The logical distance between networks based on the number of routers that must be traversed by packets sent between them. For example, in TCP/IP internetworking, the number of hops between two hosts would be the number of routers that an Internet Protocol (IP) packet would have to pass through in order to reach its destination.
The following illustration shows a network path that is 3 hops long. As the packet travels from source to destination, the header of the packet maintains information about the “hop count” (the number of hops traversed). This information is stored as a Time to Live (TTL) parameter within each packet that typically starts with a value of 128 and is decremented by 1 at each router (that is, after each hop).
If router congestion delays the packet at a router, the TTL might be decremented by more than 1 to indicate this. If the TTL is decremented to 0 before the packet reaches its ultimate destination, the next router drops the packet and retransmission is required from the source host.
Graphic H-6. Hop.
Hop counts are used by dynamic routers to determine the best route for forwarding data across a large internetwork. The route that has the smallest total number of hops is generally the best route for sending the data.
Use the tracert command in Microsoft Windows NT or Windows 2000 to watch the hop count decrease as an IP packet traverses an internetwork toward its destination.