Subnet Classless Address Extensions Linda Wu CMPT 471

Subnet & Classless Address Extensions Linda Wu (CMPT 471 • 2003 -3)

Content ¡ ¡ ¡ Motivation Transparent routers Proxy ARP Subnet addressing Classless addressing Reference: chapter 10 Notes-4 CMPT 471 2003 -3 2

Motivation Problem: network growth will exhaust IPv 4 address space eventually ¡ Solution: minimize the number of addresses used ¡ l l l Notes-4 Avoid assigning netid whenever possible Share one netid among multiple networks: transparent router, proxy ARP, subnet addressing Arbitrary assignment: classless addressing CMPT 471 2003 -3 3

Transparent Routers ¡ ¡ ¡ Notes-4 A special router T, called transparent router, connects WAN and LAN Other hosts and routers on the WAN do not know T’s existence The LAN does not have its own netid; the hosts in the LAN are assigned unused addresses in the WAN T sends packets from the WAN to the appropriate host in the LAN T accepts packets from the LAN and routes them across WAN to the destination CMPT 471 2003 -3 4

Transparent Routers (cont. ) ¡ Advantages l l ¡ Disadvantages l l Notes-4 One netid is shared by multiple networks: fewer network addresses are required Support load balancing Only work with WAN that has a large address space Do not provide all the same services as conventional routers CMPT 471 2003 -3 5

Proxy ARP ¡ ARP l l ¡ Address Resolution Protocol Maps IP addresses to physical addresses Proxy l an application that closes a straight path between 2 networks and prevents the crackers from obtaining internal addresses and details of a private network Main network A B C R Router running proxy ARP D E Hidden network Notes-4 CMPT 471 2003 -3 6

Proxy ARP (cont. ) ¡ How proxy ARP works? l l Main network and hidden network: share the same netid A router, R, connects these 2 networks R knows which hosts lie on which physical networks, and maintains the illusion that only one network exists A in the main network sends packets to E in the hidden network ¡ ¡ Notes-4 A broadcasts ARP request for E’s physical addr. R responses ARP request by sending back its own physical addr. A sends the packet destined for E to R R forwards the packet destined for E over the hidden network CMPT 471 2003 -3 7

Proxy ARP (cont. ) ¡ Advantages l l ¡ Disadvantages l l l Notes-4 One netid is shared by multiple networks Proxy ARP can be added to a single router without disturbing other hosts or routers on the network The network must use ARP for address resolution Cannot be generalized to more complex network topology Rely on managers to maintain tables of machines and addresses manually CMPT 471 2003 -3 8

Subnet Addressing Subnet addressing, subnetting ¡ A network is divided into several smaller subnets ¡ Each subnet has its own subnet address ¡ Subnets appear as a single network to the rest of the internet ¡ The router attached to the subnets knows the network is physically divided into subnets ¡ Notes-4 CMPT 471 2003 -3 9

Subnet Addressing (cont. ) ¡ Subnetting Example 141. 14. 5. 1 141. 14. 5. 2 141. 14. 5. 3 Subnet: 141. 14. 5. 0 141. 14. 64. 1 141. 14. 64. 5 141. 14. 64. 7 Subnet: 141. 14. 64. 0 R 1 netid: 141. 14. 0. 0 Class B Notes-4 141. 14. 128. 1 141. 14. 128. 2 R 2 Subnet: 141. 14. 128. 0 CMPT 471 2003 -3 To the rest of the internet 10

Subnet Addressing (cont. ) ¡ 141 Subnetting address • 14 • 191 netid • 10 Hierarchy addressing: hostid netid Without subnetting 141 • netid 14 • 192 • subnetid 192 hostid With subnetting ( 405 ) 864 Area code Exchange - 8902 Connection Hierarchy in telephone number Notes-4 CMPT 471 2003 -3 11

Subnet Addressing (cont. ) ¡ Mask l l a 32 -bit binary number that gives the network address when bitwise ANDed with an IP address e. g. IP address: 123. 24. 3. 1 (class B) Mask: 11111111 00000000 IP & mask = 123. 24. 0. 0 (network address) mask IP address Notes-4 Bitwise AND CMPT 471 2003 -3 Network address 12

Subnet Addressing (cont. ) ¡ Default masks Masks for class A, B, C addresses l 1 s: preserve the netid l 0 s: set the hostid to 0 l Number of 1 s is predetermined: 8/16/24 Class Notes-4 Binary mask Dotted-decimal mask A 1111 00000000 255. 0. 0. 0 B 11111111 00000000 255. 0. 0 C 11111111 0000 255. 0 CMPT 471 2003 -3 13

Subnet Addressing (cont. ) ¡ Subnet mask l l l Change some of the leftmost 0 s in the default mask to 1 s to make a subnet mask Preserve netid and subnetid, set hostid to 0 Contiguous subnet mask (recommended) 11111111 11000000 l Noncontiguous subnet mask 11111111 00110000 001000000 Default mask: 255. 0. 0 11111111 00000000 Subnet mask: 255. 224. 0 1111 Notes-4 1111 111 00000 CMPT 471 2003 -3 0000 14

Subnet Addressing (cont. ) Subnet mask 255. 224. 0 141. 14. 72. 24 IP address Bitwise AND 141. 14. 64. 0 Network address 72 01000 224 111 00000 010 00000 (64) Notes-4 CMPT 471 2003 -3 15

Subnet Addressing (cont. ) ¡ Subnet design example A company is granted network address 200. 16. 64. 0 (class C). It needs 6 subnets. Design the subnet. l l l Notes-4 # of 1 s in the default mask = 24 (class C) 6 subnets < 23: need 3 more 1 s in the subnet mask Total # of 1 s in the subnet mask: 24 + 3 = 27 Total # of 0 s in the subnet mask: 8 – 3 = 5 (hostid bits) Mask is: 11111111 11100000, or, 255. 224 # of hosts per subnet: 25 = 32 CMPT 471 2003 -3 16

Subnet Addressing (cont. ) l Subnet address ranges 1 st: 2 nd: 3 rd : 4 th : 5 th : 6 th : 7 th : 8 th : Notes-4 200. 16. 64. 0 200. 16. 64. 32 200. 16. 64. 96 200. 16. 64. 128 200. 16. 64. 160 200. 16. 64. 192 200. 16. 64. 224 ~ ~ ~ ~ 200. 16. 64. 31 200. 16. 64. 63 200. 16. 64. 95 200. 16. 64. 127 200. 16. 64. 159 200. 16. 64. 191 200. 16. 64. 223 200. 16. 64. 255 CMPT 471 2003 -3 17

Subnet Addressing (cont. ) ¡ Fixed-length subnetting l l ¡ Subnet bits # of subnets Hosts per subnet (class B) 0 1 (20) 65534 (216 – 2) 2 2 (22 -2) 16382 (214 – 2) 8 254 (28 -2) Variable-length subnetting l l Notes-4 All 1 s or all 0 s subnet is not recommended All 1 s and all 0 s host addresses are reserved No single subnetid partition works for all organizations An organization may select subnetid partition on a per-network basis; all hosts and routers attached to the network must follow the partition CMPT 471 2003 -3 18

Subnet Addressing (cont. ) ¡ Variable-length subnetting example A class C site has 5 subnets with host numbers: 60, 60, 30 l l 2 bits in subnetid? No, only 4 subnets. 3 bits in subnetid? No, at most 32 hosts per subnets. 62 hosts First mask (26 1 s) 62 hosts 255. 192 62 hosts router Notes-4 Second mask (27 1 s) 30 hosts 255. 224 30 hosts CMPT 471 2003 -3 19

Subnet Addressing (cont. ) ¡ Subnet broadcasting l Subnet broadcast address hostid is all 1 s ¡ 3 -tuple form: {netid, subnetid, -1}, “-1” means “all 1 s”. ¡ l {netid, -1} Means “deliver packet to all hosts with network address netid, even if they are in separate physical subnets” ¡ Operationally, such broadcasting make sense only if routers that interconnect the subnets agree to propagate the packets to all subnets ¡ Notes-4 CMPT 471 2003 -3 20

Classless Addressing ¡ ¡ ¡ Also called supernetting Combine several address blocks to create a larger address range: supernet Instead of using a single netid for multiple subnets, it allows a network’s addresses to span multiple netids l Notes-4 E. g. , an organization that needs 1000 addresses can be granted 4 class C blocks instead of 1 class B block X. Y. 32. 0 ~ X. Y. 32. 255 X. Y. 33. 0 ~ X. Y. 33. 255 X. Y. 34. 0 ~ X. Y. 34. 255 X. Y. 35. 0 ~ X. Y. 35. 255 CMPT 471 2003 -3 21

Classless Addressing (cont. ) ¡ Address block assigning l Choose address blocks randomly The routers outside of the supernet treat each block separately ¡ Each router has N entries in its routing table, N = # of blocks; therefore, increase the size of the routing table tremendously ¡ Notes-4 CMPT 471 2003 -3 22

Classless Addressing (cont. ) l Choose address blocks based on a set of rules so that each router has only one entry in the routing table: required by CIDR (Classless Inter-Domain Routing) # of blocks is a power of 2 (1, 2, 4, 8 …) ¡ The size of each block is a power of 2 ¡ The blocks are contiguous in the address space (no gaps between the blocks) ¡ The size of supernet = (# of blocks) * (size of each block): a power of 2 ¡ The first address can be evenly divisible by supernet size ¡ Notes-4 CMPT 471 2003 -3 23

Classless Addressing (cont. ) ¡ Blocks defining in different addressing schemes Block: [first address, last address] l Classful address one block, default mask is always known ¡ the first address only can define the block Subnetting ¡ the first address in the subblock (subnet) and subnet mask define the subblock Supernetting ¡ the first address of the supernet and supernet mask define the superblock ¡ IP address & supernet mask = first address (network address) ¡ l l Notes-4 CMPT 471 2003 -3 24

Classless Addressing (cont. ) ¡ Supernet mask l l The reverse of a subnet mask Has less 1 s than the default mask for this class Subnet mask Divide 1 network into 8 subnets 11111111 111 00000 Default mask (class C) 1111 Subnetting 11111111 00000 Supernetting Supernet mask Combine 8 networks into 1 supernet 1111 Notes-4 11111 000 CMPT 471 2003 -3 00000 25

Classless Addressing (cont. ) ¡ Supernet mask examples l A supernet is made out of 16 class C blocks, what is its supernet mask? Block #: 16 = 24 Change the last 4 1 s in the default mask (class C) to 0 s to get the supernet mask: 11111111 0000 111111110000 Notes-4 CMPT 471 2003 -3 26

Classless Addressing (cont. ) l A supernet with mask 255. 248. 0 includes an address 205. 16. 37. 44, what is the address range? ¡ ¡ ¡ Notes-4 First address 205. 16. 37. 44 AND 255. 248. 0 = 205. 16. 32. 0 (11001101 000100000 0000) Mask 11111111000 0000, 1 s: 21, 0 s: 11 Last address: 205. 16. 39. 255 (11001101 00010000 00100111 1111) CMPT 471 2003 -3 27

Classless Addressing (cont. ) ¡ Slash notation (CIDR notation): A. B. C. D/n l l l Notes-4 For identifying a CIDR block A. B. C. D: an IP address n: # of bits that are shared in every address in the block, i. e. , # of 1 s in the mask Prefix: common part of the address range (similar to netid), prefix length = n Suffix: varying part of the address range (similar to hostid), suffix length = 32 - n CMPT 471 2003 -3 28

Classless Addressing (cont. ) ¡ Relationship between mask and prefix length /n /1 /2 /3 Mask 128. 0. 0. 0 192. 0. 0. 0 224. 0. 0. 0 /n /12 /16 /24 Mask 255. 240. 0. 0 255. 0. 0 (class B) 255. 0 (class C) /8 255. 0. 0. 0 (class A) /32 255 Class A: a. b. c. d/8 Class B: a. b. c. d/16 Class C: a. b. c. d/24 Notes-4 CMPT 471 2003 -3 29

Classless Addressing (cont. ) ¡ Subnetting with classless addressing l Increase supernet prefix length (n) to define the subnet prefix length Example: an organization is granted the block 130. 34. 12. 64/26. It needs to have 4 subnets. What is the subnet address and address range for each subnet? ¡ Prefix length = 26, suffix length = 6 # of addresses in the block: 26 = 64 ¡ 4 subnets 16 addresses per subnet ¡ 4 subnets subnet prefix /28 (2 more 1 s in the mask) Notes-4 CMPT 471 2003 -3 30

Classless Addressing (cont. ) l Subnet address ranges 1 st: 130. 34. 12. 64/28 ~ 130. 34. 12. 79/28 ¡ 2 nd: 130. 34. 12. 80/28 ~ 130. 34. 12. 95/28 ¡ 3 rd: 130. 34. 12. 96/28 ~ 130. 34. 12. 111/28 ¡ 4 th: 130. 34. 12. 112/28 ~ 130. 34. 127/28 130. 34. 12. 64/28 130. 34. 12. 80/28 130. 34. 12. 96/28 R 1 R 2 R 3 130. 34. 12. 112/28 ¡ Site: 130. 34. 12. 64/26 Notes-4 CMPT 471 2003 -3 31

Classless Addressing (cont. ) ¡ Reserved CIDR blocks l l Notes-4 Private addresses, unroutable addresses Used with private networks Never assigned to networks in the global Internet Router in the global Internet knows they are reserved addresses, and can detect it if a packet destined to the reserved address accidentally reaches the Internet CMPT 471 2003 -3 32

Classless Addressing (cont. ) ¡ Notes-4 Reserved CIDR blocks: list Prefix First address Last address 10/8 10. 0 10. 255 172. 16/12 172. 16. 0. 0 172. 31. 255 192. 168/16 192. 168. 0. 0 192. 168. 255 169. 254/16 169. 254. 0. 0 169. 254. 255 CMPT 471 2003 -3 33