Chapter 6 VLSM and CIDR Routing Protocols and Concepts 2 Topics  Classful and Classless Addressing  Classful IP Addressing  Classful Routing Protocols  Classless IP Addressing  Classless Routing Protocols  VLSM  VLSM in Action  VLSM and IP Addresses  CIDR  Route Summarization  Calculating Route Summarization Classful and Classless Addressing  Classful IP Addressing  Classful Routing Protocols  Classless IP Addressing  Classless Routing Protocols 4 Classful and Classless Routing Protocols  Routing protocols:  classful or classless.  This is a result of the evolution from classful to classless IPv4 addressing.  As networks began to use classless addressing, classless routing protocols had to be modified or developed to include the subnet mask in the routing update. 5 Classful IP Addressing  When the ARPANET was commissioned in 1969, no one anticipated that the Internet would explode out of the humble beginnings of this research project.  Over the next decade, the number of hosts on the Internet grew exponentially, from 159,000 in October 1989 to over 72 million by the end of the millennium.  As of January 2007, there were over 433 million hosts on the Internet.  Without the introduction of VLSM and CIDR notation in 1993 (RFC 1519), Network Address Translation (NAT) in 1994 (RFC 1631), and private addressing in 1996 (RFC 1918), the IPv4 32-bit address space would now be exhausted. 6 High-Order Bits  In the original specification of IPv4 (RFC 791), released in 1981, the authors established the classes to provide three different sizes of networks for large, medium, and small organizations.  As a result, Class A, B, and C addresses were defined with a specific format for the high-order bits. 7 IPv4 Classful Addressing Structure  RFC 790 (released with RFC 791).  Subnet mask for a network is determined based on its class.  Only choices were networks with very large number of hosts, large number of hosts, or few number of hosts.  No medium sized networks  Only these three choices. 16,384 8 Classful Routing Protocol  Routing protocols, such as RIPv1, only needed to propagate the network address, not the subnet mask.  Subnet mask of a network address could be determined by the value of the first octet (or more accurately, the first 3 bits of the address).  The subnet mask was directly related to the network address. 9 Classful Routing Protocol R2 applies s0/0/0’s /24 subnet mask (same major network) R3 applies the default /16 subnet mask (different major network) R1 sends a subnet address out s0/0/0 (same major network) R2 sends a summarized route out s0/0/1 (different major network) 10 Moving Toward Classless Addressing  By 1992, members of the IETF had serious concerns about the exponential growth of the Internet and the limited scalability of Internet routing tables.  They were also concerned with the eventual exhaustion of 32-bit IPv4 address space.  1993, IETF introduced classless interdomain routing (CIDR) (RFC 1517). CIDR allowed the following:  More efficient use of IPv4 address space  Prefix aggregation, which reduced the size of routing tables [...]... octet 16 Classless Routing Protocol  Classless routing protocols include the subnet mask with the network address in their routing updates 17 Classless Routing Protocol  With a classless routing protocol:  Networks 172. 16. 0.0/ 16, 172.17.0.0/ 16, 172.18.0.0/ 16, and 172.19.0.0/ 16 can be summarized as 172. 16. 0.0/14, known as a supernet  The /14 (255.252.0.0) subnet mask is included in the routing update... mask of / 16, which gives the potential of 2 56 subnets: 10.0.0.0/ 16 10.1.0.0/ 16 10.2.0.0/ 16 20 10.255.0.0/ 16 VLSM  Any of these / 16 subnets can be subnetted further  For example the 10.1.0.0/ 16 subnet is subnetted again using the /24 mask 21 VLSM  The 10.2.0.0/ 16 subnet is also subnetted again with a /24 mask  The 10.3.0.0/ 16 subnet is subnetted again with the /28 mask  The 10.4.0.0/ 16 subnet... mask CIDR and Route Summarization  The 192. 168 .0.0/20, summarized or aggregated route includes all the networks belonging to customers A, B, C, and D  192. 168 .0.0/23, 192. 168 .2.0/23, 192. 168 .4.0/22, and 192. 168 .8.0/21 are all 15 subnets of 192. 168 .0.0/20 CIDR and Route Summarization Although the network address is the same; one is a subnet of the other The subnet mask is required in the routing update... addresses 2,097,152 host addresses 1,048,5 76 host addresses 524,288 host addresses 262 ,144 host addresses 131,072 host addresses 65 ,5 36 host addresses 32, 768 host addresses 16, 384 host addresses 8,192 host addresses 4,0 96 host addresses 2,048 host addresses 1,024 host addresses 512 host addresses 2 56 host addresses 128 host addresses 64 host addresses 32 host addresses 16 host addresses 8 host addresses 4... addresses 2,097,152 host addresses 1,048,5 76 host addresses 524,288 host addresses 262 ,144 host addresses 131,072 host addresses 65 ,5 36 host addresses 32, 768 host addresses 16, 384 host addresses 8,192 host addresses 4,0 96 host addresses 2,048 host addresses 1,024 host addresses 512 host addresses 2 56 host addresses 128 host addresses 64 host addresses 32 host addresses 16 host addresses 8 host addresses 4... routing update  With a classful routing protocol:  If R2 sends the 172. 16. 0.0 summary route without the /14 mask, R3 only knows to apply the default classful mask of / 16  Classful routing protocols cannot send supernet routes because the receiving router will apply the default classful mask to the network address in the routing update 18 VLSM  VLSM in Action  VLSM and IP Addresses VLSM  The network... by the routing table to know whether or not the destination IP address of the packet is a match with the route in the routing table A classless routing protocol includes the subnet mask with the network address in the routing update  Propagating VLSM and supernet routes requires a classless routing protocol, because the subnet mask can no longer be determined by the value of the first octet 16 Classless... that was subnetted YES! 10.2.1.55/24 10.2.0.55/ 16 NO! All other / 16 subnets are still available for use as / 16 networks or to be subnetted 31 VLSM 1 32 VLSM 1 255.255.255.240 or /28 33 VLSM 2 /30 – Gives 4 addresses - 2 usable host addresses 34 VLSM 2 – Possible / 30 options Conflicts Existing /27 Networks /30 Choices 128 64 32 16 8 4 2 1 64 0 1 0 0 0 0 0 0 96 0 1 1 0 0 0 0 0 128 1 0 0 0 0 0 0 0 ... you can go  You can choose only one mask for all your networks  With VLSM and classless routing, you have more flexibility to create 24 additional network addresses and use a mask that fits your needs VLSM: A different way to look at it  For subnet 10.1.0.0/ 16 (see Figure 6- 10), 8 more bits are borrowed again, to create 2 56 subnets with a /24 mask  This mask will allow 254 host addresses per subnet... (255.254.0.0) / 16 (255.255.0.0) /17 (255.255.128.0) /18 (255.255.192.0) /19 (255.255.224.0) /20 (255.255.240.0) /21 (255.255.248.0) /22 (255.255.252.0) /23 (255.255.254.0) /24 (255.255.255.0) /25 (255.255.255.128) / 26 (255.255.255.192) /27 (255.255.255.224) /28 (255.255.255.240) /29 (255.255.255.248) /30 (255.255.255.252) /31 (255.255.255.254) /32 (255.255.255.255) 16, 777,2 16 host addresses 8,388 ,60 8 host . Chapter 6 VLSM and CIDR Routing Protocols and Concepts 2 Topics  Classful and Classless Addressing  Classful IP Addressing  Classful Routing Protocols  Classless. Addressing  Classful Routing Protocols  Classless IP Addressing  Classless Routing Protocols 4 Classful and Classless Routing Protocols  Routing protocols:  classful