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The basic idea in CIDR is to allocate blocks of class C network addresses to each ISP. Organizations using the ISP are suballocated a block of 2" contiguous addresses. For instance, if an organization requires 2000 addresses, then it will be allocated a block of 2048 (i.e. 28) contiguous class C addresses.
Hierarchical suballocation of addresses in this manner implies that clients with addresses allocated out of a given ISP will be routed via the ISPís network. This permits all of these addresses to be advertised outside the ISPís network in an aggregate manner. As an example, let us assume that an ISP was allocated 131,072 class C network addresses starting at 220.127.116.11. That means that the lowest network address is 18.104.22.168 or 11000010 00000000 00000000 00000000 and the highest network address is 22.214.171.124 or 11000011 11111111 11111111 11111111. Any address whose first seven bits are 1100001 belongs to the group of addresses allocated to the ISP. This prefix can be calculated by performing a bit-wise AND operation between the lowest address and the mask 254.0.0.0 or 11111110 00000000 00000000 00000000. Routers outside the ISPís network are provided, therefore, only with the base address 126.96.36.199 and the mask 254.0.0.0. This information suffices in order to identify whether an address of an IP packet has the same prefix as the ISP. Calculating a prefix using a base network address and a mask is known as supernetting. Supernetting is the converse of subnetting.
The above use of contiguous addresses gives rise to better usage of the address space. Also, by only advertising a base address and a mask, the amount of information that a router has to keep in its routing table is minimized. Note that some network addresses were allocated prior to CIDR, and a router has to keep these addresses in its table as well.
To further simplify routing, blocks of addresses were also allocated according to geographic regions (see Table 6.1).
Finally, note that the class A, B, and C addresses are no longer used for routing. Instead, CIDR is applied to all addresses, which explains why this scheme is called classless.
6.1.3 ARP, RARP, and ICMP
The TCP/IP protocol suite includes other protocols such as the address resolution protocol (ARP), the reverse address resolution protocol (RARP) and the Internet control message protocol (ICMP).
Table 6.1 Allocation of addresses per region.
Region Lower Higher
Europe 188.8.131.52 184.108.40.206
North America 220.127.116.11 18.104.22.168
Central/South America 22.214.171.124 126.96.36.199
Pacific Rim 188.8.131.52 184.108.40.206
THE MULTI-PROTOCOL LABEL SWITCHING (MPLS) ARCHITECTURE
ARP is used to translate a hostís IP address to its corresponding hardware address. This address translation is known as address resolution. The ARP standard defines two basic messages: a request and a response. A request message contains an IP address and requests the corresponding hardware address. A reply message contains the IP address sent in the request and the hardware address.
RARP does the opposite to ARP. It identifies the IP address of a host that corresponds to a known hardware address.
ICMP defines several error and information messages used in the Internet to report various types of errors or send various types of information. Some of the principal messages are: source quench, time exceeded, destination unreachable, redirect, fragmentation required, parameter problem, echo request/reply, and timestamp request/reply.
A source quench message is sent by a router when it has run out of buffer space and it cannot accept more datagrams. A time exceeded message is sent by a router when the time to live field in a datagram is 0. The datagram is dropped by the router. The same message is also used by a host if the reassembly timer expires before all fragments from a given datagram have arrived. A destination unreachable message is sent by a router to a host that created a datagram, when it decides that the datagram cannot be delivered to its final destination. A redirect message is sent by a router to the host that originated a datagram, if the router believes that the datagram should have been sent to another router. A fragmentation required message is sent by a router to the host of a datagram, if it finds that the datagram is larger than the maximum transfer unit (MTU) of the network over which it must be sent. The datagram is rejected by the router. A parameter problem message is used to indicate that an illegal value has been discovered in the IP header of a datagram. Echo reply and echo request is used to test if a user destination is reachable and alive. Timestamp request and timestamp reply are similar to the echo request/reply messages except that the arrival time of the request message and the departure time of the reply message are also recorded.