[CUT]

Hello Kevin and thanx for answering,

I actually use Socket.Bind in order to bind to a specific network card on
the machine, we had problems when in the past where more than a network card
existed on the machine. I did get same samples browsing in MSDN for VS.2003
looking at MulticastOption class.

The problem on that particular x64 machine is that I get the error very
seldom and for a limited amount of time (some minutes) looks like it's a NIC
driver issue (or perhaps x64 issue?), as we have the same service installed
100+ times and behaving well (all on 32bit machines, tho).

Anyway, here is the complete routine I use to send UDP datagrams, maybe u
can have a look at it and give me some clues.

TIA
Paolo

   Public Shared Function Send(ByVal LocalAddress As IPAddress, ByVal
LocalPort As Integer, ByVal GroupAddress As IPAddress, ByVal GroupPort As
Integer, ByVal ttl As Integer, ByVal message As String) As Boolean
     Dim UdpSender As Socket
     Dim groupEP As IPEndPoint
     Dim bytes As Byte()

     Try
       UdpSender = New Socket(AddressFamily.InterNetwork, SocketType.Dgram,
ProtocolType.Udp)
       UdpSender.Bind(New IPEndPoint(LocalAddress, LocalPort))

       UdpSender.SetSocketOption(SocketOptionLevel.IP,
SocketOptionName.AddMembership, New MulticastOption(GroupAddress,
LocalAddress))
       UdpSender.SetSocketOption(SocketOptionLevel.IP,
SocketOptionName.MulticastInterface, LocalAddress.GetAddressBytes)
       UdpSender.SetSocketOption(SocketOptionLevel.IP,
SocketOptionName.MulticastTimeToLive, ttl)

       groupEP = New IPEndPoint(GroupAddress, GroupPort)

       bytes = Encoding.ASCII.GetBytes(message)

       UdpSender.SendTo(bytes, bytes.Length, SocketFlags.None, groupEP)

       Return True
     Catch ex As Exception
       Return False
     Finally
       If (Not UdpSender Is Nothing) Then
         UdpSender.Close()
         UdpSender = Nothing
       End If

       groupEP = Nothing
     End Try