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Understanding MTU on Windows

In Windows or Linux, the highest packet you can specify before fragmentation is 1472. Linux is a little better and reports the complete packet size:

user@server:~$ ping 67.192.224.122 -M do -s 1472 -c 1
PING 67.192.224.122 (67.192.224.122) 1472(1500) bytes of data.
1480 bytes from 67.192.224.122: icmp_seq=1 ttl=119 time=13.3 ms

You can notice that on this linux server it shows the size you specified: 1472 and in addition the total size: 1500
As you can see I pinged with 1472 which is the maximum size you can ping with before the 28 bytes IP/ICMP header information is added, which brings it up to 1500.

When you specify the packet size in windows:

Pinging www.dslreports.com 209.123.109.175 with 1472 bytes of data:

Reply from 209.123.109.175: bytes=1472 time=42ms TTL=49

It does not show the actual packet size but only the size you specified.

You can ping with a much bigger packet size than 1500 but this is the maximum size before the packet is fragmented which you can limit fragmentation by specifying the "-f" option when pinging:

C:\>ping -f -l 1473 www.dslreports.com
Pinging www.dslreports.com 209.123.109.175 with 1473 bytes of data:

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

As you can see the "-f" option was specified, ping was set to not fragment the packet when sending.

By default most net devices have a MTU value of 1500:

You can verify this on the firewall:

sh run | i mtu
mtu outside 1500
mtu inside 1500

References:
TCP/IP and NBT configuration parameters for Windows

http://support.microsoft.com/kb/314053/en-us

Max MTU: How do I find mine? (#695)

http://www.dslreports.com/faq/695

Windows TCP/IP Registry Entries

http://support.microsoft.com/kb/158474/en-us

Tools:

http://www.dslreports.com/tweakr

http://www.dslreports.com/drtcp