A Flow label in IPv6 is a value given to a sequential flow of packets. To better understand what a flow label is, we need to know what a flow is first. A flow is a series of packets sent from a combination of a source address to a destination address. The routers configured to deal with IPv6 addresses handle these flows. Therefore, a flow label makes delivery of packets that belong to other similar sequential packets from source to destination or multiple destinations a priority.
