Is there a difference between <winsock.h> and <winsock2.h>?

@cost's answer links to a discussion that, amongst other things, asks this question that was never answered:

Is there a reason why I can't include windows.h before winsock2.h, it gives me tons of errors, but once I switch their order everything is okay... why is that?

windows.h includes winsock.h by default (if WIN32_LEAN_AND_MEAN is not defined). The problem is not limited to just windows.h, though. Any time winsock.h gets included before winsock2.h, there will be compiler errors.

The reason is because the two files DO NOT co-exist very well. winsock2.h was designed to replace winsock.h, not extend it. Everything that is defined in winsock.h is also defined in winsock2.h.

If winsock2.h is included before winsock.h, winsock2.h defines _WINSOCKAPI_ to prevent the compiler from processing subsequent winsock.h includes, and all is fine.

But if winsock.h is included before winsock2.h, winsock2.h does not detect that and tries to re-define everything that winsock.h has already defined, causing the compile to fail.

You have to be very careful when mixing code that uses winsock.h with code that uses winsock2.h in the same project. For instance, when writing your own socket code that uses winsock2.h, and using third-party libraries that still use winsock.h.

You should use winsock2.h unless you want to use Winsock 1.1. winsock2.h is for Winsock 2.

You can read more about it at the Wikipedia Winsock page.