com/arduino/ArduinoCore-samd/#developing)
+
+## Donations
+
+This open source code is maintained by Arduino with the help of the community. We invest a considerable amount of time in testing code, optimizing it and introducing new features. Please consider [donating](https://www.arduino.cc/en/donate/) or [sponsoring](https://github.com/sponsors/arduino) to support our work, as well as [buying original Arduino boards](https://store.arduino.cc) which is the best way to make sure our effort can continue in the long term.
+
+## License and credits
+
+This code is licensed under the terms of the GNU LGPL 2.1. If you have questions about licensing please contact us at [license@arduino.cc](mailto:license@arduino.cc).
+
diff --git a/api/IPAddress.cpp b/api/IPAddress.cpp
index 5cf62d5e..4b729319 100644
--- a/api/IPAddress.cpp
+++ b/api/IPAddress.cpp
@@ -22,36 +22,93 @@
 
 using namespace arduino;
 
-IPAddress::IPAddress()
+IPAddress::IPAddress() : IPAddress(IPv4) {}
+
+IPAddress::IPAddress(IPType ip_type)
 {
-    _address.dword = 0;
+    _type = ip_type;
+    memset(_address.bytes, 0, sizeof(_address.bytes));
 }
 
 IPAddress::IPAddress(uint8_t first_octet, uint8_t second_octet, uint8_t third_octet, uint8_t fourth_octet)
 {
-    _address.bytes[0] = first_octet;
-    _address.bytes[1] = second_octet;
-    _address.bytes[2] = third_octet;
-    _address.bytes[3] = fourth_octet;
+    _type = IPv4;
+    memset(_address.bytes, 0, sizeof(_address.bytes));
+    _address.bytes[IPADDRESS_V4_BYTES_INDEX] = first_octet;
+    _address.bytes[IPADDRESS_V4_BYTES_INDEX + 1] = second_octet;
+    _address.bytes[IPADDRESS_V4_BYTES_INDEX + 2] = third_octet;
+    _address.bytes[IPADDRESS_V4_BYTES_INDEX + 3] = fourth_octet;
+}
+
+IPAddress::IPAddress(uint8_t o1, uint8_t o2, uint8_t o3, uint8_t o4, uint8_t o5, uint8_t o6, uint8_t o7, uint8_t o8, uint8_t o9, uint8_t o10, uint8_t o11, uint8_t o12, uint8_t o13, uint8_t o14, uint8_t o15, uint8_t o16) {
+    _type = IPv6;
+    _address.bytes[0] = o1;
+    _address.bytes[1] = o2;
+    _address.bytes[2] = o3;
+    _address.bytes[3] = o4;
+    _address.bytes[4] = o5;
+    _address.bytes[5] = o6;
+    _address.bytes[6] = o7;
+    _address.bytes[7] = o8;
+    _address.bytes[8] = o9;
+    _address.bytes[9] = o10;
+    _address.bytes[10] = o11;
+    _address.bytes[11] = o12;
+    _address.bytes[12] = o13;
+    _address.bytes[13] = o14;
+    _address.bytes[14] = o15;
+    _address.bytes[15] = o16;
 }
 
 IPAddress::IPAddress(uint32_t address)
 {
-    _address.dword = address;
+    // IPv4 only
+    _type = IPv4;
+    memset(_address.bytes, 0, sizeof(_address.bytes));
+    _address.dword[IPADDRESS_V4_DWORD_INDEX] = address;
+
+    // NOTE on conversion/comparison and uint32_t:
+    // These conversions are host platform dependent.
+    // There is a defined integer representation of IPv4 addresses,
+    // based on network byte order (will be the value on big endian systems),
+    // e.g. http://2398766798 is the same as http://142.250.70.206,
+    // However on little endian systems the octets 0x83, 0xFA, 0x46, 0xCE,
+    // in that order, will form the integer (uint32_t) 3460758158 .
+}
+
+IPAddress::IPAddress(const uint8_t *address) : IPAddress(IPv4, address) {}
+
+IPAddress::IPAddress(IPType ip_type, const uint8_t *address)
+{
+    _type = ip_type;
+    if (ip_type == IPv4) {
+        memset(_address.bytes, 0, sizeof(_address.bytes));
+        memcpy(&_address.bytes[IPADDRESS_V4_BYTES_INDEX], address, sizeof(uint32_t));
+    } else {
+        memcpy(_address.bytes, address, sizeof(_address.bytes));
+    }
 }
 
-IPAddress::IPAddress(const uint8_t *address)
+IPAddress::IPAddress(const char *address)
 {
-    memcpy(_address.bytes, address, sizeof(_address.bytes));
+    fromString(address);
 }
 
-bool IPAddress::fromString(const char *address)
+bool IPAddress::fromString(const char *address) {
+    if (!fromString4(address)) {
+        return fromString6(address);
+    }
+    return true;
+}
+
+bool IPAddress::fromString4(const char *address)
 {
     // TODO: add support for "a", "a.b", "a.b.c" formats
 
     int16_t acc = -1; // Accumulator
     uint8_t dots = 0;
 
+    memset(_address.bytes, 0, sizeof(_address.bytes));
     while (*address)
     {
         char c = *address++;
@@ -73,7 +130,7 @@ bool IPAddress::fromString(const char *address)
                 /* No value between dots, e.g. '1..' */
                 return false;
             }
-            _address.bytes[dots++] = acc;
+            _address.bytes[IPADDRESS_V4_BYTES_INDEX + dots++] = acc;
             acc = -1;
         }
         else
@@ -91,37 +148,194 @@ bool IPAddress::fromString(const char *address)
         /* No value between dots, e.g. '1..' */
         return false;
     }
-    _address.bytes[3] = acc;
+    _address.bytes[IPADDRESS_V4_BYTES_INDEX + 3] = acc;
+    _type = IPv4;
+    return true;
+}
+
+bool IPAddress::fromString6(const char *address) {
+    uint32_t acc = 0; // Accumulator
+    int colons = 0, double_colons = -1;
+
+    while (*address)
+    {
+        char c = tolower(*address++);
+        if (isalnum(c) && c <= 'f') {
+            if (c >= 'a')
+                c -= 'a' - '0' - 10;
+            acc = acc * 16 + (c - '0');
+            if (acc > 0xffff)
+                // Value out of range
+                return false;
+        }
+        else if (c == ':') {
+            if (*address == ':') {
+                if (double_colons >= 0) {
+                    // :: allowed once
+                    return false;
+                }
+                if (*address != '\0' && *(address + 1) == ':') {
+                    // ::: not allowed
+                    return false;
+                }
+                // remember location
+                double_colons = colons + !!acc;
+                address++;
+            } else if (*address == '\0') {
+                // can't end with a single colon
+                return false;
+            }
+            if (colons == 7)
+                // too many separators
+                return false;
+            _address.bytes[colons * 2] = acc >> 8;
+            _address.bytes[colons * 2 + 1] = acc & 0xff;
+            colons++;
+            acc = 0;
+        }
+        else
+            // Invalid char
+            return false;
+    }
+
+    if (double_colons == -1 && colons != 7) {
+        // Too few separators
+        return false;
+    }
+    if (double_colons > -1 && colons > 6) {
+        // Too many segments (double colon must be at least one zero field)
+        return false;
+    }
+    _address.bytes[colons * 2] = acc >> 8;
+    _address.bytes[colons * 2 + 1] = acc & 0xff;
+    colons++;
+
+    if (double_colons != -1) {
+        for (int i = colons * 2 - double_colons * 2 - 1; i >= 0; i--)
+            _address.bytes[16 - colons * 2 + double_colons * 2 + i] = _address.bytes[double_colons * 2 + i];
+        for (int i = double_colons * 2; i < 16 - colons * 2 + double_colons * 2; i++)
+            _address.bytes[i] = 0;
+    }
+
+    _type = IPv6;
     return true;
 }
 
 IPAddress& IPAddress::operator=(const uint8_t *address)
 {
-    memcpy(_address.bytes, address, sizeof(_address.bytes));
+    // IPv4 only conversion from byte pointer
+    _type = IPv4;
+    memset(_address.bytes, 0, sizeof(_address.bytes));
+    memcpy(&_address.bytes[IPADDRESS_V4_BYTES_INDEX], address, sizeof(uint32_t));
+    return *this;
+}
+
+IPAddress& IPAddress::operator=(const char *address)
+{
+    fromString(address);
     return *this;
 }
 
 IPAddress& IPAddress::operator=(uint32_t address)
 {
-    _address.dword = address;
+    // IPv4 conversion
+    // See note on conversion/comparison and uint32_t
+    _type = IPv4;
+    memset(_address.bytes, 0, sizeof(_address.bytes));
+    _address.dword[IPADDRESS_V4_DWORD_INDEX] = address;
     return *this;
 }
 
+bool IPAddress::operator==(const IPAddress& addr) const {
+    return (addr._type == _type)
+        && (memcmp(addr._address.bytes, _address.bytes, sizeof(_address.bytes)) == 0);
+}
+
 bool IPAddress::operator==(const uint8_t* addr) const
 {
-    return memcmp(addr, _address.bytes, sizeof(_address.bytes)) == 0;
+    // IPv4 only comparison to byte pointer
+    // Can't support IPv6 as we know our type, but not the length of the pointer
+    return _type == IPv4 && memcmp(addr, &_address.bytes[IPADDRESS_V4_BYTES_INDEX], sizeof(uint32_t)) == 0;
+}
+
+uint8_t IPAddress::operator[](int index) const {
+    if (_type == IPv4) {
+        return _address.bytes[IPADDRESS_V4_BYTES_INDEX + index];
+    }
+    return _address.bytes[index];
+}
+
+uint8_t& IPAddress::operator[](int index) {
+    if (_type == IPv4) {
+        return _address.bytes[IPADDRESS_V4_BYTES_INDEX + index];
+    }
+    return _address.bytes[index];
 }
 
 size_t IPAddress::printTo(Print& p) const
 {
     size_t n = 0;
+
+    if (_type == IPv6) {
+        // IPv6 IETF canonical format: compress left-most longest run of two or more zero fields, lower case
+        int8_t longest_start = -1;
+        int8_t longest_length = 1;
+        int8_t current_start = -1;
+        int8_t current_length = 0;
+        for (int8_t f = 0; f < 8; f++) {
+            if (_address.bytes[f * 2] == 0 && _address.bytes[f * 2 + 1] == 0) {
+                if (current_start == -1) {
+                    current_start = f;
+                    current_length = 1;
+                } else {
+                    current_length++;
+                }
+                if (current_length > longest_length) {
+                    longest_start = current_start;
+                    longest_length = current_length;
+                }
+            } else {
+                current_start = -1;
+            }
+        }
+        for (int f = 0; f < 8; f++) {
+            if (f < longest_start || f >= longest_start + longest_length) {
+                uint8_t c1 = _address.bytes[f * 2] >> 4;
+                uint8_t c2 = _address.bytes[f * 2] & 0xf;
+                uint8_t c3 = _address.bytes[f * 2 + 1] >> 4;
+                uint8_t c4 = _address.bytes[f * 2 + 1] & 0xf;
+                if (c1 > 0) {
+                    n += p.print((char)(c1 < 10 ? '0' + c1 : 'a' + c1 - 10));
+                }
+                if (c1 > 0 || c2 > 0) {
+                    n += p.print((char)(c2 < 10 ? '0' + c2 : 'a' + c2 - 10));
+                }
+                if (c1 > 0 || c2 > 0 || c3 > 0) {
+                    n += p.print((char)(c3 < 10 ? '0' + c3 : 'a' + c3 - 10));
+                }
+                n += p.print((char)(c4 < 10 ? '0' + c4 : 'a' + c4 - 10));
+                if (f < 7) {
+                    n += p.print(':');
+                }
+            } else if (f == longest_start) {
+                if (longest_start == 0) {
+                    n += p.print(':');
+                }
+                n += p.print(':');
+            }
+        }
+        return n;
+    }
+
+    // IPv4
     for (int i =0; i < 3; i++)
     {
-        n += p.print(_address.bytes[i], DEC);
+        n += p.print(_address.bytes[IPADDRESS_V4_BYTES_INDEX + i], DEC);
         n += p.print('.');
     }
-    n += p.print(_address.bytes[3], DEC);
+    n += p.print(_address.bytes[IPADDRESS_V4_BYTES_INDEX + 3], DEC);
     return n;
 }
 
+const IPAddress arduino::IN6ADDR_ANY(IPv6);
 const IPAddress arduino::INADDR_NONE(0,0,0,0);
diff --git a/api/IPAddress.h b/api/IPAddress.h
index d70783ca..964faa67 100644
--- a/api/IPAddress.h
+++ b/api/IPAddress.h
@@ -23,6 +23,9 @@
 #include "Printable.h"
 #include "String.h"
 
+#define IPADDRESS_V4_BYTES_INDEX 12
+#define IPADDRESS_V4_DWORD_INDEX 3
+
 // forward declarations of global name space friend classes
 class EthernetClass;
 class DhcpClass;
@@ -32,46 +35,70 @@ namespace arduino {
 
 // A class to make it easier to handle and pass around IP addresses
 
+enum IPType {
+    IPv4,
+    IPv6
+};
+
 class IPAddress : public Printable {
 private:
     union {
-	uint8_t bytes[4];  // IPv4 address
-	uint32_t dword;
+        uint8_t bytes[16];
+        uint32_t dword[4];
     } _address;
+    IPType _type;
 
     // Access the raw byte array containing the address.  Because this returns a pointer
     // to the internal structure rather than a copy of the address this function should only
     // be used when you know that the usage of the returned uint8_t* will be transient and not
     // stored.
-    uint8_t* raw_address() { return _address.bytes; };
+    uint8_t* raw_address() { return _type == IPv4 ? &_address.bytes[IPADDRESS_V4_BYTES_INDEX] : _address.bytes; }
 
 public:
     // Constructors
+
+    // Default IPv4
     IPAddress();
+    IPAddress(IPType ip_type);
     IPAddress(uint8_t first_octet, uint8_t second_octet, uint8_t third_octet, uint8_t fourth_octet);
+    IPAddress(uint8_t o1, uint8_t o2, uint8_t o3, uint8_t o4, uint8_t o5, uint8_t o6, uint8_t o7, uint8_t o8, uint8_t o9, uint8_t o10, uint8_t o11, uint8_t o12, uint8_t o13, uint8_t o14, uint8_t o15, uint8_t o16);
+     // IPv4; see implementation note
     IPAddress(uint32_t address);
+     // Default IPv4
     IPAddress(const uint8_t *address);
+    IPAddress(IPType ip_type, const uint8_t *address);
+    // If IPv4 fails tries IPv6 see fromString function
+    IPAddress(const char *address);
 
     bool fromString(const char *address);
     bool fromString(const String &address) { return fromString(address.c_str()); }
 
-    // Overloaded cast operator to allow IPAddress objects to be used where a pointer
-    // to a four-byte uint8_t array is expected
-    operator uint32_t() const { return _address.dword; };
-    bool operator==(const IPAddress& addr) const { return _address.dword == addr._address.dword; };
-    bool operator!=(const IPAddress& addr) const { return _address.dword != addr._address.dword; };
+    // Overloaded cast operator to allow IPAddress objects to be used where a uint32_t is expected
+    // NOTE: IPv4 only; see implementation note
+    operator uint32_t() const { return _type == IPv4 ? _address.dword[IPADDRESS_V4_DWORD_INDEX] : 0; };
+
+    bool operator==(const IPAddress& addr) const;
+    bool operator!=(const IPAddress& addr) const { return !(*this == addr); };
+
+    // NOTE: IPv4 only; we don't know the length of the pointer
     bool operator==(const uint8_t* addr) const;
 
     // Overloaded index operator to allow getting and setting individual octets of the address
-    uint8_t operator[](int index) const { return _address.bytes[index]; };
-    uint8_t& operator[](int index) { return _address.bytes[index]; };
+    uint8_t operator[](int index) const;
+    uint8_t& operator[](int index);
 
     // Overloaded copy operators to allow initialisation of IPAddress objects from other types
+    // NOTE: IPv4 only
     IPAddress& operator=(const uint8_t *address);
+    // NOTE: IPv4 only; see implementation note
     IPAddress& operator=(uint32_t address);
+    // If IPv4 fails tries IPv6 see fromString function
+    IPAddress& operator=(const char *address);
 
     virtual size_t printTo(Print& p) const;
 
+    IPType type() { return _type; }
+
     friend class UDP;
     friend class Client;
     friend class Server;
@@ -79,8 +106,13 @@ class IPAddress : public Printable {
     friend ::EthernetClass;
     friend ::DhcpClass;
     friend ::DNSClient;
+
+protected:
+    bool fromString4(const char *address);
+    bool fromString6(const char *address);
 };
 
+extern const IPAddress IN6ADDR_ANY;
 extern const IPAddress INADDR_NONE;
 }
 
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index f731b577..bd0b6821 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -30,11 +30,16 @@ set(TEST_SRCS
   src/Common/test_max.cpp
   src/Common/test_min.cpp
   src/IPAddress/test_fromString.cpp
+  src/IPAddress/test_fromString6.cpp
   src/IPAddress/test_IPAddress.cpp
+  src/IPAddress/test_IPAddress6.cpp
   src/IPAddress/test_operator_assignment.cpp
   src/IPAddress/test_operator_comparison.cpp
+  src/IPAddress/test_operator_comparison6.cpp
   src/IPAddress/test_operator_parentheses.cpp
+  src/IPAddress/test_operator_parentheses6.cpp
   src/IPAddress/test_printTo.cpp
+  src/IPAddress/test_printTo6.cpp
   src/Print/test_clearWriteError.cpp
   src/Print/test_getWriteError.cpp
   src/Print/test_print.cpp
diff --git a/test/src/IPAddress/test_IPAddress6.cpp b/test/src/IPAddress/test_IPAddress6.cpp
new file mode 100644
index 00000000..a6941b71
--- /dev/null
+++ b/test/src/IPAddress/test_IPAddress6.cpp
@@ -0,0 +1,85 @@
+/*
+ * Copyright (c) 2020 Arduino.  All rights reserved.
+ */
+
+/**************************************************************************************
+ * INCLUDE
+ **************************************************************************************/
+
+#include <catch.hpp>
+
+#include <IPAddress.h>
+
+/**************************************************************************************
+ * TEST CODE
+ **************************************************************************************/
+
+TEST_CASE ("Testing IPAddress(type) constructor()", "[IPAddress6-Ctor-01]")
+{
+  arduino::IPAddress ip (arduino::IPType::IPv6);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0);
+  REQUIRE(ip[1] == 0);
+  REQUIRE(ip[2] == 0);
+  REQUIRE(ip[3] == 0);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 0);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 0);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 0);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 0);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 0);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 0);
+}
+
+TEST_CASE ("Testing IPAddress(o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o) constructor", "[IPAddress-Ctor6-02]")
+{
+  arduino::IPAddress ip(0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0x20);
+  REQUIRE(ip[1] == 0x01);
+  REQUIRE(ip[2] == 0xd);
+  REQUIRE(ip[3] == 0xb8);
+  REQUIRE(ip[4] == 1);
+  REQUIRE(ip[5] == 2);
+  REQUIRE(ip[6] == 3);
+  REQUIRE(ip[7] == 4);
+  REQUIRE(ip[8] == 5);
+  REQUIRE(ip[9] == 6);
+  REQUIRE(ip[10] == 7);
+  REQUIRE(ip[11] == 8);
+  REQUIRE(ip[12] == 9);
+  REQUIRE(ip[13] == 0xa);
+  REQUIRE(ip[14] == 0xb);
+  REQUIRE(ip[15] == 0xc);
+}
+
+TEST_CASE ("Testing IPAddress(type, a *) constructor", "[IPAddress6-Ctor-03]")
+{
+  uint8_t const ip_addr_array[] = {0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc};
+  arduino::IPAddress ip(arduino::IPType::IPv6, ip_addr_array);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0x20);
+  REQUIRE(ip[1] == 0x01);
+  REQUIRE(ip[2] == 0xd);
+  REQUIRE(ip[3] == 0xb8);
+  REQUIRE(ip[4] == 1);
+  REQUIRE(ip[5] == 2);
+  REQUIRE(ip[6] == 3);
+  REQUIRE(ip[7] == 4);
+  REQUIRE(ip[8] == 5);
+  REQUIRE(ip[9] == 6);
+  REQUIRE(ip[10] == 7);
+  REQUIRE(ip[11] == 8);
+  REQUIRE(ip[12] == 9);
+  REQUIRE(ip[13] == 0xa);
+  REQUIRE(ip[14] == 0xb);
+  REQUIRE(ip[15] == 0xc);
+}
diff --git a/test/src/IPAddress/test_fromString6.cpp b/test/src/IPAddress/test_fromString6.cpp
new file mode 100644
index 00000000..a0f09c73
--- /dev/null
+++ b/test/src/IPAddress/test_fromString6.cpp
@@ -0,0 +1,395 @@
+/*
+ * Copyright (c) 2020 Arduino.  All rights reserved.
+ */
+
+/**************************************************************************************
+ * INCLUDE
+ **************************************************************************************/
+
+#include <catch.hpp>
+
+#include <String.h>
+#include <IPAddress.h>
+
+/**************************************************************************************
+ * TEST CODE
+ **************************************************************************************/
+
+TEST_CASE ("Extract valid IPv6 address 'fromString(const char *)'", "[IPAddress-fromString-01]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("2001:db8:102:304:506:708:90a:b0c") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0x20);
+  REQUIRE(ip[1] == 0x01);
+  REQUIRE(ip[2] == 0xd);
+  REQUIRE(ip[3] == 0xb8);
+  REQUIRE(ip[4] == 1);
+  REQUIRE(ip[5] == 2);
+  REQUIRE(ip[6] == 3);
+  REQUIRE(ip[7] == 4);
+  REQUIRE(ip[8] == 5);
+  REQUIRE(ip[9] == 6);
+  REQUIRE(ip[10] == 7);
+  REQUIRE(ip[11] == 8);
+  REQUIRE(ip[12] == 9);
+  REQUIRE(ip[13] == 0xa);
+  REQUIRE(ip[14] == 0xb);
+  REQUIRE(ip[15] == 0xc);
+}
+
+TEST_CASE ("Extract valid IPv6 address 'fromString(const String &)'", "[IPAddress-fromString-02]")
+{
+  arduino::IPAddress ip;
+
+  arduino::String const ip_addr_str("2001:db8:102:304:506:708:90a:b0c");
+
+  REQUIRE(ip.fromString(ip_addr_str) == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0x20);
+  REQUIRE(ip[1] == 0x01);
+  REQUIRE(ip[2] == 0xd);
+  REQUIRE(ip[3] == 0xb8);
+  REQUIRE(ip[4] == 1);
+  REQUIRE(ip[5] == 2);
+  REQUIRE(ip[6] == 3);
+  REQUIRE(ip[7] == 4);
+  REQUIRE(ip[8] == 5);
+  REQUIRE(ip[9] == 6);
+  REQUIRE(ip[10] == 7);
+  REQUIRE(ip[11] == 8);
+  REQUIRE(ip[12] == 9);
+  REQUIRE(ip[13] == 0xa);
+  REQUIRE(ip[14] == 0xb);
+  REQUIRE(ip[15] == 0xc);
+}
+
+TEST_CASE ("Extract valid IPv6 any address", "[IPAddress-fromString-03]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("::") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0);
+  REQUIRE(ip[1] == 0);
+  REQUIRE(ip[2] == 0);
+  REQUIRE(ip[3] == 0);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 0);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 0);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 0);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 0);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 0);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 0);
+}
+
+TEST_CASE ("Extract valid IPv6 localhost address", "[IPAddress-fromString-04]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("::1") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0);
+  REQUIRE(ip[1] == 0);
+  REQUIRE(ip[2] == 0);
+  REQUIRE(ip[3] == 0);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 0);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 0);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 0);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 0);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 0);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 1);
+}
+
+TEST_CASE ("Extract valid IPv6 different length segments", "[IPAddress-fromString-05]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("abcd:ef1:23:0:4::") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0xab);
+  REQUIRE(ip[1] == 0xcd);
+  REQUIRE(ip[2] == 0xe);
+  REQUIRE(ip[3] == 0xf1);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 0x23);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 0);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 4);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 0);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 0);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 0);
+}
+
+
+TEST_CASE ("Extract valid IPv6 zero start", "[IPAddress-fromString-06]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("0:2:3:4:5:6:7:8") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0);
+  REQUIRE(ip[1] == 0);
+  REQUIRE(ip[2] == 0);
+  REQUIRE(ip[3] == 2);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 3);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 4);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 5);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 6);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 7);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 8);
+}
+
+
+TEST_CASE ("Extract valid IPv6 zero end", "[IPAddress-fromString-07]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("1:2:3:4:5:6:7:0") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0);
+  REQUIRE(ip[1] == 1);
+  REQUIRE(ip[2] == 0);
+  REQUIRE(ip[3] == 2);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 3);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 4);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 5);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 6);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 7);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 0);
+}
+
+
+TEST_CASE ("Extract valid IPv6 two zero start", "[IPAddress-fromString-08]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("::3:4:5:6:7:0") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0);
+  REQUIRE(ip[1] == 0);
+  REQUIRE(ip[2] == 0);
+  REQUIRE(ip[3] == 0);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 3);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 4);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 5);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 6);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 7);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 0);
+}
+
+
+TEST_CASE ("Extract valid IPv6 two zero end", "[IPAddress-fromString-9]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("0:2:3:4:5:6::") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0);
+  REQUIRE(ip[1] == 0);
+  REQUIRE(ip[2] == 0);
+  REQUIRE(ip[3] == 2);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 3);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 4);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 5);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 6);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 0);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 0);
+}
+
+// Non-canonical
+
+TEST_CASE ("Extract valid IPv6 any full long form", "[IPAddress-fromString-10]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("0:0:0:0:0:0:0:0") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0);
+  REQUIRE(ip[1] == 0);
+  REQUIRE(ip[2] == 0);
+  REQUIRE(ip[3] == 0);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 0);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 0);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 0);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 0);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 0);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 0);
+}
+
+TEST_CASE ("Extract valid IPv6 upper case", "[IPAddress-fromString-11]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("2001:DB8:102:304:506:708:90A:B0C") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0x20);
+  REQUIRE(ip[1] == 0x01);
+  REQUIRE(ip[2] == 0xd);
+  REQUIRE(ip[3] == 0xb8);
+  REQUIRE(ip[4] == 1);
+  REQUIRE(ip[5] == 2);
+  REQUIRE(ip[6] == 3);
+  REQUIRE(ip[7] == 4);
+  REQUIRE(ip[8] == 5);
+  REQUIRE(ip[9] == 6);
+  REQUIRE(ip[10] == 7);
+  REQUIRE(ip[11] == 8);
+  REQUIRE(ip[12] == 9);
+  REQUIRE(ip[13] == 0xa);
+  REQUIRE(ip[14] == 0xb);
+  REQUIRE(ip[15] == 0xc);
+}
+
+TEST_CASE ("Extract valid IPv6 explicit start zero", "[IPAddress-fromString-10]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("0::") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0);
+  REQUIRE(ip[1] == 0);
+  REQUIRE(ip[2] == 0);
+  REQUIRE(ip[3] == 0);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 0);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 0);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 0);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 0);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 0);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 0);
+}
+
+TEST_CASE ("Extract valid IPv6 explicit end zero", "[IPAddress-fromString-10]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("::0") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0);
+  REQUIRE(ip[1] == 0);
+  REQUIRE(ip[2] == 0);
+  REQUIRE(ip[3] == 0);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 0);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 0);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 0);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 0);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 0);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 0);
+}
+
+TEST_CASE ("Extract valid IPv6 compression of one group of zero", "[IPAddress-fromString-10]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString("1::3:4:5:6:7:8") == true);
+
+  REQUIRE(ip.type() == arduino::IPType::IPv6);
+  REQUIRE(ip[0] == 0);
+  REQUIRE(ip[1] == 1);
+  REQUIRE(ip[2] == 0);
+  REQUIRE(ip[3] == 0);
+  REQUIRE(ip[4] == 0);
+  REQUIRE(ip[5] == 3);
+  REQUIRE(ip[6] == 0);
+  REQUIRE(ip[7] == 4);
+  REQUIRE(ip[8] == 0);
+  REQUIRE(ip[9] == 5);
+  REQUIRE(ip[10] == 0);
+  REQUIRE(ip[11] == 6);
+  REQUIRE(ip[12] == 0);
+  REQUIRE(ip[13] == 7);
+  REQUIRE(ip[14] == 0);
+  REQUIRE(ip[15] == 8);
+}
+
+// Invalid cases
+
+TEST_CASE ("Extract invalid IPv6 address", "[IPAddress-fromString-12]")
+{
+  arduino::IPAddress ip;
+
+  REQUIRE(ip.fromString(":::") == false); // three colons by self
+  REQUIRE(ip.fromString("::3:4:5:6::") == false); // two compressions
+  REQUIRE(ip.fromString("2001:db8:102:10304:506:708:90a:b0c") == false); // 5 character field
+  REQUIRE(ip.fromString("200x:db8:102:304:506:708:90a:b0c") == false); // invalid character
+  REQUIRE(ip.fromString("2001:db8:102:304::506:708:90a:b0c") == false); // double colon with 8 other fields (so not a compression)
+  REQUIRE(ip.fromString("2001:db8:102:304:::708:90a:b0c") == false); // three colons in middle
+  REQUIRE(ip.fromString("2001:db8:102:304:506:708:90a:b0c:d0e") == false); // 9 fields
+  REQUIRE(ip.fromString("2001:db8:102:304:506:708:90a:") == false); // missing last group (but has a colon)
+  REQUIRE(ip.fromString("2001:db8:102:304:506:708:90a") == false); // only seven groups
+  REQUIRE(ip.fromString("0:0:0:0:0:0:0:0:0") == false); // nine zeros
+  REQUIRE(ip.fromString("0:0:0:0:0:0:0:0:") == false); // extra colon
+  REQUIRE(ip.fromString("0:0:0:0:0:0:0:") == false); // missing last group (but has a colon)
+  REQUIRE(ip.fromString("0:0:0:0:0:0:0") == false); // only seven groups
+}
diff --git a/test/src/IPAddress/test_operator_assignment.cpp b/test/src/IPAddress/test_operator_assignment.cpp
index f7afece2..e9fc8691 100644
--- a/test/src/IPAddress/test_operator_assignment.cpp
+++ b/test/src/IPAddress/test_operator_assignment.cpp
@@ -29,5 +29,6 @@ TEST_CASE ("Testing IPAddress::operator = (uint32_t a)", "[IPAddress-Operator-=-
   uint32_t const ip2 = 192 | (168 << 8) | (1 << 16) | (2 << 24);
 
   ip1 = ip2;
+  // NOTE: Only correct on little-endian systems
   REQUIRE(ip1 == arduino::IPAddress(192,168,1,2));
 }
diff --git a/test/src/IPAddress/test_operator_comparison6.cpp b/test/src/IPAddress/test_operator_comparison6.cpp
new file mode 100644
index 00000000..a5e1b87c
--- /dev/null
+++ b/test/src/IPAddress/test_operator_comparison6.cpp
@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2020 Arduino.  All rights reserved.
+ */
+
+/**************************************************************************************
+ * INCLUDE
+ **************************************************************************************/
+
+#include <catch.hpp>
+
+#include <IPAddress.h>
+
+/**************************************************************************************
+ * TEST CODE
+ **************************************************************************************/
+
+TEST_CASE ("Testing two basic constructs the same", "[IPAddress6-Operator-==-01]")
+{
+  arduino::IPAddress ip1(0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc), ip2(0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc);
+  REQUIRE((ip1 == ip2) == true);
+}
+
+TEST_CASE ("Testing two addresses different", "[IPAddress-Operator-==-02]")
+{
+  arduino::IPAddress ip1(0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc), ip2(0xfd,0x12, 0x34,0x56, 0x78,0x9a, 0,1, 0,0, 0,0, 0,0, 0,1);
+  REQUIRE((ip1 == ip2) == false);
+}
+
+TEST_CASE ("Testing not equals different address is true", "[IPAddress-Operator-==-03]")
+{
+  arduino::IPAddress ip1(0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc), ip2(0xfd,0x12, 0x34,0x56, 0x78,0x9a, 0,1, 0,0, 0,0, 0,0, 0,1);
+  REQUIRE((ip1 != ip2) == true);
+}
+
+TEST_CASE ("Testing not equals same address is false", "[IPAddress-Operator-==-04]")
+{
+  arduino::IPAddress ip1(0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc), ip2(0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc);
+  REQUIRE((ip1 != ip2) == false);
+}
+
+// IPv4 and IPv6 differ based on type (irrespective of bytes)
+
+TEST_CASE ("Testing IPv4 vs IPv6", "[IPAddress6-Operator-==-05]")
+{
+  arduino::IPAddress ip1(10, 0, 0, 1), ip2(0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc);
+  REQUIRE((ip1 == ip2) == false);
+}
+
+TEST_CASE ("Testing IPv4 vs IPv6 equivalent IPv4-compatible address (deprecated)", "[IPAddress6-Operator-==-05]")
+{
+  arduino::IPAddress ip1(10, 0, 0, 1), ip2(0,0, 0,0, 0,0, 0,0, 0,0, 0,0, 10,0, 0,1);
+  REQUIRE((ip1 == ip2) == false);
+}
+
+TEST_CASE ("Testing IPv4 vs IPv6 localhost", "[IPAddress6-Operator-==-05]")
+{
+  arduino::IPAddress ip1(127, 0, 0, 1), ip2(0,0, 0,0, 0,0, 0,0, 0,0, 0,0, 10,0, 0,1);
+  REQUIRE((ip1 == ip2) == false);
+}
+
+TEST_CASE ("Testing IPv4 equivalent compatible address vs IPv6 localhost", "[IPAddress6-Operator-==-05]")
+{
+  arduino::IPAddress ip1(0, 0, 0, 1), ip2(0,0, 0,0, 0,0, 0,0, 0,0, 0,0, 0,0, 0,1);
+  REQUIRE((ip1 == ip2) == false);
+}
+
+TEST_CASE ("Testing IPv6 never matches as raw byte sequence assumed to be length 4", "[IPAddress6-Operator-==-06]")
+{
+  arduino::IPAddress ip1(0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc);
+  uint8_t const ip2[] = {0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc};
+  REQUIRE((ip1 == ip2) == false);
+}
diff --git a/test/src/IPAddress/test_operator_parentheses.cpp b/test/src/IPAddress/test_operator_parentheses.cpp
index e9f1c0af..27fce3a5 100644
--- a/test/src/IPAddress/test_operator_parentheses.cpp
+++ b/test/src/IPAddress/test_operator_parentheses.cpp
@@ -19,5 +19,6 @@ TEST_CASE ("Testing IPAddress::operator uint32_t() const", "[IPAddress-Operator-
   arduino::IPAddress ip(129,168,1,2);
   uint32_t const val_expected = ip;
   uint32_t const val_actual = (129 | (168 << 8) | (1 << 16) | (2 << 24));
+  // NOTE: Only correct on little-endian systems
   REQUIRE(val_expected == val_actual);
 }
diff --git a/test/src/IPAddress/test_operator_parentheses6.cpp b/test/src/IPAddress/test_operator_parentheses6.cpp
new file mode 100644
index 00000000..5b4740c8
--- /dev/null
+++ b/test/src/IPAddress/test_operator_parentheses6.cpp
@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2020 Arduino.  All rights reserved.
+ */
+
+/**************************************************************************************
+ * INCLUDE
+ **************************************************************************************/
+
+#include <catch.hpp>
+
+#include <IPAddress.h>
+
+/**************************************************************************************
+ * TEST CODE
+ **************************************************************************************/
+
+// These comparisons should always return false, as you can't compare an IPv6 to an int32_t
+
+TEST_CASE ("Testing implicit cast of IPv6 compatible (little endian) to uint32_t always false", "[IPAddress6-Operator-()-01]")
+{
+  // On little endian systems, considering only last four octets (ignoring the rest)
+  arduino::IPAddress ip(0,0, 0,0, 0,0, 0,0, 0,0, 0,0, 129,168, 1,2);
+  uint32_t const val_expected = ip;
+  uint32_t const val_actual = (129 | (168 << 8) | (1 << 16) | (2 << 24));
+  REQUIRE((val_expected == val_actual) == false);
+}
+
+TEST_CASE ("Testing implicit cast of IPv6 full little endian to uint32_t always false", "[IPAddress6-Operator-()-01]")
+{
+  // On little endian systems (full value)
+  arduino::IPAddress ip(129,168, 1,2, 0,0, 0,0, 0,0, 0,0, 0,0, 0,0);
+  uint32_t const val_expected = ip;
+  uint32_t const val_actual = (129 | (168 << 8) | (1 << 16) | (2 << 24));
+  REQUIRE((val_expected == val_actual) == false);
+}
+
+TEST_CASE ("Testing implicit cast of IPv6 to uint32_t always false", "[IPAddress6-Operator-()-01]")
+{
+  // Actual value of the 128-bit IPv6 address, which is network byte order
+  arduino::IPAddress ip(0,0, 0,0, 0,0, 0,0, 0,0, 0,0, 129,168, 1,2);
+  uint32_t const val_expected = ip;
+  uint32_t const val_actual = ((129 << 24) | (168 << 16) | (1 << 8) | 2);
+  REQUIRE((val_expected == val_actual) == false);
+}
diff --git a/test/src/IPAddress/test_printTo6.cpp b/test/src/IPAddress/test_printTo6.cpp
new file mode 100644
index 00000000..621008af
--- /dev/null
+++ b/test/src/IPAddress/test_printTo6.cpp
@@ -0,0 +1,137 @@
+/*
+ * Copyright (c) 2020 Arduino.  All rights reserved.
+ */
+
+/**************************************************************************************
+ * INCLUDE
+ **************************************************************************************/
+
+#include <catch.hpp>
+
+#include <IPAddress.h>
+#include <PrintMock.h>
+
+/**************************************************************************************
+ * TEST CODE
+ **************************************************************************************/
+
+TEST_CASE ("Print IPv6", "[IPAddress-printTo6-01]")
+{
+  PrintMock mock;
+  arduino::IPAddress ip(0x20,0x01, 0xd,0xb8, 1,2, 3,4, 5,6, 7,8, 9,0xa, 0xb,0xc);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "2001:db8:102:304:506:708:90a:b0c");
+}
+
+TEST_CASE ("Print IPv6 any", "[IPAddress-printTo6-02]")
+{
+  PrintMock mock;
+  arduino::IPAddress const ip(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "::");
+}
+
+TEST_CASE ("Print IPv6 localhost", "[IPAddress-printTo6-03]")
+{
+  PrintMock mock;
+  arduino::IPAddress const ip(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "::1");
+}
+
+TEST_CASE ("Print IPv6 different length segments", "[IPAddress-printTo6-04]")
+{
+  PrintMock mock;
+  arduino::IPAddress const ip(0xab,0xcd, 0x0e,0xf1, 0x00,0x23, 0,0, 0x00,0x04, 0,0, 0,0, 0,0);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "abcd:ef1:23:0:4::");
+}
+
+TEST_CASE ("Print IPv6 zero longest run end", "[IPAddress-printTo6-05]")
+{
+  PrintMock mock;
+  arduino::IPAddress const ip(0,0, 0,1, 0,0, 0,0, 0,2, 0,0, 0,0, 0,0);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "0:1:0:0:2::");
+}
+
+TEST_CASE ("Print IPv6 zero longest run mid", "[IPAddress-printTo6-06]")
+{
+  PrintMock mock;
+  arduino::IPAddress const ip(0,0, 0,1, 0,0, 0,0, 0,0, 0,2, 0,0, 0,0);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "0:1::2:0:0");
+}
+
+TEST_CASE ("Print IPv6 start zero", "[IPAddress-printTo6-07]")
+{
+  PrintMock mock;
+  arduino::IPAddress const ip(0,0, 0,2, 0,3, 0,4, 0,5, 0,6, 0,7, 0,8);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "0:2:3:4:5:6:7:8");
+}
+
+TEST_CASE ("Print IPv6 ending zero", "[IPAddress-printTo6-08]")
+{
+  PrintMock mock;
+  arduino::IPAddress const ip(0,1, 0,2, 0,3, 0,4, 0,5, 0,6, 0,7, 0,0);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "1:2:3:4:5:6:7:0");
+}
+
+TEST_CASE ("Print IPv6 start two zero", "[IPAddress-printTo6-09]")
+{
+  PrintMock mock;
+  arduino::IPAddress const ip(0,0, 0,0, 0,3, 0,4, 0,5, 0,6, 0,7, 0,0);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "::3:4:5:6:7:0");
+}
+
+TEST_CASE ("Print IPv6 ending two zero", "[IPAddress-printTo6-10]")
+{
+  PrintMock mock;
+  arduino::IPAddress const ip(0,0, 0,2, 0,3, 0,4, 0,5, 0,6, 0,0, 0,0);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "0:2:3:4:5:6::");
+}
+
+TEST_CASE ("Print IPv6 first out of same length", "[IPAddress-printTo6-11]")
+{
+  PrintMock mock;
+  arduino::IPAddress const ip(0,1, 0,0, 0,0, 0,4, 0,5, 0,0, 0,0, 0,8);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "1::4:5:0:0:8");
+}
+
+
+TEST_CASE ("Print IPv6 single zeros not compressed", "[IPAddress-printTo6-12]")
+{
+  PrintMock mock;
+  arduino::IPAddress const ip(0,1, 0,0, 0,3, 0,0, 0,5, 0,0, 0,7, 0,8);
+
+  mock.print(ip);
+
+  REQUIRE(mock._str  == "1:0:3:0:5:0:7:8");
+}