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.

In the example network, isolde has 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 is 192.168.100.254. Importantly, the IP network address, 192.168.100.0/24, is reachable through the directly connected Ethernet interface (refer to classification 2). Therefore, 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 ^[16] and then with the ip route show ^[17] 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 route”, consult Section 8.3, “The Main Routing Table”.

[root@isolde]# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.100.0   0.0.0.0         255.255.255.0   U     0      0        0 eth0
127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        0 lo
0.0.0.0         192.168.100.254 0.0.0.0         UG    0      0        0 eth0
[root@isolde]# ip route show
192.168.100.0/24 dev eth0  scope link 
127.0.0.0/8 dev lo  scope link 
default via 192.168.100.254 dev eth0
      

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 192.168.100.17 (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 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 directly, because 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 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 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.