The IPv4 default route is 0.0.0.0. This default route is a “catch all” route to route packets when a more specific route is not available. The use of this address also reserves all addresses in the 0.0.0.0 /8 address block (0.0.0.0–0.255.255.255).
Another reserved address block is 127.0.0.0 /8 (127.0.0.0 to 127.255.255.255). This is reserved in the IPv4 hosts for the 127.0.0.1 loopback. The loopback is a special address that hosts use to direct traffic to themselves.
IPv4 addresses in the 169.254.0.0 /16 address blocks (169.254.0.0 to 169.254.255.255) are designated as link-local addresses. These addresses can be automatically assigned to the local host by the operating system in environments where no IP configuration is available.
The test-net addresses are set aside for teaching and learning purposes. This is the address block 192.0.2.0 /24 (192.0.2.0 to 192.0.2.255). These addresses can be used in documentation and network examples.
Figure 5-12, this gateway address is the address of a router interface that is connected to the same network as the host.
The router interface is actually a host on the local network, so the host IP address and the default gateway address must be on the same network.
Figure 5-12 shows that default gateways are members of their own local networks.
The default gateway is configured on a host.
On a Windows computer, the Internet Protocol (TCP/IP) Properties tools are used to enter the default gateway IPv4 address.
Both the host IPv4 address and the gateway address must have the same network (and subnet, if used) portion of their respective addresses.
The next hop is the address of the device that will process the packet next. For a host on a network, the address of the default gateway (router interface) is the next hop for all packets destined for another network.
As each packet arrives at a router, the destination network address is examined and compared to the routes in the routing table.
The routing table lists an IP address for the next-hop router for the routes it knows. If a matching route is determined, the router then forwards the packet out the interface to which the next-hop router is connected.
Example 5-6 outlines the association of routes with next hops and router interfaces.