TCP/IP Tutorial and Technical Overview - IBM Redbooks

different physical device, it will reply to the ARP as though it were host B, saying
that the network address of host B is that of the router R itself.
Host A receives this ARP reply, puts it in its cache, and will send future IP
packets for host B to the router R. The router will forward such packets to the
correct subnet.
The result is transparent subnetting:
► Normal hosts (such as A and B) do not know about subnetting, so they use
the “old” IP routing algorithm.
► The routers between subnets have to:
– Use the subnet IP routing algorithm.
– Use a modified ARP module, which can reply on behalf of other hosts.
See Figure 3-42 for more details.
A
"old" IP
ro uting
IP
subnet routing
and m odified ARP
Figure 3-42 ARP: Proxy-ARP router
3.5 Reverse Address Resolution Protocol (RARP)
Reverse Address Resolution Protocol (RARP) is a network-specific standard
protocol. It is described in RFC 903.
Some network hosts, such as diskless workstations, do not know their own IP
address when they are booted. To determine their own IP address, they use a
mechanism similar to ARP, but now the hardware address of the host is the
known parameter and the IP address the queried parameter. It differs more
fundamentally from ARP in the fact that a RARP server must exist in the network
that maintains that a database of mappings from hardware address to protocol
address must be preconfigured.
124 TCP/IP Tutorial and Technical Overview
R
B