Confusing MTU

[drummer]

I'm slightly confused about this, so maybe someone can shed some light.

Been using the TCP/IP Optimizer tool for some years with my router's MTU always set at 1500.  I've only ever chosen the Optimal settings for TCP Optimizer which always suggests my OS's MTU should be 1500.

Rebooting the computer and going to www.speedguide.net/analyzer.php always showed that everything was hunky dory with a 1500 MTU and my computer/router couldn't get any better for interwebby connections.

Did a fresh install of XP Pro on a lappy earlier today and TCP/IP Optimizer again suggested that an MTU of 1500 was optimal.  After the reboot, I went back to the TCP analyzer page which says my MTU is now 1460.  Odd in itself, but all four computers on the network are now suddenly showing the same MTU (1460).

Any clues?

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[Sebby]

I suspect that the TCP analyser is just reporting the figure wrong for some reason. There certainly doesn't seem to be an issue with your settings.

[Rik]

The site regularly reports 1460 as the MTU, Drummer. It's my belief that somewhere on the route to the server there is a choke point which sets the 'working' MTU to 1460 dynamically. Elsewhere, 1500 is reported correctly.

[drummer]

Thanks Sebby and Rik for the reassurance.

MTU for me is just set, check and forget so I've never made a habit of returning to the analyser tool.

I sometimes wish I knew nothing about how ADSL works.  Only sometimes mind.

[Sebby]

I can understand that.

[Rik]

Me too.