How to determine if IPv6 address is private?

Question

I am trying to determine if a given IPv6 address is private or not in C# and I was tempted to simply use the 'IsIPv6SiteLocal' property on the IPAddress class. However, as explained in this comment, the logic implemented in this property is deprecated. I ran the following unit test:

[TestMethod]
public void IsPrivate_ipv6_True()
{
    // This sample private IPv6 address was generated using: http://unique-local-ipv6.com/
    var ip = IPAddress.Parse("fd44:fda4:e1ba::1");
    Assert.IsTrue(ip.IsIPv6SiteLocal);
}

The assertion in the unit test fails which confirms that IsIPv6SiteLocal does not correctly determines if an address is local. So I need an alternative.

I wrote the following extension method and I was wondering if anybody can think of a scenario where it would not properly determine if the address is private/public.

public static bool IsPrivateIPv6(this IPAddress address)
{
    var addressAsString = address.ToString();
    var firstWord = addressAsString.Split(new[] { ':' }, StringSplitOptions.RemoveEmptyEntries)[0];

    // Make sure we are dealing with an IPv6 address
    if (address.AddressFamily != AddressFamily.InterNetworkV6) return false;

    // The original IPv6 Site Local addresses (fec0::/10) are deprecated. Unfortunately IsIPv6SiteLocal only checks for the original deprecated version:
    else if (address.IsIPv6SiteLocal) return true;

    // These days Unique Local Addresses (ULA) are used in place of Site Local. 
    // ULA has two variants: 
    //      fc00::/8 is not defined yet, but might be used in the future for internal-use addresses that are registered in a central place (ULA Central). 
    //      fd00::/8 is in use and does not have to registered anywhere.
    else if (firstWord.Substring(0, 2) == "fc" && firstWord.Length >= 4) return true;
    else if (firstWord.Substring(0, 2) == "fd" && firstWord.Length >= 4) return true;

    // Link local addresses (prefixed with fe80) are not routable
    else if (firstWord == "fe80") return true;

    // Discard Prefix
    else if (firstWord == "100") return true;

    // Any other IP address is not Unique Local Address (ULA)
    else return false;
}

EDITED 2/13/2016:

• make sure the first word is at least 4 characters long as suggested by @RonMaupin
• improved comment above 'else return false' as suggested by @RonMaupin
• check for 'fe80' prefix as suggested by @KevinBurdett
• check for 'Discard' prefix as suggested by @KevinBurdett

Answer 1

Improved @desautelsj's answer by adding a special case for ::1 and avoiding an ArgumentException from his solution (that would happen in the Substring() call):

public static bool IsPrivateIPv6(IPAddress address)
{
    // Make sure we are dealing with an IPv6 address
    if (address.AddressFamily != AddressFamily.InterNetworkV6)
        throw new ArgumentException("IP address is not V6", "address");

    var addressAsString = address.ToString();
    var firstWord = addressAsString.Split(new[] { ':' }, StringSplitOptions.RemoveEmptyEntries)[0];

    // equivalent of 127.0.0.1 in IPv6
    if (addressAsString == "::1")
        return true;

    // The original IPv6 Site Local addresses (fec0::/10) are deprecated. Unfortunately IsIPv6SiteLocal only checks for the original deprecated version:
    else if (address.IsIPv6SiteLocal)
        return true;

    // These days Unique Local Addresses (ULA) are used in place of Site Local. 
    // ULA has two variants: 
    //      fc00::/8 is not defined yet, but might be used in the future for internal-use addresses that are registered in a central place (ULA Central). 
    //      fd00::/8 is in use and does not have to registered anywhere.
    else if (firstWord.Length >= 4 && firstWord.Substring(0, 2) == "fc")
        return true;
    else if (firstWord.Length >= 4 && firstWord.Substring(0, 2) == "fd")
        return true;

    // Link local addresses (prefixed with fe80) are not routable
    else if (firstWord == "fe80")
        return true;

    // Discard Prefix
    else if (firstWord == "100")
        return true;

    // Any other IP address is not Unique Local Address (ULA)
    return false;
}

And in F#:

let private IsIpv6AddressPrivate (address: IPAddress) =
    if address.AddressFamily = AddressFamily.InterNetwork then
        invalidArg "address" "address must be IPv6"

    // The original IPv6 Site Local addresses (fec0::/10) are deprecated. Unfortunately IsIPv6SiteLocal only checks for the original deprecated version:
    elif address.IsIPv6SiteLocal then
        true
    else
        let addressAsString = address.ToString()

        // equivalent of 127.0.0.1 in IPv6
        if addressAsString = "::1" then
            true
        else
            let firstWord = addressAsString.Split([|':'|], StringSplitOptions.RemoveEmptyEntries).[0]
            // These days Unique Local Addresses (ULA) are used in place of Site Local. 
            // ULA has two variants: 
            //      fc00::/8 is not defined yet, but might be used in the future for internal-use addresses that are registered in a central place (ULA Central). 
            //      fd00::/8 is in use and does not have to registered anywhere.
            if (firstWord.Length >= 4 && firstWord.Substring(0, 2) = "fc") ||
               (firstWord.Length >= 4 && firstWord.Substring(0, 2) = "fd") ||
               // Link local addresses (prefixed with fe80) are not routable
               (firstWord = "fe80") ||
               // Discard Prefix
               (firstWord = "100") then
                true
            else
                false

Answer 2

While not particularly elegant... I can't really think of anything better :)

However, I would also check for link local addresses, prefixed with fe80:. They are not routable, but if you are pulling the IP directly from an interface, it may still report the link local when no other IPv6 addresses are present.

The wikipedia article on IPv6 (https://en.wikipedia.org/wiki/Reserved_IP_addresses#IPv6) also shows 100: as a discard prefix. Depending on what you are trying to accomplish, you may want to check for these too.