Question

I’m deserializing data from a web page generated by php that is using ip2long(). However, when I try to make a new ip address by using the integer value in the constructor of IPAddress the dotted version of the ip address is in reverse order?

ex:

4.3.2.1 should really be 1.2.3.4

Any ideas of how to fix this?

Answer 1

It sounds like someone is using little-endian and someone is using network byte order (big-endian) for the packed value. For instance the octect sequence compromising an integer, AA,BB,CC,DD in LE is DD,CC,BB,AA in BE/NBO — a nice symmetrical reverse!

Since the IPAddress(Int64) constructor documentations says:

The Int64 value is assumed to be in network byte order.

I would imagine that ip2long in PHP is generating a value in little-endian. Good thing IPAddress also takes byte[] for the constructor, now get those elbows greasy… just pass the bytes in the “correct” order.

Happy coding.

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The code at How to convert an int to a little endian byte array? should give some ideas.

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Or, as Josh points out, there is a HostToNetworkOrder method to do this.