So when I discover the maximum mtu which allows packet to be transmitted in an unfragmented form, why does that value not continue to lead to packets being transmitted in an unfragmented form - why in other words, when I run "ping -f -l", do I get the message that the packets need to be fragmented when previously, before I set the new, lower mtu, that value resulted in no fragmentation?

Maybe something is automatically tuning things - Vista for example.

I am using xp - but I have got a clearer picture of what is going on. It seems that the actual mtu is 28bytes lower than the one I set (In other words, the mtu I set is 1432 but the maximu size packet I can send without fragmentation is 1404). Something to do with overheads?