What is Hop?
A hop is one passage of a packet through a router or other layer-3 forwarding device as it travels from source to destination across an internetwork.
A hop is the fundamental unit of progress in packet-switched networks. Each time a packet passes from one router to the next, it completes a single hop. The path from source to destination is a sequence of hops, where the source host and each intermediate router constitute a hop boundary. The destination host itself is not counted as a hop in most contexts, though exceptions exist in traceroute implementations where the final host responds with an ICMP port unreachable.
Hops are counted and measured at the Internet layer (OSI layer 3). The IPv4 header contains a Time to Live (TTL) field, originally specified as an 8-bit field in RFC 791 (1981). Each router decrements the TTL by at least 1 per hop. When TTL reaches 0, the router discards the packet and sends an ICMP Time Exceeded message back to the source. In IPv6, the identical function is called the Hop Limit (RFC 2460). The hop count thus serves as both a routing metric and a loop-prevention mechanism.
Hop count is a simple metric used by routing protocols such as RIP (Routing Information Protocol), which defines a maximum hop count of 15 (RFC 1058). A route with a metric of 16 is considered unreachable. More sophisticated protocols like OSPF or BGP use composite metrics. The hop count is also the basis for traceroute, a diagnostic tool that sends packets with progressively increasing TTL values to map the router-level path to a destination.
Key facts
- IPv4 TTL and IPv6 Hop Limit are both decremented by at least 1 per router.
- RIP uses hop count as its sole metric with a maximum of 15 hops.
- Traceroute exploits TTL expiration to discover each hop in the path.
- A hop always involves layer-3 forwarding, not layer-2 bridging or switching.
- The term originates from early ARPANET packet-switching nodes.
How it works in practice
Related terms
References
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