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Subnet & Classless Address Extensions

Subnet & Classless Address Extensions. Linda Wu (CMPT 471 • 2003-3). Content. Motivation Transparent routers Proxy ARP Subnet addressing Classless addressing Reference: chapter 10. Motivation. Problem: network growth will exhaust IPv4 address space eventually

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Subnet & Classless Address Extensions

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

  2. Content • Motivation • Transparent routers • Proxy ARP • Subnet addressing • Classless addressing Reference: chapter 10 CMPT 471  2003-3

  3. Motivation • Problem: network growth will exhaust IPv4 address space eventually • Solution: minimize the number of addresses used • 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

  4. Transparent Routers • 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

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

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

  7. Proxy ARP (cont.) • How proxy ARP works? • 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 • 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

  8. Proxy ARP (cont.) • Advantages • 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 • Disadvantages • 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

  9. 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 CMPT 471  2003-3

  10. 141.14.5.1 141.14.5.2 141.14.5.3 141.14.64.1 141.14.64.5 141.14.64.7 Subnet: 141.14.5.0 Subnet: 141.14.64.0 R1 R2 141.14.128.1 141.14.128.2 netid: 141.14.0.0 Class B Subnet: 141.14.128.0 To the rest of the internet Subnet Addressing (cont.) • Subnetting Example CMPT 471  2003-3

  11. Subnet Addressing (cont.) • Subnetting address Hierarchy addressing: netid subnetid hostid CMPT 471  2003-3

  12. mask Bitwise AND Network address IP address Subnet Addressing (cont.) • Mask • 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 11111111 00000000 00000000 IP & mask = 123.24.0.0 (network address) CMPT 471  2003-3

  13. Subnet Addressing (cont.) • Default masks Masks for class A, B, C addresses • 1s: preserve the netid • 0s: set the hostid to 0 • Number of 1s is predetermined: 8/16/24 CMPT 471  2003-3

  14. Subnet Addressing (cont.) • Subnet mask • Change some of the leftmost 0s in the default mask to 1s to make a subnet mask • Preserve netid and subnetid, set hostid to 0 • Contiguous subnet mask (recommended) 11111111 11111111 11000000 00000000 • Noncontiguous subnet mask 11111111 11111111 00110000 001000000 CMPT 471  2003-3

  15. 72  010 01000 224  111 00000 010 00000 (64) Subnet Addressing (cont.) Subnet mask 255.255.224.0 141.14.72.24 IP address Bitwise AND 141.14.64.0 Network address CMPT 471  2003-3

  16. 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. • # of 1s in the default mask = 24 (class C) • 6 subnets < 23: need 3 more 1s in the subnet mask • Total # of 1s in the subnet mask: 24 + 3 = 27 • Total # of 0s in the subnet mask: 8 – 3 = 5 (hostid bits) • Mask is: 11111111 11111111 11111111 11100000, or, 255.255.255.224 • # of hosts per subnet: 25 = 32 CMPT 471  2003-3

  17. Subnet Addressing (cont.) • Subnet address ranges CMPT 471  2003-3

  18. Subnet Addressing (cont.) • Fixed-length subnetting • All 1s or all 0s subnet is not recommended • All 1s and all 0s host addresses are reserved • Variable-length subnetting • 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

  19. 62 hosts 62 hosts First mask (26 1s) 255.255.255.192 62 hosts Second mask (27 1s) 255.255.255.224 30 hosts 30 hosts Subnet Addressing (cont.) • Variable-length subnetting example A class C site has 5 subnets with host numbers: 60, 60, 60, 30, 30 • 2 bits in subnetid? No, only 4 subnets. • 3 bits in subnetid? No, at most 32 hosts per subnets. router CMPT 471  2003-3

  20. Subnet Addressing (cont.) • Subnet broadcasting • Subnet broadcast address • hostid is all 1s • 3-tuple form: {netid, subnetid, -1}, “-1” means “all 1s”. • {netid, -1, -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 CMPT 471  2003-3

  21. 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 • 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

  22. Classless Addressing (cont.) • Address block assigning • 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 CMPT 471  2003-3

  23. Classless Addressing (cont.) • 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 CMPT 471  2003-3

  24. Classless Addressing (cont.) • Blocks defining in different addressing schemes Block: [first address, last address] • 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) CMPT 471  2003-3

  25. Classless Addressing (cont.) • Supernet mask • The reverse of a subnet mask • Has less 1s than the default mask for this class Subnetting Supernetting CMPT 471  2003-3

  26. Classless Addressing (cont.) • Supernet mask examples • A supernet is made out of 16 class C blocks, what is its supernet mask? Block #: 16 = 24 Change the last 4 1s in the default mask (class C) to 0s to get the supernet mask: 11111111 11111111 11111111 00000000 11111111 11111111 11110000 00000000 CMPT 471  2003-3

  27. Classless Addressing (cont.) • A supernet with mask 255.255.248.0 includes an address 205.16.37.44, what is the address range? • First address 205.16.37.44 AND 255.255.248.0 = 205.16.32.0 (11001101 00010000 00100000 00000000) • Mask 11111111 11111111 11111000 00000000, 1s: 21, 0s: 11 • Last address: 205.16.39.255 (11001101 00010000 00100111 11111111) CMPT 471  2003-3

  28. Classless Addressing (cont.) • Slash notation (CIDR notation): A.B.C.D/n • 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 1s 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

  29. Classless Addressing (cont.) • Relationship between mask and prefix length Class A: a.b.c.d/8 Class B: a.b.c.d/16 Class C: a.b.c.d/24 CMPT 471  2003-3

  30. Classless Addressing (cont.) • Subnetting with classless addressing • 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 1s in the mask) CMPT 471  2003-3

  31. 130.34.12.64/28 R1 130.34.12.112/28 130.34.12.80/28 R2 130.34.12.96/28 R3 Site: 130.34.12.64/26 Classless Addressing (cont.) • Subnet address ranges • 1st: 130.34.12.64/28 ~ 130.34.12.79/28 • 2nd: 130.34.12.80/28 ~ 130.34.12.95/28 • 3rd: 130.34.12.96/28 ~ 130.34.12.111/28 • 4th: 130.34.12.112/28 ~ 130.34.12.127/28 CMPT 471  2003-3

  32. Classless Addressing (cont.) • Reserved CIDR blocks • 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

  33. Classless Addressing (cont.) • Reserved CIDR blocks: list CMPT 471  2003-3

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