I attached a small patch for libs\asio\test\ip\multicast.cpp that should fix the last problems for asio on Mobile 5. It addresses two things: - apparently WinCE will let you get the IP_MULTICAST_LOOP option, but not set it. I updated the expected result of the get_option tests appropriately. - Although WinCE 5.0 support multicast group membership, it has a bug that restricts the range of multicast addresses it can use: It appears to me that any group address which does not have 0's in the b and c positions of a.b.c.d will not work. Further, any address, even if b and c are zero, where d has bit 0 or 1 set doesn't seem to work, either. So a.0.0.4 works, but a.0.0.5 does not and neither does a.0.0.6. Strange bug... (See microsoft.public.windowsce.app.development under topic 'Multicast and WinCE') As a result, I've changed the v4 multicast address from 239.255.0.1 to 239.0.0.4 as long as that does not cause any problems. Thanks, -Dave