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What is fragmentation? How is it handled in IPv4 and IPv6?

What is fragmentation? How is it handled in IPv4 and IPv6? Fragmentation is the process in which a device (usually...

a router or some type of gateway) breaks incoming packets into smaller pieces during transit.

Each network has its Maximum Transmission Unit (MTU). The value of the MTU depends on the type of network and protocols it is using. The MTU declares the largest possible packet that is allowed to be sent on the network and is always respected by all hosts and devices that transmit data on that network.

To give you an example of a fragmentation let's consider the following two networks:

    Network A		   Network B
     MTU 1500		    MTU1400
			Router
Host A----****-----/==/----*---***----Host B
	  ----->		  -->  -->
	  Packet 1		Packet

Host A resides on Network A, which has a MTU of 1500. Host B on the other side is part of Network B where the MTU there is 1400.

Host A generates a packet (****) with a size of 1500 bytes which is equal to the network's MTU (1500). When the packet transits the router that connects the networks, the router knows that Network B's MTU is 1400 bytes, so it needs to split the 1500 byte packet into two smaller packets and transmit them on the network, so it creates the first packet which is 1400 bytes long (***) and then sticks the rest of the original packet into the second packet(*) which is just 100 bytes.

When Host B receives the packets, it will realize it needs to assemble the two packets in order to retrieve their data, and that's exactly what happens.

Now the above example is valid for an IPv4 networks, and to the best of my knowledge, it should also work the same with an IPv6 since only the addressing scheme changes between the two.
 

Do you have more questions about networking, VPN security or VoIP? Then visit Firewall.cx, one of the few websites recommended by Cisco Systems in its world class Cisco Academy program.

This was last published in June 2003

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