Lecture11-Internetworking

8/18/2019 Lecture11-Internetworking

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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 “naturalmask” – mask created by the definition ofthe network

class A natural mask 255.0.0.0class B natural mask 255.255.0.0

class C natural mask 255.255.255.0

►By using masks, networks can be dividedinto subnetworksextends the network portion of the address

into host portion

increases 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 subnets

without 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 wasdesigned as a remedy for

class B exhaustion

routing table explosion• as more networks get connected -- more memory

is needed for storing routing tables

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

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

CIDR

►Classless addressesmain observation: many organizations need

more than a class C network but does not

have enough hosts to efficiently utilize a classB

idea: give such organizations multiple class Caddresses

in the CIDR strategy, the class C addressesare contiguous and share the same “mostsignificant bits” -- the same prefixes

if the routing protocols can route based onthese 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 prefixboundary 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-capableare called “classless” – traditional routing

“classfull” routing


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Route Aggregation in CIDR Route Aggregation in CIDR

► Aggregation may not work always

customers having IP addresses that do not

belong to their provider’s range

some 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 whentrying 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 keptaddresses of the previous provider

►Renumbering should be done – if not

provider cannot aggregate as efficiently – holeis punched in the address space

new provider cannot aggregate the addresseither


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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 (proxyaggregation) can be tricky

unless aggregating party is a superset

or parties are in total agreement





► ISP2 sends an aggregate summarizes Jamesnet

and 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




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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 multihomed

organizations 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


taken from Different Providers

Lecture11-Internetworking

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