Introduction to TCP/IP and routing : Routing : When do you route ?
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2.2. When do you route ?

If all hosts were directly reachable through the local network we could, using ARP, locate other hosts on the network and send data to them.

The problem is: how do you reach hosts that are not on the same physical network (link) as you, and what do you do to reach them ?

Another question: how do you know that the host isn't on the local network ?

The answer is: compare the address/netmask of the source address with the address of the destination.

The rule is that for other hosts to be considered directly reachable is:

The IP address of the destination must be within the range defined by the local network number and netmask.

Example 1

  Source: 192.168.1.10, netmask is /24 (or 255.255.255.0)
  Dest  : 192.168.1.50

The netmask, /24, indicates that the network part of the source address is 24 bits, or 3 bytes wide. Therefore, the network number here is:

  192.168.1.0

And the range of the network is:

  192.168.1.0 - 192.168.1.255

The destination host's address, .50, is withing the range and can therefore be reached directly via a local link.

Example 2

Now consider the following:

  Source: 192.168.1.35, netmask is /28 (or 255.255.255.240)
  Dest  : 192.168.1.75

The netmask, /28, indicates that the network number of the source address is:

  192.168.1.32

With a range of:

	192.168.1.32 - 192.168.1.47	(14 hosts)

As we can see, the address 192.168.1.75 is NOT included in the range, therefore it is not directly reachable.

Example 3

Yet another case:

  Source: 192.168.1.10, netmask is /16 (or 255.255.255.240)
  Dest  : 192.168.2.30

The network associated with the source address/netmask is:

  192.168.0.0

With a range of:

  192.168.0.0 - 192.168.255.255 (65534 hosts)

Address 192.168.2.30 is contained within the range, and directly reachable.

Example 4

  Source: 10.2.15.4, netmask is /8 (or 255.0.0.0)
  Dest  : 192.168.2.30

Range:

  10.0.0.0 - 10.255.255.255 (16777214 hosts)

Address is 192.168.2.30 is not contained, and not directly reachable.


Introduction to TCP/IP and routing : Routing : When do you route ?
Previous: What is routing ? A router ?
Next: Where do you route ?