11.4 Internet Addresses in Practice

Like telephone numbers, there are physical constraints on how we assign numerical IP addresses. Every address contains two parts: the “network bits” and the “host bits” in the address. The upper bits in the address form the network address. The lower bits form the host address.

The distinction between network and host bits is essential for routing packets on the internet. Routing tables describe how to locate networks, not hosts. Once the packet arrives on its destination LAN, we can use ARP to find the right host. The challenge is to reach the network from elsewhere on the internet.

When a router receives a packet, it extracts the packet’s destination IP address, then it sets the host bits to zero, leaving ...

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