Are you seeing a lot of SSDP message on your network monitoring software? (wireshark, network monitor). This could be because of the Windows Messenger is trying to broadcast to an internet host.

SSDP: Method = M-SEARCH
SSDP: Uniform Resource Identifier = *
SSDP: HTTP Protocol Version = HTTP/1.1
SSDP: Host = 239.255.255.250:1900
SSDP: Search Target = urn:schemas-upnp-org:device:InternetGatewayDevice:1
SSDP: Mandatory Extension = “ssdp:discover”
SSDP: Maximum Wait = 3

You can disabled the Windows Messenger broadcast by modifying the computer registry.

Hive: HKEY_LOCAL_MACHINE
Key: Software\Microsoft\DirectPlayNATHelp\DPNHUPnP
Name: UPnPMode
Type: REG_DWORD
Value: 2 disabled