TCP/IP Tutorial and Technical Overview - IBM Redbooks

To differentiate between subnets, the IP routing algorithm is updated, as shown
in Figure 3-8.
Figure 3-8 IP: Routing with subnets
Some implications of this change include:
► This algorithm represents a change to the general IP algorithm. Therefore, to
be able to operate this way, the particular gateway must contain the new
algorithm. Some implementations might still use the general algorithm, and
will not function within a subnetted network, although they can still
communicate with hosts in other networks that are subnetted.
► As IP routing is used in all of the hosts (and not just the routers), all of the
hosts in the subnet must have:
– An IP routing algorithm that supports subnetting
– The same subnet mask (unless subnets are formed within the subnet)
► If the IP implementation on any of the hosts does not support subnetting, that
host will be able to communicate with any host in its own subnet but not with
any machine on another subnet within the same network. This is because the
host sees only one IP network and its routing cannot differentiate between an
IP datagram directed to a host on the local subnet and a datagram that should
be sent through a router to a different subnet.
In case one or more hosts do not support subnetting, an alternative way to
achieve the same goal exists in the form of proxy-ARP. This does not require any
changes to the IP routing algorithm for single-homed hosts. It does require
changes on routers between subnets in the network (refer to 3.4.4, “Proxy-ARP
or transparent subnetting” on page 123).
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