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Internet Address Depletion and CIDR

Introduction

► A subnet is a subset of class A, B, or C networks

► IP addresses are formed of a network and host portions – network mask used to separate the information

Introduction► Each class of address has its own “natural

mask” – mask created by the definition of the network

class A natural mask 255.0.0.0class B natural mask 255.255.0.0class C natural mask 255.255.255.0

► By using masks, networks can be divided into subnetworks

extends the network portion of the address into host portionincreases the number of subnetworks and reduces the number of hosts

Introduction

► Mask of 255.255.0.0 is applied to network 10.0.0.0

divides the IP address 10.0.0.1 into a network portion of 10, subnet portion of 0, host portion of 0.1

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Variable Length Subnet Mask

► VLSM allows a network to be be configured with different masks

adds more flexibility in dividing the network into multiple subnetswithout VLSM a mask may have too few subnets or hosts

► Suppose we want to split 192.214.11.0 (class C) into three subnets with 100 hosts in one subnet and 50 hosts in each remaining subnet

Variable Length Subnet Mask

CIDR

► Classless Inter-Domain Routing was designed as a remedy for

class B exhaustionrouting table explosion

• as more networks get connected -- more memory is needed for storing routing tables

• most high performance routers “cache” portions of routing tables at the interface board themselves --to speedup forwarding

• some extreme designs had fast memories that were in stand-alone mode at the interface boards

CIDR► Classless addresses

main observation: many organizations need more than a class C network but does not have enough hosts to efficiently utilize a class Bidea: give such organizations multiple class C addressesin the CIDR strategy, the class C addresses are contiguous and share the same “most significant bits” -- the same prefixesif the routing protocols can route based on these prefixes, they need only one block of network numbers

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CIDR

by allocating addresses intelligently -- we can group numbers by region

► In CIDR, an IP network is represented by a prefix

IP address + some indication of the left-most contiguous significant bits within this address

► A network is called “supernet” when prefix boundary contains fewer bits than the networks natural mask

CIDR

► CIDR notation enables lumping of specific routes into aggregates

► Aggregate denotes any summary route► Supernet denotes a summary route with

shorter prefix length than the natural mask

CIDR CIDR

► Networks that are subset of an aggregate or a CIDR block are called “more specific”

► Routing domains that are CIDR-capable are called “classless” – traditional routing “classfull” routing

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Route Aggregation in CIDR Route Aggregation in CIDR

► Aggregation may not work alwayscustomers having IP addresses that do not belong to their provider’s rangesome customers (ISPs) need to connect to multiple providers at the same time

► A router with 198.32.1.0/24 and 198.32.0.0/16 will match 198.32.1.0 when trying to deliver traffic to 198.32.1.1

Longest Prefix Match

► Destinations connected to multiple domains must be explicitly announced – in most specific forms

Single Homing: Address Outside Provider’s Address Space

► Customer connected to single provider► IP address space different from provider’s► Customer changed providers and kept

addresses of the previous provider► Renumbering should be done – if not

provider cannot aggregate as efficiently – hole is punched in the address spacenew provider cannot aggregate the address either

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Multihoming Scenerio: Addresses taken from one provider

► Customers are connected to multiple providers – small enough to take addresses only from one

► Aggregate advertisement can lead to black holes

► Aggregating someone else’s routes (proxy aggregation) can be tricky

unless aggregating party is a supersetor parties are in total agreement

Multihoming Scenerio: Addresses taken from one provider

Multihoming Scenerio: Addresses taken from one provider

► ISP2 sends an aggregate summarizes Jamesnetand Lindanet into one update 198.24.0.0/18

► Stubnet which is a customer for ISP1 has an address space falling in 198.24.0.0/18

► Traffic for Stubnet 198.24.16.0/21 will perform longest match and endup in ISP2

► Solution:ISP2 should specifically list each of the IP ranges that it has in common with ISP1 on top of its own address space 198.32.0.0/13

Multihoming Scenerio: Addresses taken from one provider

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Multihoming Scenerio: Addresses taken from one provider

Multihoming Scenerio: Addresses taken from Different Providers

► Large domains can take addresses from different providers

► Each provider aggregates its own address space without listing specific ranges from other provider

drawback – backup routes to multihomedorganizations not maintained – redundancy is one of the reasons for multi-homing!traffic using the addresses taken from provider will be unable to reach the destination if the provider is down – even if the destination is reachable via “other”provider

Multihoming Scenerio: Addresses taken from Different Providers