IP hosts may be located far apart. It is usually not possible to send data packets from one host to the other directly. Routers are special IP hosts which receive and forward packets. An IP packet may go through many routers, or hops, before arriving at a destination, if at all. Figure 13.4 shows the topology of a TCP/IP network.

Fig. 13.4 TCP/IP network

Each IP packet has an 8-bit Time-To-Live (TTL) count in the IP header, which has a maximum value of 255. At each router, the TTL is reduced by 1. If the TTL reduces to 0 and the packet still has not reached the destination, it is simply dropped. This prevents any packets from circulating around indefinitely in the IP network.

Source: Wang K.C. (2018), Systems Programming in Unix/Linux, Springer; 1st ed. 2018 edition.

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