TCP/IP Tutorial and Technical Overview - IBM Redbooks

Because D is directly attached to network 128.15.0.0, it maintains a direct route
for this network. To reach networks 129.7.0.0 and 128.10.0.0, however, it must
have an indirect route through E and B, respectively, because these networks
are not directly attached to it.
The routing table of host F might contain the following (symbolic) entries
(Table 3-3).
Table 3-3 Host F sample entries
Destination Router Interface
129.7.0.0 F wan0
default E wan0
127.0.0.1 loopback lo
Because every host not on the 129.7.0.0 network must be reached through host
E, host F simply maintains a default route through E.
IP routing algorithm
IP uses a unique algorithm to route datagrams, as illustrated in Figure 3-7.
Figure 3-7 IP: Routing without subnets
Chapter 3. Internetworking protocols 81