Subnetting

1

• Subnetting
• and Classless Routing

2
Last time Subnetting
Take a network address and break it up into
subnets that can be assigned to individual
physical networks.
Define a subnet mask to help create a new level
of hierarchy in the addressing scheme.
The bitwise AND of the subnet mask with the full
address gives the subnet number.
Example Take host address 128.96.34.15 and
subnet mask 255.255.255.0 and compute the subnet
number.
3
Example of Subnetting
All hosts in a subnet are configured with the
same subnet mask.
Question What does subnetting do to routing as
we know it? What happens to the format of
forwarding tables?
Question What does subnetting do to ARP?
4
Datagram Forwarding Algorithm

• D destination IP address
• for each forwarding table entry
  ltSubnetNumber,SubnetMask,NextHopgt
• D1 SubnetMask D
• if (NextHop is an interface)
• deliver datagram directly to destination
• else
• deliver datagram to NextHop (a router)

5
Fine Points on Subnetting

• The subnet mask does not need to align on byte
  boundaries. (You dont even have to have
  contiguous 1s, although that is not
  recommended.)
• It is possible to put multiple subnets on the
  same physical network, but hosts on the same
  physical network may then have to go through a
  router to talk to each other.
• From outside the subnetted domain, the whole
  thing is viewed as a single network. For this
  reason, subnets should be kept geographically
  close.

6
Classless Routing
7
Address Assignment Efficiency

• 2-host network assigned a Class C address
• Efficiency 2/254 0.78
• 256-host network assigned a Class B address
• Efficiency 256/65,534 0.039

How efficiently is the IP address space being
used?
8
Autonomous Systems and Interdomain Routing
In order to provide network scalability, we
approach the problem of routing using a
hierarchical view of the system. We define
domains or autonomous systems as interconnected
collections of physical networks. Inside an AS,
routers use protocols like RIP or OSPF. Once we
interconnect ASs, we need to define another level
of routing.
9
Possible Policies on Class B Address Assignment

• Scenario An Autonomous System (AS) requests a
  Class B address.
• 1) If number of hosts is close to 64K, grant
  request, otherwise deny it.
• 2) Instead of giving one Class B address, give
  multiple Class C addresses to cover all the hosts.

10
The Multiple Class C Addresses Solution

• Scenario One AS called X a 16 Class C network
  numbers.

Question How many entries for AS X in an
Internet backbone router?
Question What if we had given it only one Class
B network? What would the address assignment
efficiency be?
Question How could we reduce the impact of
multiple Class C addresses on routing table size?
11
Classless Interdomain Routing (CIDR)

• Goal Minimize the number of routes that a router
  needs to know while keeping address assignment
  efficiency high by giving out contiguous blocks
  of Class C addresses.

Example
top 20 bits remain constant
20 bits
12 bits
192.4.16.0
11000000 00000100 0001
Network
Host
192.4.17.0

16 bits
8 bits
192.4.31.0
Host
Host
CLASS B
CLASS C
12
CIDR

• In order to make this work, we need to
• Assign contiguous blocks of Class C addresses.
• Each block must contain a number of class C
  networks that is a power of 2.
• Use interdomain routing that understands
  classless addresses (network number can be of any
  length in number of bits).
• Routing protocols see network numbers as
• This is analogous to subnetting

ltlength, valuegt
ltmask, valuegt
13
CIDR vs. Subnetting
Multiple physical networks
Multiple network numbers
Physical Network 0
192.4.16.0
Physical Network 1
One address
192.4.17.0
128.96.34.0
11000000 00000100 0001

Physical Network 2
One network number
192.4.31.0
Physical Network 3
Route aggregation, or supernetting.
14
Enhanced Route Aggregation
Both X and Y are accessible via the same provider
network, the ISP advertises a single route for
both using the 19-bit prefix they share. If
addresses are assigned carefully, better route
aggregation becomes possible.