Internet Security - Dang Thanh Binh's Page

32 INTERNET SECURITY
H
Host Server
H
Host
Physical address
is given. Request
IP address
RARP
request
M1 M2 M3
(a) Request for the physical address by broadcast
IP address
RARP
reply
(b) Reply IP address by unicast
Reply IP
Address
Figure 2.6 RARP dynamic mapping.
S
S
Server
reality, RARP is a protocol of dynamic mapping in which a given physical address is
associated with a logical IP address, as shown in Figure 2.6.
To get the IP address, a RARP request is broadcast to all systems on the network.
Every host or router on the physical network will receive the RARP request packet, but
the RARP server will only answer it as shown in Figure 2.6(b). The server sends a RARP
reply packet including the IP address of the requestor.
2.1.4 Classless Interdomain Routing (CIDR)
CIDR is the standard that specifies the details of both classless addressing and an associated
routing scheme. Accordingly, the name is slightly inaccurate designation because
CIDR specifies addressing as well as routing.
The original IPv4 model built on network classes was a useful mechanism for allocating
identifiers (netid and hostid) when the primary users of the Internet were academic and
research organisations. But, this mode proved insufficiently flexible and inefficient as
the Internet grew rapidly to include gateways into corporate enterprises with complex