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

H4

10.0.0.0

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

RRouter running proxy ARP

Main network

Hidden network

H1 H2 H3

H4 H5 H6

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

H1Rest of

the internet

All traffic to

128.10.0.0

Network 128.10.1.0

Network 128.10.2.0

H2

H3 H4

128.10.1.1 128.10.1.2

128.10.2.1 128.10.2.2

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

Subnet Addressing (cont)

• Regular (Class B) IP address:

• New interpretation (locally only):

0 8 16 24 311 0 netid hostid

0 8 16 24 311 0 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

0 8 16 24 311 0 netid subnet hostid

0 8 16 19 311 0 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: – 11111111 11111111 11111111 00000000

0 8 16 24 311 0 netid subnet hostid

Subnet Masks

• Subnet bits do not have to be contiguous:– Mask = 11111111 11111111 00001010 10000000

= subnet id

= host id

0 8 16 24 311 0 netid

Representing Subnet Masks in Dotted Decimal Notation

• Example - a class B network:

• Subnet mask: – 11111111 11111111 11111111 00000000

• Dotted Decimal:– 255.255.255.0

0 8 16 24 311 0 netid subnet hostid

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}

Routing in the Presence of Subnets

• All hosts and routers must use a subnet routing algorithm

R2R1 H

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– (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)

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

elsefor each entry (M,N,NH) in the routing table {

I = M&Dif (I == N) then send datagram to NH}

if no matches were found declare a routing error

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}

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