`IPAddress.TryParse` return wrong result for `192.168.1`

[SMAH1]

Description

I use "IPAddress.TryParse" to validate the IP. When I validate 192.168.1 se result 192.168.0.1.

Reproduction Steps

System.Net.IPAddress.TryParse("192.168.1", out var ip);
if (ip.ToString() == "192.168.0.1")
    Console.WriteLine("WHY?");

Expected behavior

Return invalid IP.

Actual behavior

Return wrong IP.

Regression?

No response

Known Workarounds

No response

Configuration

OS: Windows 10 22H2
.NET SDK: 9.0.200
IDE: Microsoft Visual Studio Community 2022 (64-bit) Version 17.13.0
Project Type: Web Project

Other information

No response

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[SMAH1]

AND
192.168.1.020 convert to 192.168.1.16! (020 parse in base 8)

[huoyaoyuan]

It's explicitly specified as valid IP address in test:

runtime/src/libraries/System.Net.Primitives/tests/FunctionalTests/IPAddressParsing.cs

Lines 171 to 190 in 48b566c

	public static readonly object[][] ValidIpv4Addresses =
	{
	// Decimal
	new object[] { "192.168.0.1", "192.168.0.1" },
	new object[] { "0.0.0.0", "0.0.0.0" },
	new object[] { "0", "0.0.0.0" },
	new object[] { "12", "0.0.0.12" },
	new object[] { "12.1.7", "12.1.0.7" },
	new object[] { "12.1.7", "12.1.0.7" },
	new object[] { "255.255.255.255", "255.255.255.255" },
	new object[] { "20.65535", "20.0.255.255" },
	new object[] { "157.3873051", "157.59.25.27" },
	new object[] { "157.6427", "157.0.25.27" },
	new object[] { "65535", "0.0.255.255" },
	new object[] { "65536", "0.1.0.0" },
	new object[] { "1434328179", "85.126.28.115" },
	new object[] { "2637895963", "157.59.25.27" },
	new object[] { "3397943208", "202.136.127.168" },
	new object[] { "4294967294", "255.255.255.254" },
	new object[] { "4294967295", "255.255.255.255" },

I believe they are all allowed IPv4 formats. You can try to find from the original IPv4 specification.

[huoyaoyuan]

Also reminder that IPAddress.IsValid is coming in .NET 10 (#111282).

[elgonzo]

Console.WriteLine("WHY?");

As to why it behaves likes that, why not take a look into the official documentation for the IPAddress.TryParse method (https://learn.microsoft.com/en-us/dotnet/api/system.net.ipaddress.tryparse):

The number of parts (each part is separated by a period) in ipString determines how the IP address is constructed. A one part address is stored directly in the network address. A two part address, convenient for specifying a class A address, puts the leading part in the first byte and the trailing part in the right-most three bytes of the network address. A three part address, convenient for specifying a class B address, puts the first part in the first byte, the second part in the second byte, and the final part in the right-most two bytes of the network address.

This also means that IPAddress.IsValid would be unlikely to really help you here for detecting accidentally given one-, two- or three-part ipStrings.

192.168.1.020 convert to 192.168.1.16! (020 parse in base 8)

Yeah, this is documented behavior, but as far as the documentation for IPAddress.TryParse is concerned it only documents it by example. In the documentation i linked to, note the table with example ipStrings and how they will be parsed. You will notice that it includes examples where parts are using octal or hexadecimal notation. But it is really easy to miss when browsing the documentation...

(As an aside, even Wikipedia features a remark regarding octal and hexadecimal numbers in dotted-quad address notation: "It also allowed the numbers to be written in hexadecimal and octal representations, by prefixing them with 0x and 0, respectively. These features continue to be supported in some software, even though they are considered as non-standard. This means addresses with a component written with a leading zero digit may be interpreted differently in programs that do or do not recognize such formats." [link])

[SMAH1]

So I need to write my own validation. :😀

[tannergooding]

@SMAH1 it's worth noting that if you're trying to write validation that states 192.168.1 is invalid, then you're going to end up rejecting valid IP addresses.

Tools across both Linux and Windows, such as ping, ssh, etc, accept such IP addresses (as do all major browsers). Such IP addresses can be encountered via user input, serialized data, etc.

Windows:

D:\repos> ping 192.168.1

Pinging 192.168.0.1 with 32 bytes of data:
...

D:\repos> ping 1

Pinging 0.0.0.1 with 32 bytes of data:
...

Linux:

tagoo@GOODING-ZEN4:~$ ping 192.168.1
PING 192.168.1 (192.168.0.1) 56(84) bytes of data.
...

tagoo@GOODING-ZEN4:~$ ping 1
PING 1 (0.0.0.1) 56(84) bytes of data.
...

you can also easily validate this yourself in the same tools, in the browsers, etc.