Internet Security - Dang Thanh Binh's Page

H
Host
H
TCP/IP SUITE AND INTERNET STACK PROTOCOLS 29
IP address
(a) Request for the physical address by broadcast
Physical address
ARP
request
M1 M2 M3
ARP
reply
Server
Host Server
(b) Reply for the physical address by unicast
Figure 2.4 ARP dynamic mapping.
S
S
Physical
address
Figure 2.4 shows an example of simplified ARP dynamic mapping. Let a host or router
call a machine. A machine uses ARP to find the physical address of another machine by
broadcasting an ARP request. The request contains the IP address of the machine for which
a physical address is needed. All machines (M1, M2, M3, ...) on the network receive an
ARP request. If the request matches a M2 machine’s IP address, the machine responds
by sending a reply that contains the requested physical address. Note that Ethernet uses
the 48-bit address of all 1’s (FFFFFFFFFFFF) as the broadcast address.
A proxy ARP is an ARP that acts on behalf of a set of hosts. Proxy ARP can be used
to create a subnetting effect. In proxy ARP, a router represents a set of hosts. When an
ARP request seeks the physical address of any host in this set, the router sends its own
physical address. This creates a subnetting effect. Whenever looking for the IP address of
one of these hosts, the router sends an ARP reply announcing its own physical address.
To make address resolution easy, choose both IP and physical addresses the same
length. Address resolution is difficult for Ethernet-like networks because the physical
address of the Ethernet interface is 48 bits long and the high-level IP address is 32 bits
long. In order for the 48-bit physical address to encode a 32-bit IP address, the next
generation of IP is being designed to allow 48-bit physical (hardware) addresses P to be
encoded in IP addresses I by the functional relationship of P = f (I). Conceptually, it will
be necessary to choose a numbering scheme that makes address resolution efficient by
selecting a function f that maps IP addresses to physical addresses.
As shown in Figure 2.5, the ARP software package consists of the following five
components: