A hop is a network section and is used as a counting unit. The number of hops indicates how many network sections the data packets were transmitted over. Such network sections can be defined by routers or other nodes.
Hops are used as a counting unit for determining and limiting the lifetime of a data packet. The hop method is used in the IP protocol, in the Next Hop Resolution Protocol( NHRP) and also in the Hybrid Wireless Mesh Protocol( HWMP).
The idea is to prevent undeliverable datagrams from wandering endlessly around the network and generating additional traffic. For this reason, the individual steps of a data packet are entered and counted in a header field. In the IP protocol, it is the Time To Live( TTL) data field in which the hop values are entered; in IPv6, it is the Hop Limit field. Each time a data packet passes through a router, the previously set hop value is reduced by one time or counting unit. For example, three hops means that the data packet passes through three routers on its way from the source to the destination.
The time unit is set in seconds or counting units. For example, if the hop count value is entered as 255, it means that the maximum time is 255 seconds. Each time the datagram passes through a router, the value is reduced by one second. The datagram can therefore pass through 255 routers until it is set to zero, cancelled and destroyed.