What is an MTU and why is it important?

Finding my networks MTU?

• Your network’s maximum packet size (including headers):1518 Overhead of the Data Link layer header and trailer: Overhead of the IP header:20 bytes Overhead of an ICMP header in an Echo Request packet: Your network MTU: OK, this is the question I have for a homework assignment but im completely lost and really want to know how this works. What I have open is a packet capture from Wireshark that has a bunch of frames that are ICMP and are (592) bits on the wire, I also know that ethernet networks can support 1,518 bytes. What I cant figure out is the overhead of the data link layer header and trailer and the overhead of the ICMP header in an echo request packet. Once i figure these two out I should be able to add them together to get my networks MTU correct?

• Answer:

  You can test the MTU with the ping command like this >ping -f -l 1472 192.168.10.1 Pinging 192.168.10.1 with 1472 bytes of data: Reply from 192.168.10.1: bytes=1472 time<1ms TTL=64 Reply from 192.168.10.1: bytes=1472 time<1ms TTL=64 Reply from 192.168.10.1: bytes=1472 time<1ms TTL=64 Reply from 192.168.10.1: bytes=1472 time<1ms TTL=64 Ping statistics for 192.168.10.1: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 0ms, Maximum = 0ms, Average = 0ms >ping -f -l 1473 192.168.10.1 Pinging 192.168.10.1 with 1473 bytes of data: Packet needs to be fragmented but DF set. the -f says don't fragment the packet. the -l says use a packet of this length. What I proved was that for my network, the MTU is 1472. Why you need to know this is for things like a VPN connection for example. Due to the overhead, the MTU just might be far lower. All the header stuff is specified in the various RFC's. Learning to download them and pickout the data you need is a VERY useful skill. You can also download and use one of the free promiscuous network packet listeners. They capture the entire packet. Then you can break them down to see what bytes are what.