hopcount

Hopcount is a measure used in computer networking to denote the number of intermediate devices, typically routers, that a data packet must pass through to reach a destination. In routing, hop count is a simple metric used by some distance-vector protocols to determine the best path. A hop is counted each time the packet is forwarded by a router.

In IPv4 networks, the concept is closely tied to the Time to Live (TTL) field. The IP layer decrements TTL by one at every hop; if TTL reaches zero, the router discards the packet and may send an ICMP Time Exceeded message. The observed number of hops to a destination can be inferred by diagnostic tools such as traceroute. In IPv6, the analogous field is called the Hop Limit.

Hop count as a routing metric is most famously used by the Routing Information Protocol (RIP), where a route’s value is the number of hops and a maximum of 15 hops means the destination is unreachable. Other routing protocols use more complex metrics or combine hop count with bandwidth, delay, or cost. For example, OSPF uses a cost metric, while BGP relies on policy decisions and AS-path length rather than pure hop count.

Applications and limitations: hopcount is simple and easy to compute, but it does not reflect link speed, reliability, or congestion. Networks with equal-cost paths or asymmetric routes can complicate interpretation. Diagnostics such as traceroute rely on hop counts to map paths, though firewalls, load balancers, and path variations can affect accuracy. See also TTL, Hop Limit, ICMP, traceroute, RIP.