Understanding the network.pdf - Back to Home

Routers are also called intermediate systems (ISs) because a router acts as a
forwarding intermediary. Layer 2 protocols and functions will be covered in Chapter
4, "LAN Internetworking Technologies," and Chapter 5, "WAN Internetworking
Technologies."
Here, the destination host is not local to the source, so the datagram is forwarded to
an intermediate router. The intermediate router then determines where to forward
the IP datagram next. If the host is local to the intermediate router, it forwards the
datagram on to the destination host (as in this example). If it's not local, the router
forwards the IP datagram on to a router that it believes can deliver the datagram.
Routing table lookups make these determinations.
IP Routing and IP Datagram Forwarding
All IP datagram delivery starts with a router table lookup. Every host that is part of
a TCP/IP network has a routing table. The routing table is created either by static or
dynamic means. There are four basic values that make up an IP routing table entry:
• Interface—The interface with which the route is associated
• Network address—The network portion of IP address for the destination
network
• Subnet mask—The bitmask used to distinguish the network address from the
host address
• Gateway—The next hop to which the interface forwards the IP datagram for
delivery
All IP-based hosts create a local routing table at startup. Almost all end-station
routing tables are created statically. Every device that uses TCP/IP for host-to-host
communication has at least one static route: the default gateway.
Table 2.10 provides the routing table for the network illustrated in Figure 2.9. The
routing table has entries for all its connected interfaces and the default gateway,
which is defined with a network address of 0.0.0.0 and a network mask of 0.0.0.0.