W H I T E PA P E R Understanding IP Addressing Everything You Ever Wanted To Know Understanding IP Addressing Everything You Ever Wanted To Know CONTENTS Internet Scaling Problems 1 Classful IP Addressing 3 Subnetting 7 Variable Length Subnet Masks VLSM 18 Classless Inter Domain Routing CIDR 31 New Solutions for Scaling the Internet Address Space 39 IPv6 Resolves IPv4 Issues 42 Additional IPv6 Features 49 Keeping Current on Internet Addressing Issues 50 Appendix A References 52 Appendix B Classful IP Addressing 55 Appendix C Subnetting Exercises 57 Appendix D VLSM Exercise 61 Appendix E CIDR Exercises 66 III Understanding IP Addressing Everything You Ever Wanted To Know The Internet continues to grow at a phenomenal rate 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 ways of doing business It is clear that expanding business and public awareness will continue to increase demand for access to resources on the Internet 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 IP version 4 IPv4 address space The need to route traffic between the ever increasing number of net works that comprise the Internet The first problem is concerned with the eventual depletion of the IP address space IPv4 defines a 32 bit address which means that there are only 232 4 294 967 296 IPv4 addresses available As the Internet continues to grow this 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 Internet Engineering Task Force 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 More than half of all possible IPv4 addresses have been assigned to ISPs corporations and government agencies but only an estimated 69 million addresses are actually in use F I G U R E 1 N e t w o r k N u m b e r G ro w t h 1 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 1995 there were more than 30 000 routes and in December 2000 more than 100 000 routes F I G U R E 2 G ro w t h o f I n t e r n e t R o u t i n g Ta b l e s 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 Currently IPv6 is being tested and implemented on the 6Bone network which is an informal collaborative project covering North America Europe and Japan 6Bone supports the routing of IPv6 packets since that function has not yet been integrated into many production routers Until IPv6 can be deployed worldwide IPv4 patches will need to be used and modified to continue to provide the universal connectivity users have come to expect U N D E R S TA N D I N G I P A D D R E S S I N G 2 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 Systems that have interfaces to more than one network require 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 creates the two level addressing hierarchy that is illustrated in Figure 3 F I G U R E 3 Tw o L e v e l I n t e r n e t A d d re s s S t r u c t u re 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 To provide the flexibility required to support networks of varying sizes the Internet designers decided that the IP address space should be divided into three address classes Class A Class B and Class C This is often referred to as classful addressing 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 F I G U R E 4 P r i n c i p l e C l a s s f u l I P A d d re s s F o r m a t s 3 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