Classless inter domain routing

It is best presentation for Classless inter domain routing

1. CIDR Classless Inter-Domain Routing

2. 2 Address Allocation Problem Exhaustion of the class B network address space The lack of a network class of size which is appropriate for mid-sizes organization class C, with a max of 254 hosts, too small While class B, with a max of 65534 hosts, too large Allocate block of class C instead and downside is more routes entry in routing table

3. CIDR - Classless Interdomain Routing IP backbone routers have one routing table entry for each network address: With subnetting, a backbone router only needs to know one entry for each network This is acceptable for Class A and Class B networks 2 7 = 128 Class A networks 2 14 = 16,384 Class B networks But this is not acceptable for Class C networks 2 21 = 2,097,152 Class C networks In 1993, the size of the routing tables started to outgrow the capacity of routers Consequence: The Class-based assignment of IP addresses had to be abandoned

4. CIDR - Classless Interdomain Routing Goals: Restructure IP address assignments to increase efficiency Hierarchical routing aggregation to minimize route table entries Key Concept: The length of the network id (prefix) in IP addresses is arbitrary/flexible and is defined by the network hierarchy . Consequence: Routers use the IP address and the length of the prefix for forwarding. All advertised IP addresses must include a prefix

5. CIDR Example CIDR notation of a network address: 192.0.2.0/18 "18" says that the first 18 bits are the network part of the address The network part is called the network prefix Example: Assume that a site requires an IP network domain that can support 1000 IP host addresses With CIDR, the network is assigned a continuous block of 1024 = 2 10 (>1000) addresses with a 32-10 = 22-bit long prefix

6. IPv6 vs. IPv4: Address Comparison IPv4 has a maximum of 2 32  4 billion addresses IPv6 has a maximum of 2 128 = (2 32 ) 4  4 billion x 4 billion x 4 billion x 4 billion addresses

7. Notation of IPv6 addresses Convention : The 128-bit IPv6 address is written as eight 16-bit integers (using hexadecimal digits for each integer) CEDF:BP76:3245:4464:FACE:2E50:3025:DF12 Short notation: Abbreviations of leading zeroes: CEDF:BP76:0000:0000:009E:0000:3025:DF12  CEDF:BP76:0:0:9E :0:3025:DF12 “ :0000:0000 ” can be written as “ :: ” CEDF:BP76:0:0:FACE:0:3025:DF12  CEDF:BP76::FACE:0:3025:DF12 IPv6 addresses derived from IPv4 addresses have different formats. Convention allows to use IPv4 notation for the last 32 bits. 128.143.137.144 -> 0:0:0:0:0:ffff:808F:8990 or 128.143.137.144 -> 2002:808f:8990:0:0:0:0:0 (called 6to4 address)

8. IPv6 Provider-Based Addresses The first IPv6 addresses will be allocated to a provider-based plan Type: Set to “ 010 ” for provider-based addresses Registry : identifies the agency that registered the address The following fields have a variable length (recommeded length in “ () ” ) Provider : Id of Internet access provider (16 bits) Subscriber: Id of the organization at provider (24 bits) Subnetwork : Id of subnet within organization (32 bits) Interface : identifies an interface at a node (48 bits) Registry ID Provider ID 010 Subscriber ID Interface ID Subnetwork ID

9. More on IPv6 Addresses The provider-based addresses have a similar flavor as CIDR addresses IPv6 provides address formats for: Unicast – identifies a single interface Multicast – identifies a group. Datagrams sent to a multicast address are sent to all members of the group Anycast – identifies a group. Datagrams sent to an anycast address are sent to one of the members in the group.

10. 10 Packet Forwarding

11. 11 Hop-by-Hop Packet Forwarding Each router has a forwarding table Maps destination addresses… … to outgoing interfaces Upon receiving a packet Inspect the destination IP address in the header Index into the table Determine the outgoing interface Forward the packet out that interface Then, the next router in the path repeats And the packet travels along the path to the destination

12. 12 Separate Entry Per 24-bit Prefix If the router had an entry per 24-bit prefix Look only at the top 24 bits of the destination address Index into the table to determine the next-hop interface host host host LAN 1 ... host host host LAN ... router router router WAN WAN 1.2.3.4 1.2.3.7 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.212 1.2.3.0/24 5.6.7.0/24 forwarding table

13. 13 Separate Entry Classful Address If the router had an entry per classful prefix Mixture of Class A, B, and C addresses Depends on the first couple of bits of the destination Identify the mask automatically from the address First bit of 0: class A address (/8) First two bits of 10: class B address (/16) First three bits of 110: class C address (/24) Then, look in the forwarding table for the match E.g., 1.2.3.4 maps to 1.2.3.0/24 Then, look up the entry for 1.2.3.0/24 … to identify the outgoing interface