The Internet Encyclopedia (Volume 3)

P1: 211

Zarki WL040/Bidgolio-Vol I WL040-Sample.cls September 15, 2003 8:55 Char Count= 0

CURRENTSTATE OF THEINTERNET 833

Table 2World Internet Users

Millions of

subscribers 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

World 55 101 150 201 407 518.5 664.7 813.7 978.8 1156 a

Source: Nua Ltd (up to year 2001) and eTForecasts (forecasts from 2002–2005).

aExtrapolated from estimates for 2004.

Classless Addressing

Each node or a device on the Internet must have a unique

IP address. Messages are routed through the IP network

using the IP address of the destination. The most widely

deployed version of IP today is version 4 (IPv4) (Postel,

1981a), which uses 32 bits of address space. The IP ad-

dress is typically written as four separate numbers, each

coded using 8 bits, separated by periods (an example

would be 128.200.222.100). In an isolated network, any

IP address that matches the format may be used (except

for certain reserved addresses), as long as each node or

device has a unique address. However, to connect to the

Internet, it is necessary to obtain addresses not being used

so as to avoid having duplicate addresses with other net-

works and nodes.

In the early days, the InterNIC assigned to anyone who

asked a range of addresses that belongs to one of three

classes. Class A is the largest class, which supports up to

16 million hosts on each of 127 networks. A Class B net-

work supports up to 65,000 hosts, and a Class C network

supports up to 254 hosts. These classes were also used to

route packets (see the section Dynamic Routing). Hence,

this type of address allocation scheme has been termed

classful addressing or routing. Although simple, this allo-

cation scheme has proven to be very inefficient in terms

of actual use of IP addresses. For example, a modestly

sized network may need to support up to 3,000 nodes in

its own network. Since a Class C network supports only

254 hosts, a Class B network would have been assigned

even though less than 5% of the assigned addresses will

be used. The possibility of running out of IP addresses

became quite real as the growth of the Internet exploded.

Although IPv6 (Deering & Hinden, 1998) can solve the ad-

dress crunch by increasing the address space to 128 bits,

it does not resolve the nearer term problems described

next.

Since Class A networks are far too large for most or-

ganizations, and Class C networks are too small, Class B

networks were the most commonly requested and granted

type. In August 1990 during the Vancouver Internet Engi-

neering Task Force (IETF) meeting, Frank Solensky, Phill

Gross, and Sue Hares projected that the Class B space

would be depleted by March 1994. This led the InterNIC to

force many smaller organizations to accept several Class C

networks rather than a single Class B network. In the exa-

mple above, 3,000 nodes would require at least 12 Class

C networks. The problem with this situation is that each

network must be routed individually when classful rout-

ing is used, so instead of having a single entry for the net-

work in the routing table, there would now be 12 entries

in the table. This led to extremely large routing tables in

backbone routers, slowing down the whole network.

Classless Interdomain Routing

To address this problem, classless interdomain routing

(CIDR) (Hinden, 1993) and classless addressing (Rekhter

& Li, 1993) were introduced in 1993. Instead of using one

of just three partitions between the network and the host

portion of the IP address (determined by the “class” of

the network), it became possible to have variable length

network identifiers or “prefixes.” CIDR can have any pre-

fix length between 13 and 27 bits instead of just 8, 16,

or 24 bits in classful routing. Thus, the smallest network

can have up to 32 hosts and the largest network can have

more than 500,000 hosts. A CIDR address includes the

standard 32-bit IP address as well as the information on

how many bits are used as the network prefix. Increasing

the efficiency of address allocation has reduced not only

the address crunch, but also the size of the routing table.

CIDR can also be used for route aggregation, which fur-

ther reduces the size of the routing table. Deployment of

CIDR has contributed greatly to the continued growth of

the Internet.

QoS-enabled

wireless backbone

Laptop Router network

PDA

Database

(^123456789) *8#
Host
QoS-enabled IP network
Figure 2: Architecture of packet-switched wireline/wireless multimedia network.