Any IP network is defined by two sets of numbers: network address and netmask. By convention, there are two ways to represent these two numbers. Netmask notation is the convention and tradition in IP networking although the more succinct CIDR notation is gaining popularity.
IP address 192.168.100.17.
In CIDR notation,
isolde's address is 192.168.100.17/24, and in
traditional netmask notation, 192.168.100.17/255.255.255.0.
Any of the
IP calculators, confirms that the
first usable IP address is 192.168.100.1 and the last usable IP address
Importantly, the IP network address, 192.168.100.0/24, is reachable
through the directly connected Ethernet interface (refer to
isolde should be able to reach any IP address in
this range directly on the locally connected Ethernet segment.
Below is the routing table for
isolde, first shown with the
conventional route -n output
and then with the
ip route show
command. Each of these tools conveys
the same routing table and operates on the same kernel routing table.
For more on the routing table displayed in
Example 4.3, “Identifying the locally connected networks with
Section 4.8.3, “The Main Routing Table”.
Example 4.3. Identifying the locally connected networks with route
In the above example, the locally reachable destination is
192.168.100.0/255.255.255.0 which can also be written 192.168.100.0/24
as in ip route show. In classful networking
terms, the network to which
isolde is directly connected is called a
class C sized network.
When a process on
isolde needs to send a packet to another
machine on the locally connected network, packets will be sent from
isolde's IP). The kernel will consult
the routing table to determine the route and the source address to use
when sending this packet.
Assuming the destination is 192.168.100.32, the kernel will find that
192.168.100.32 falls inside the IP address range 192.168.100.0/24 and
will select this route for the outbound packet. For further details on
source address selection, see
Section 4.6, “Source Address Selection”. The source address on the
outbound packet conveys vital information to the host receiving the
packet. In order for the packet to be able to return,
isolde has to
use an IP address that is locally available, 192.168.100.32 has to have
a route to
isolde and neither host must block the packet.
The packet will be sent to the locally connected network segment
isolde interprets from the routing table
that 192.168.100.32 is directly reachable through the physical network
connection on eth0.
Occasionally, a machine will be directly connected to two different IP networks on the same device. The routing table will show that both networks are reachable through the same physical device. For more on this topic, see Section 9.2, “Multiple IP Networks on one Ethernet Segment”. Similarly, multi-homed hosts will have routes for all locally connected networks through the locally-connected network interface. For more on this sort of configuration, see Section 9.6, “Multihomed Hosts”.
This covers the classification of IP destinations which are available on a locally connected network. This highlights the importance of an accurate netmask and network address. The next section will cover IP ranges which are neither locally hosted nor fall in the range of the locally reachable networks. These destinations must be reached through a router.
 The route -n output can also be produced with netstat -rn and is commonly used by admininstrators who rely on platform independent behaviour across heterogeneous Unix and Unix-like systems. This traditional routing table output uses conventional netmask notation to denote network size.