Introduction Internet Address Depletion and 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 255.0.0.0 class B natural mask 255.255.0.0 class 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 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 1
Variable Length Subnet Mask 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 Classless Inter-Domain Routing was designed 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 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 Classless addresses main observation: many organizations need more than a class C network but does not have enough hosts to efficiently utilize a class B idea: give such organizations multiple class C addresses in the strategy, the class C addresses are contiguous and share the same most significant bits -- the same prefixes if the routing protocols can route based on these prefixes, they need only one block of network numbers 2
by allocating addresses intelligently -- we can group numbers by region In, 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 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 Networks that are subset of an aggregate or a block are called more specific Routing domains that are -capable are called classless traditional routing classfull routing 3
Route Aggregation in Route Aggregation in 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 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 space new provider cannot aggregate the address either 4
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 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 5
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 6