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CIDR and VLSM

CIDR and VLSM. Erkki Kukk. Classful IP Addressing. original specification of IPv4 (RFC 791), released in 1981. Subnet Mask. Why does a host need to know what network it belongs to? So, it knows whether to encapsulate the IP packet into an Ethernet frame with:

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CIDR and VLSM

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  1. CIDR and VLSM Erkki Kukk

  2. Classful IP Addressing • original specification of IPv4 (RFC 791), released in 1981

  3. Subnet Mask • Why does a host need to know what network it belongs to? • So, it knows whether to encapsulate the IP packet into an Ethernet frame with: • The Destination MAC Address of the default gateway • Must know the default gateway’s IP address • The Destination MAC Address of the host with the Destination IP address of the packet Host: “I’m a host on the 192.168.1.0/24 network.”

  4. When the ARPANET was founded 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 • 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

  5. Classful and Classless Routing Protocols • Routing protocols: • classful or classless. • 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.

  6. Classful IP Addressing • Classful Routing Updates -classful routing protocols (i.e. RIPv1) do not sendsubnet masks in their routing updates The reason is that the Subnet mask isdirectly related to the network address

  7. 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

  8. CIDR • Classless interdomain routing (CIDR) is a prefix-based standard for the interpretation of IP addresses. • CIDR allows routing protocols to summarize multiple networks, a block of addresses, as a single route. • With CIDR, IP addresses and their subnet masks are written as four octets, separated by periods, and followed by a forward slash and a number that represents the subnet mask (slash notation). • An example is 172.16.1.0/24.

  9. ISPs no longer restricted to three classes. Can now allocate a large range of network addresses based on customer requirements 11111111.00000000.00000000.00000000 /8 (255.0.0.0) 16,777,216 host addresses 11111111.10000000.00000000.00000000 /9 (255.128.0.0) 8,388,608 host addresses 11111111.11000000.00000000.00000000 /10 (255.192.0.0) 4,194,304 host addresses 11111111.11100000.00000000.00000000 /11 (255.224.0.0) 2,097,152 host addresses 11111111.11110000.00000000.00000000 /12 (255.240.0.0) 1,048,576 host addresses 11111111.11111000.00000000.00000000 /13 (255.248.0.0) 524,288 host addresses 11111111.11111100.00000000.00000000 /14 (255.252.0.0) 262,144 host addresses 11111111.11111110.00000000.00000000 /15 (255.254.0.0) 131,072 host addresses 11111111.11111111.00000000.00000000 /16 (255.255.0.0) 65,536 host addresses 11111111.11111111.10000000.00000000 /17 (255.255.128.0) 32,768 host addresses 11111111.11111111.11000000.00000000 /18 (255.255.192.0) 16,384 host addresses 11111111.11111111.11100000.00000000 /19 (255.255.224.0) 8,192 host addresses 11111111.11111111.11110000.00000000 /20 (255.255.240.0) 4,096 host addresses 11111111.11111111.11111000.00000000 /21 (255.255.248.0) 2,048 host addresses 11111111.11111111.11111100.00000000 /22 (255.255.252.0) 1,024 host addresses 11111111.11111111.11111110.00000000 /23 (255.255.254.0) 512 host addresses 11111111.11111111.11111111.00000000 /24 (255.255.255.0) 256 host addresses 11111111.11111111.11111111.10000000 /25 (255.255.255.128) 128 host addresses 11111111.11111111.11111111.11000000 /26 (255.255.255.192) 64 host addresses 11111111.11111111.11111111.11100000 /27 (255.255.255.224) 32 host addresses 11111111.11111111.11111111.11110000 /28 (255.255.255.240) 16 host addresses 11111111.11111111.11111111.11111000 /29 (255.255.255.248) 8 host addresses 11111111.11111111.11111111.11111100 /30 (255.255.255.252) 4 host addresses 11111111.11111111.11111111.11111110 /31 (255.255.255.254) 2 host addresses 11111111.11111111.11111111.11111111 /32 (255.255.255.255) “Host Route”

  10. Classless RoutingProtocol • With a classless routing protocol: • The /14 (255.252.0.0) subnet mask is included in the 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.

  11. Classful and Classless IP Addressing • Classless Routing Protocol

  12. Subnetting and Subnet Masks Formalized in 1985, the subnet mask breaks a single network in to smaller pieces. • By subnetting we borrow bits from the host part • Allows network administrators to divide their network into small networks or subnets

  13. Calculating the number of subnets/hosts needed • Network 172.16.1.0/24 • Need: • As many subnets as possible, 60 hosts per subnet 172.16.1.0 255.255.255.0 Host Network

  14. Calculating the number of subnets/hosts needed • Network 172.16.1.0/24 • Need: • As many subnets as possible, 60 hosts per subnet • New Subnet Mask: 255.255.255.192 (/26) • Number of Hosts per subnet: 6 bits, 64-2 hosts, 62 hosts • Number of Subnets: 2 bits or 4 subnets Number of subnets 172.16.1. 0 0 0 0 0 0 0 0 255.255.255. 1 1 0 0 0 0 0 0 255.255.255.192 6 host bits Network Host

  15. Calculating the number of subnets/hosts needed 172.16.1.0 network 172.16.1.64 network 172.16.1.128 network 172.16.1.192 network

  16. Calculating the number of subnets/hosts needed • Network 172.16.1.0/24 • Need: • As many subnets as possible, 12 hosts per subnet 172.16.1.0 255.255.255.0 Network Host

  17. VLSM VLSM in Action VLSM and IP Addresses

  18. VLSM • VLSM – the process of sub-netting a subnet to fit your needs -Example: Subnet 10.1.0.0/16, 8 more bits are borrowed again, to create 256 subnets with a /24 mask. -Mask allows for 254 host addresses per subnet -Subnets range from: 10.1.0.0 / 24 to 10.1.255.0 / 24

  19. VLSM • The network 10.0.0.0/8 has been subnetted using the subnet mask of /16, which gives the potential of 256 subnets: • 10.0.0.0/16 • 10.1.0.0/16 • 10.2.0.0/16 • . • . • 10.255.0.0/16

  20. 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 is subnetted again with the /20 mask.

  22. VLSM 1

  23. VLSM 1 255.255.255.240 or /28

  24. VLSM 2

  25. VLSM 2

  26. 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.

  27. Route Summarization • Steps to calculate a route summary -List networks in binary format -Count number of left most matching bits to determine summary route’s mask -Copy the matching bits and add zero bits to determine the summarized network address

  28. Route Summarization (Example 2) -Routes are summarized with masks that are lessthan that of the default classful mask -172.16.0.0/13is the summarizedroute for the 172.16.0.0/16 to 172.23.0.0 /16 classful networks

  29. Lab exercises Subnetting scenario 1

  30. SubnettingScenario 1 You have been given the network address 192.168.9.0/24 to subnet and The network has the following addressing requirements: • The BRANCH1 LAN 1 will require 10 host IP addresses. • The BRANCH1 LAN 2 will require 10 host IP addresses. • The BRANCH2 LAN 1 will require 10 host IP addresses. • The BRANCH2 LAN 2 will require 10 host IP addresses. • The HQ LAN will require 20 host IP addresses. • The link from HQ to BRANCH1 will require an IP address for each end of the link. • The link from HQ to BRANCH2 will require an IP address for each end of the link.

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