can't find my Maximum Transmission Unit

[Guest]

Hi everybody,
I saw some software called TCP optimizer( by speedguide ) on the windows
market place. In order to use the software you frist have to determine what
your
maximum transmission unit is. So far I have been unable to do so.
I have windows xp pro and cox high speed internet. Whenever I use system
information to run a network Diagnostics test it comes back as a failure on
the IP address check. It uses this address:68.102.79.205 . When I use command
prompt to ping I get the same failure for the address. However ,cox high
speed told me because i have a Norton firewall, my address
is:68.103.144.249.When I use that address all packets are recieved.
I was advised to use this command prompt :-f -l
1500www.yourcableprovider.com to determine my MTU.When I use my IP address I
get a bad option -1. error message. When I just use the ping with only the-f
and no -1 I get a 100% lost report. When I type in my internet service
provider , instead of my address I get either a bad command error message or
a couldn't find any one by that name.
Also, speedguide said I cound check my regisrty for my current MTU setting
at:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\NetTrans , I
checked this and there is no such regisrty seeting on my computer.
I'm not having any internet problems, I just wanted to speed up my
downloads.
Speedguide said that windows xp MTU is set by default to a phone connection,
which can slow you down if you have cable. And I'm starting to wonder if I
have a problem with my network settings.
Your feedback will be very appreciated! Thank you.

 

[Johan Sandqvist]

You'll find your maximum MTU by using the ping command as indicated, but
by using a lower case L (i.e. not the number 1).

The -l option sets the date size in bytes, and in combination with the
-f (do not fragment) you can use it to determine the maximum MTU by
lowering the data size until you get a reply.

I.e. ping -f -l 1500 192.168.22.1 results in "Packet needs to be
fragmented but DF set", meaning that the MTU on my local network is
lower than 1500. Subsequent testing revealed that my max MTU is 1472
indicated by "Reply from 192.168.22.1: bytes=1472 time<1ms TTL=255".

Good ping addresses would be either some known address at your ISP, or
the closes gateway (usually ip address x.y.z.1 where x.y.z. are the
first parts of your current IP address).

// Johan

--
Johan Sandqvist
Microsoft Exchange MVP

http://www.microsoft.com/exchange/community/default.mspx
http://blogs.technet.com/exchange/
http://www.MSExchange.org/

 

[Guest]

Thanks alot for the tip! You were right, the little "l" was the key.
By the way, my MTU is 1472.