Maximum transmission unit (MTU) is the max number of bytes of a data packet
that a device can accept. The max size of MTU size is 1500 bytes.
What happens when the packet is larger than MTU
If the packet is larger than a specified MTU for a specific device it will be dropped off or fragmented into chunks. The packet is fragmented by the sender and adds certain fields in the IP header to indicate that the packet is fragmented.
IPv6 cannot fragment packets, so anything that exceeds the MTU will be
dropped. The same can be achieved in
IPv4 when the “Don’t Fragment” flag is
specified in the packet header.
How a device knows the receiver MTU.
Devices use the Path MTU Discovery which discovers the MTU of all the devices, routers, and switches in the network path. This is done by sending test packets with the “Don’t Fragment” flag and if one of the hops drops the packet a message is sent back with an ICMP message with its MTU.