Classless and Subnet Address Extensions (CIDR)

1. Classless and Subnet Address Extensions (CIDR) • Topics: • There are problems with the IP addressing scheme we’ve studied • We’ll study some ways to get around these problems

2. Review: IP Addresses

3. Problems with IP Addresses • The designers of IP addresses did not foresee the Internet’s tremendous growth • Higher overhead to manage network addresses • Larger routing tables • IP addresses might one day be exhausted

4. Solution to IP Addresses Problems • The same IP network prefix can be shared by multiple physical networks • A site can choose to assign and use IP addresses in unusual ways internally as long as: • All hosts and routers at the site honor the site’s addressing scheme • The site’s addressing scheme is transparent to other sites on the internet

5. H1 H2 T 10.0.0.0 H3 H4 Strategy 1: Transparent Routers • A network with a class A IP address can be extended:

6. Transparent Routers (cont) • Hosts on LAN are assigned IP addresses as if they were on WAN • LAN does not need its own network prefix • Traffic for hosts on LAN is multiplexed through T • Other hosts and routers on the WAN do not know T exists

7. Transparent Routers • Advantages • Require fewer network addresses (LAN doesn’t need a separate network prefix) • Load balancing • Disadvantages • Require a large address space • Do not provide all the services of standard routers

8. H1 H2 R H3 H4 Strategy 2: Proxy ARP • Using ARP, map a single network prefix into two physical addresses Main network Router running proxy ARP H5 H6 Hidden network

9. Proxy ARP (cont) • Gives the illusion that all hosts are on the same physical network • Router R answers ARP requests on each network for hosts on the other • R answers ARPs with its own hardware address (it lies) • When R receives a datagram it forwards it to the correct physical address

10. Proxy ARP • Advantages • Require fewer network addresses • Only the router running proxy ARP needs to know what’s going on • Disadvantages • Can only be used if the network uses ARP for address resolution • Allows spoofing

11. H1 H2 H3 H4 R Strategy 3: Subnet Addressing • Hierarchical addressing Network 128.10.1.0 Rest of the internet 128.10.1.1 128.10.1.2 Network 128.10.2.0 All traffic to 128.10.0.0 128.10.2.1 128.10.2.2

12. Subnet Addressing (cont) • R receives all traffic for network 128.10.0.0 • R routes the datagram to a physical network based on bits in the hostid field of the IP address • Another level has been added to the addressing hierarchy

13. 0 8 16 24 31 0 8 16 24 31 1 0 netid hostid 1 0 netid subnet hostid Subnet Addressing (cont) • Regular (Class B) IP address: • New interpretation (locally only):

14. Subnet Addressing (cont) • Advantages • Minimizes network address usage • Accommodates growth • Disadvantages • Added layer of complexity • Difficult to change once hierarchy is established

15. 0 8 16 24 31 0 8 16 19 31 1 0 netid subnet hostid 1 0 netid sub hostid Subnet Addressing (cont) • Flexible Allows 256 physical networks with 256 hosts each Allows 8 physical networks with 8192 hosts each

16. 0 8 16 24 31 1 0 netid subnet hostid Subnet Masks • 32 bits • 1 if the bit is part of the network address • 0 if the bit is part of the host address • Example - a class B network: • Subnet mask: • 11111111 11111111 11111111 00000000

17. 0 8 16 24 31 1 0 netid Subnet Masks • Subnet bits do not have to be contiguous: • Mask = 11111111 11111111 00001010 10000000 = subnet id = host id

18. 0 8 16 24 31 1 0 netid subnet hostid Representing Subnet Masks in Dotted Decimal Notation • Example - a class B network: • Subnet mask: • 11111111 11111111 11111111 00000000 • Dotted Decimal: • 255.255.255.0

19. Representing Subnet Masks in 3-tuple Notation • Subnet mask: • 11111111 11111111 11111111 00000000 • 3-tuple notation • {<netid>,<subnet id>,<hostid>} • -1 means “all ones” • {-1,-1,0}

20. R2 R1 H Routing in the Presence of Subnets • All hosts and routers must use a subnet routing algorithm Net 1 (not a subnet address) Net 2 (subnet of address N) Net 3 (subnet of address N)

21. The Subnet Routing Algorithm • Recall the standard routing table: • (netid, next hop) • N = netid portion of IP address • Compare N with netid • Match = send datagram to next hop • Routing when subnets are in use: • (subnet mask, netid, next hop) • N = IP address & subnet mask • Compare N with netid • Match = send datagram to next hop

22. Using Subnet Masks for Routing • Host-specific routes • (20.0.0.3, 30.0.0.7) • (255.255.255.255 , 20.0.0.3 , 30.0.0.7) • Default routes • (default, 40.0.0.8) • (0.0.0.0 , 0.0.0.0 , 40.0.0.8) • Standard, non-subnet class B network • (128.0.0.0, 10.0.0.3) • (255.255.0.0 , 128.0.0.0 , 10.0.0.3)

23. A Unified Routing Algorithm Extract the destination IP address, D, from the datagram and compute the netid, N If N matches any directly connected network address deliver the datagram directly over that network else for each entry (M,N,NH) in the routing table { I = M&D if (I == N) then send datagram to NH } if no matches were found declare a routing error

24. Broadcasting to Subnets • IP address = 128.0.255.255 • Broadcast to all hosts on network 128 • What if network 128 has subnets? • Routers that interconnect the subnets must propagate the datagram to all physical networks • But the routers must take care not to route the datagrams in loops (reverse path forwarding) • Can you broadcast to just one subnet? • Yes: {network, subnet, -1}

25. Summary • Problem: IP v4 addresses (especially class B) would be exhausted • Solutions: • Subnet addressing - conserve network addresses by using the same network address for multiple physical networks • New version of IP (v6) with larger addresses • Supernet addressing - conserve class B network addresses by allowing a single organization to use multiple class C network addresses