Computer networks route data packets between a source and a destination. In most cases, the source of a packet is not directly attached to the destination, the packet needs to travel through other stations in between the two. Each time a packet travels through a router, or a gateway it takes one hop. To prevent network congestion, the maximum number of hops a packet can take is limited. With IPv4, this count is known as time to live, with IPv6 it is called hop count. Each time the packet takes one hop, the count is . When it reaches zero, the packet is if it has not reached its destination.