Understanding IP Addressing 1.pdf

Understanding IP Addressing:

Everything You Ever Wanted To Know

Chuck Semeria

NSD Marketing

3Com Corporation

April 26, 1996

Introduction

In the mid-1990's, the Internet is a dramatically different network than when it was first

established in the early 1980's. Today, the Internet has entered the public consciousness

as the world's largest public data network, doubling in size every nine months. This is

reflected in the tremendous popularity of the World Wide Web (WWW), the

opportunities that businesses see in reaching customers from virtual storefronts, and the

emergence of new types and methods of doing business. It is clear that expanding

business and social awareness will continue to increase public demand for access to

resources on the Internet.

There is a direct relationship between the value of the Internet and the number of sites

connected to the Internet. As the Internet grows, the value of each site's connection to

the Internet increases because it provides the organization with access to an ever

expanding user/customer population.

Internet Scaling Problems

Over the past few years, the Internet has experienced two major scaling issues as it has

struggled to provide continuous and uninterrupted growth:

- The eventual exhaustion of the IPv4 address space

- The ability to route traffic between the ever increasing number of networks that

comprise the Internet

The first problem is concerned with the eventual depletion of the IP address space. The

current version of IP, IP version 4 (IPv4), defines a 32-bit address which means that

there are only 2 32 (4,294,967,296) IPv4 addresses available. This might seem like a

large number of addresses, but as new markets open and a significant portion of the

world's population becomes candidates for IP addresses, the finite number of IP

addresses will eventually be exhausted.

The address shortage problem is aggravated by the fact that portions of the IP address

space have not been efficiently allocated. Also, the traditional model of classful

addressing does not allow the address space to be used to its maximum potential. The

Address Lifetime Expectancy (ALE) Working Group of the IETF has expressed

concerns that if the current address allocation policies are not modified, the Internet will

experience a near to medium term exhaustion of its unallocated address pool. If the

Internet's address supply problem is not solved, new users may be unable to connect to

the global Internet!

Class A

Class B

Class C

Figure 1: Assigned and Allocated Network Numbers

The second problem is caused by the rapid growth in the size of the Internet routing

tables. Internet backbone routers are required to maintain complete routing information

for the Internet. Over recent years, routing tables have experienced exponential growth

as increasing numbers of organizations connect to the Internet - in December 1990 there

were 2,190 routes, in December 1992 there were 8,500 routes, and in December 1995

there were 30,000+ routes.

Figure 2: Growth of Internet Routing Tables

Unfortunately, the routing problem cannot be solved by simply installing more router

memory and increasing the size of the routing tables. Other factors related to the

capacity problem include the growing demand for CPU horsepower to compute routing

table/topology changes, the increasingly dynamic nature of WWW connections and their

effect on router forwarding caches, and the sheer volume of information that needs to be

managed by people and machines. If the number of entries in the global routing table is

allowed to increase without bounds, core routers will be forced to drop routes and

portions of the Internet will become unreachable!

The long term solution to these problems can be found in the widespread deployment of

IP Next Generation (IPng or IPv6) towards the turn of the century. However, while the

Internet community waits for IPng, IPv4 will need to be patched and modified so that

the Internet can continue to provide the universal connectivity we have come to expect.

This patching process may cause a tremendous amount of pain and may alter some of

our fundamental concepts about the Internet.

Classful IP Addressing

When IP was first standardized in September 1981, the specification required that each

system attached to an IP-based internet be assigned a unique, 32-bit Internet address

value. Some systems, such as routers which have interfaces to more than one network,

must be assigned a unique IP address for each network interface.

The first part of an Internet address identifies the network on which the host resides,

while the second part identifies the particular host on the given network. This created the

two-level addressing hierarchy which is illustrated in Figure 3.

Network-Number

Host-Number

Network-Prefix

Host-Number

Figure 3: Two-Level Internet Address Structure

In recent years, the network-number field has been referred to as the "network-prefix"

because the leading portion of each IP address identifies the network number. All hosts

on a given network share the same network-prefix but must have a unique host-number.

Similarly, any two hosts on different networks must have different network-prefixes but

may have the same host-number.

Primary Address Classes

In order to provide the flexibility required to support different size networks, the

designers decided that the IP address space should be divided into three different address

classes - Class A, Class B, and Class C. This is often referred to as "classful"

addressing because the address space is split into three predefined classes, groupings, or

categories. Each class fixes the boundary between the network-prefix and the host-

number at a different point within the 32-bit address. The formats of the fundamental

address classes are illustrated in Figure 4.

Class A

bit #

0 1

7 8

Network-

Number

Host-Number

Class B

bit #

0 2

15 16

Network-Number

Host-Number

Class C

bit #

0 3

23 24

110

Network-Number

Host-

Number

Figure 4: Principle Classful IP Address Formats

One of the fundamental features of classful IP addressing is that each address contains a

self-encoding key that identifies the dividing point between the network-prefix and the

host-number. For example, if the first two bits of an IP address are 1-0, the dividing

point falls between the 15th and 16th bits. This simplified the routing system during the

early years of the Internet because the original routing protocols did not supply a

"deciphering key" or "mask" with each route to identify the length of the network-prefix.

Class A Networks (/8 Prefixes)

Each Class A network address has an 8-bit network-prefix with the highest order bit set

to 0 and a seven-bit network number, followed by a 24-bit host-number. Today, it is no

longer considered 'modern' to refer to a Class A network. Class A networks are now

referred to as "/8s" (pronounced "slash eight" or just "eights") since they have an 8-bit

network-prefix.

A maximum of 126 (2 7 -2) /8 networks can be defined. The calculation requires that the

2 is subtracted because the /8 network 0.0.0.0 is reserved for use as the default route and

the /8 network 127.0.0.0 (also written 127/8 or 127.0.0.0/8) has been reserved for the

"loopback" function. Each /8 supports a maximum of 16,777,214 (2 24 -2) hosts per

network. The host calculation requires that 2 is subtracted because the all-0s ("this

network") and all-1s ("broadcast") host-numbers may not be assigned to individual

hosts.

Since the /8 address block contains 2 31 (2,147,483,648 ) individual addresses and the

IPv4 address space contains a maximum of 2 32 (4,294,967,296) addresses, the /8

address space is 50% of the total IPv4 unicast address space.

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