The ExternalAddress property returns the IP address
assigned to the router that connects the local host to the Internet.
This is typically used by an application executing on a system in a
local network that uses a router which performs Network Address
Translation (NAT). In that network configuration, the
LocalAddress property will only return the IP address for the
local system on the LAN side of the network unless a connection has
already been established to a remote host. The ExternalAddress
property can be used to determine the IP address assigned to the
router on the Internet side of the connection and can be particularly
useful for servers running on a system behind a NAT router. Note that
you should not assign the LocalAddress property to the value
returned by the ExternalAddress property. If the server is
running behind a NAT router, the router must be configured to forward
incoming connections to the appropriate address on the LAN.
Using this property requires that you have an active connection to
the Internet; checking the value of this property on a system that
uses dial-up networking may cause the operating system to
automatically connect to the Internet service provider. The control
may be unable to determine the external IP address for the local host
for a number of reasons, particularly if the system is behind a
firewall or uses a proxy server that restricts access to external
sites on the Internet. If the external address for the local host
cannot be determined, the property will return an empty string.
If the control is able to obtain a valid external address for the
local host, that address will be cached for sixty minutes. Because
dial-up connections typically have different IP addresses assigned to
them each time the system is connected to the Internet, it is
recommended that this property only be used in conjunction with
persistent broadband connections.
It is important to note that checking this property value may
cause the thread to block until the external IP address can be
resolved and should never be used in conjunction with non-blocking
(asynchronous) socket connections. If you need to check this property
value in an application which uses asynchronous sockets, it is
recommended that you create a new thread and access the property from
within that thread.