Connections not closed?

[JoaoAfonso]

Good evening.
A fast question:
I have a server which was made to receive windows telnet clients (or similar). Anyway, when I order server to disconnect a client, client does not receive the message os socket closed, eventually, while client assumes it is closed. This just happens using internet, because in a lan, when I close a socket, the client receives the message and returns the info "you lost connection", as supposed.
Any clue?
thanks in advance