TCP/IP Tutorial and Technical Overview - IBM Redbooks

3.4.3 ARP and subnets
The ARP protocol remains unchanged in the presence of subnets. Remember
that each IP datagram first goes through the IP routing algorithm. This algorithm
selects the hardware device driver that should send out the packet. Only then,
the ARP module associated with that device driver is consulted.
3.4.4 Proxy-ARP or transparent subnetting
Proxy-ARP is described in RFC 1027, which is a subset of the method proposed
in RFC 925. It is another method to construct local subnets, without the need for
a modification to the IP routing algorithm, but with modifications to the routers
that interconnect the subnets.
Proxy-ARP concept
Consider one IP network that is divided into subnets and interconnected by
routers. We use the “old” IP routing algorithm, which means that no host knows
about the existence of multiple physical networks. Consider hosts A and B, which
are on different physical networks within the same IP network, and a router R
between the two subnetworks as illustrated in Figure 3-41.
A
R
B
Figure 3-41 ARP: Hosts interconnected by a router
When host A wants to send an IP datagram to host B, it first has to determine the
physical network address of host B through the use of the ARP protocol.
Because host A cannot differentiate between the physical networks, its IP routing
algorithm thinks that host B is on the local physical network and sends out a
broadcast ARP request. Host B does not receive this broadcast, but router R
does. Router R understands subnets, that is, it runs the subnet version of the IP
routing algorithm and it will be able to see that the destination of the ARP request
(from the target protocol address field) is on another physical network. If router
R's routing tables specify that the next hop to that other network is through a
Chapter 3. Internetworking protocols 123