Classful addressing. Default Route

A company or organization was assigned an entire class A, class B, or class C address block. This use of address space is referred to as classful addressing.

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.

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  Figure 5-12, this gateway address is the address of a router interface that is connected to the same network as the host.
  
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  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.
  
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  Figure 5-12 shows that default gateways are members of their own local networks.
  

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  The default gateway is configured on a host.
  

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  On a Windows computer, the Internet Protocol (TCP/IP) Properties tools are used to enter the default gateway IPv4 address.
  

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  Both the host IPv4 address and the gateway address must have the same network (and subnet, if used) portion of their respective addresses.
  

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  Remember that a default route is the route used if no specific route is available to be selected for delivery.
  
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  In IPv4 networks, the address 0.0.0.0 is used for this purpose. Packets with
  

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  The default route is also known as the gateway of last resort.
  

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  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.
  
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  As each packet arrives at a router, the destination network address is examined and compared to the routes in the routing table.
  
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  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.
  
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  Example 5-6 outlines the association of routes with next hops and router interfaces.