IPv6 defaults to 64 Cidr about ipnetwork

[lduchosal]

At first sight, it looks like you found a bug. Thanks for reporting it, I'll investigate this case further.

[Deantwo]

This done in the TryGuessCidr(string ip, out byte cidr) method, which attempts to find classful netmasks for IPv4 addresses. But since IPv6 was never classful this makes no sense for it. So someone seem to have picked /64 as the default CIDR for IPv6.
The exact line is here: System.Net.IPNetwork/IPNetwork.cs#L1778.

I see no reference to there being a default CIDR for IPv6, but /64 seem to be mentioned a lot in documention and preferred by most. So this does make sense as a default CIDR.

I think the real issue here is that most network engineers would assume that an IP address with no network mask or CIDR is always a single IP address and not an IP network, so therefore defaulting to /32 for IPv4 and /128 for IPv6 makes more sense.

The code in the opening post should have been written as:

IPNetwork rangeToCheck = IPNetwork.Parse("2001:db8:abcd:0012:0000:0000:0000:0000/112");
IPAddress ipToCheck = IPAddress.Parse("2001:db8:abcd:0012:0000:0000:0000:0001");

rangeToCheck.Contains(ipToCheck); // returns true

As shown in Example 2 (IPv6) of the ReadMe.

[lduchosal]

This seem like a default behaviour in standard tools.

Have a look at af_inet6.c for IPV6.

static void
in6_postproc(int s, const struct afswtch *afp)
{
	if (explicit_prefix == 0) {
		/* Aggregatable address architecture defines all prefixes
		   are 64. So, it is convenient to set prefixlen to 64 if
		   it is not specified. */
		setifprefixlen("64", 0, s, afp);
		/* in6_getprefix("64", MASK) if MASK is available here... */
	}
}