Lecture Slide Rizwan Rehman, CCS

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

Review: IP Addresses

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

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

Strategy 1: Transparent Routers A network with a class A IP address can be extended: T H1 H2 H3 H

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

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

Strategy 2: Proxy ARP Using ARP, map a single network prefix into two physical addresses R Router running proxy ARP Main network Hidden network H1H2H3 H4H5H6

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

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

Strategy 3: Subnet Addressing Hierarchical addressing R H1 Rest of the internet All traffic to Network Network H2 H3H

Subnet Addressing (cont) R receives all traffic for network 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

Subnet Addressing (cont) Regular (Class B) IP address: New interpretation (locally only): netid hostid netid subnet hostid

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

Subnet Addressing (cont) Flexible Allows 256 physical networks with 256 hosts each Allows 8 physical networks with 8192 hosts each netid subnet hostid netid sub 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: – netid subnet hostid

Subnet Masks Subnet bits do not have to be contiguous: –Mask = = subnet id = host id netid

Representing Subnet Masks in Dotted Decimal Notation Example - a class B network: Subnet mask: – Dotted Decimal: – netid subnet hostid

Representing Subnet Masks in 3-tuple Notation Subnet mask: – tuple notation –{,, } –-1 means “all ones” –{-1,-1,0}

Routing in the Presence of Subnets All hosts and routers must use a subnet routing algorithm R2R1H Net 3 (subnet of address N)Net 2 (subnet of address N) Net 1 (not a subnet address)

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

Using Subnet Masks for Routing Host-specific routes –( , ) –( , , ) Default routes –(default, ) –( , , ) Standard, non-subnet class B network –( , ) –( , , )

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

Broadcasting to Subnets IP address = –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}

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