In computer networking, a hop is one portion of the path between source and destination. Data packets pass through bridges, routers and gateways as they travel between source and destination. Each time packets are passed to the next network device, a hop occurs. The hop count refers to the number of intermediate devices through which data must pass between source and destination.
The hop count refers to the number of intermediate network devices through which data must pass between source and destination. Hop count is a rough measure of distance between two hosts. A hop count of n means that n network devices separate the source host from the destination host.
On a layer 3 network such as Internet Protocol (IP), each router along the data path constitutes a hop. By itself, this metric is, however, not useful for determining the optimum network path, as it does not take into consideration the speed, load, reliability, or latency of any particular hop, but merely the total count. Nevertheless, some routing protocols, such as Routing Information Protocol (RIP), use hop count as their sole metric.
Each time a router receives a packet, it modifies the packet, decrementing the time to live (TTL). The router discards any packets received with a zero TTL value. This prevents packets from endlessly bouncing around the network in the event of routing errors. Routers are capable of managing hop counts, but other types of network devices (e.g. Ethernet hubs and bridges) are not.
Known as time to live (TTL) in IPv4, and hop limit in IPv6, this field specifies a limit on the number of hops a packet is allowed before being discarded. Routers modify IP packets as they are forwarded, decrementing the respective TTL or hop limit fields. Routers do not forward packets with a resultant field of 0 or less. This prevents packets from following a loop forever.
Next hop is the next gateway to which packets should be forwarded along the path to their final destination. A routing table usually contains the IP address of a destination network and the IP address of the next gateway along the path to the final network destination. By only storing next-hop information, next-hop routing or next-hop forwarding reduces the size of routing tables. A given gateway only knows one step along the path, not the complete path to a destination. It is also key to know that the next hops listed in a routing table are on networks to which the gateway is directly connected to.
The traceroute command can be used to measure the number of router hops from one host to another. Hop counts are often useful to find faults in a network, or to discover if routing is indeed correct.