When I ask the GetIpAddrTable function to sort the results, how are they sorted?

This post has been republished via RSS; it originally appeared at: Microsoft Developer Blogs.

A customer had a question about the way the GetIpAddrTable function sorts the results. The documentation says that if you pass TRUE as the bOrder parameter, then the mapping table will be sorted upon return. What exactly is this sorting order? The customer observed that in practice, they got the IP addresses in this order: Public IP addresses, then internal addresses, and then local addresses. They were interested in obtaining the public IP address, so they just asked for the results to be sorted and the grabbed the first one. That worked great until one day, they grabbed the first sorted address and got the local address 127.0.0.1. Did this mean that the system didn't have any public IP addresses? The customer is trying to figure out why there was no public address, or at least no public address that the GetIpAddrTable function could find. The problem is that their assumption wasn't supported by the documentation. The documentation says that the bOrder parameter controls "whether the returned mapping table should be sorted in ascending order by IPv4 address." The sorting is done by IPv4 address, not by scope or availability or subnet or routing or broadcast. Specifically, the sorting is done in lexicographical order by the IPv4 address in network byte order. The following table lists the IPv4 addresses in sorted order (not to scale): Dotted notation Network byte order dwAddress Notes 0.0.0.0 00 00 00 00 0x00000000 Local 0.0.0.1 00 00 00 01 0x01000000 ⋮ ⋮ ⋮ 0.255.255.254 00 FF FF FE 0xFEFFFF00 0.255.255.255 00 FF FF FF 0xFFFFFF00 1.0.0.0 01 00 00 00 0x00000001 Public 1.0.0.1 01 00 00 01 0x01000001 ⋮ ⋮ ⋮ 9.255.255.255 09 FF FF FF 0xFFFFFF09 10.0.0.0 0A 00 00 00 0x0000000A Private 10.0.0.1 0A 00 00 01 0x0100000A ⋮ ⋮ ⋮ 10.255.255.255 0A FF FF FF 0xFFFFFF0A 11.0.0.0 0B 00 00 00 0x0000000B Public (mostly) 11.0.0.1 0B 00 00 01 0x0100000B ⋮ ⋮ ⋮ 126.255.255.255 7E FF FF FF 0xFFFFFF7E 127.0.0.0 7F 00 00 00 0x0000007F Loopback 127.0.0.1 7F 00 00 01 0x0100007F ⋮ ⋮ ⋮ 127.255.255.255 7F FF FF FF 0xFFFFFF7F 128.0.0.0 80 00 00 00 0x00000080 Public (mostly) 128.0.0.1 80 00 00 01 0x01000080 ⋮ ⋮ ⋮ 255.255.255.255 FF FF FF FF 0xFFFFFFFF Note that the areas marked "Public (mostly)" contain islands of private or other special addresses within them. The purpose of this list was not to break down the entire IPv4 address range. It was to highlight that lexicographical ordering by IPv4 address in network byte order has no relation to the nature of the address. I suspect what happened is that the company's public IP address assignment moved from an address less than 127.0.0.0 to one greater than 128.0.0.0, which means that 127.0.0.1 is now the numerically lowest IP address. The sorting performed by the GetIpAddrTable is purely numerical by IPv4 address. If you want to fish out your system's public IP address, you'll have to do your own filtering. Bonus chatter: I listed IPv4 addresses like 0.0.0.1, even though 0.0.0.1 is strictly speaking not a valid IPv4 address. The IP_MULTICAST_IF socket option uses values of this form to mean "Not an address, but an interface index."