1. CSS432 Subnetting and CIDR Textbook Ch3. 2
CSS432 Subnetting and CIDR Textbook Ch Global Internet Textbook Ch4.1
Prof. Athirai Irissappane
CSS 432: Subnetting, CIDR, and Global Internet

2. CSS 432: Subnetting, CIDR, and Global Internet
Internet Structure
NSFNET backbone
Stanford
BARRNET
regional
Berkeley P
ARC
NCAR UA
UNM
Westnet
UNL KU
ISU
MidNet …
Autonomous System (AS):
Administered independently of other AS
Have a different routing protocol and metrics
Do we really need to give an independent class A/B/C network number to every single AS?
CSS 432: Subnetting, CIDR, and Global Internet

3. Global Addresses IP address Class
Network
Host 7 24 A: 14 16 1 B: 21 8 C:
Global Addresses
IP address
Class
Identified using first few bits as shown in fig
Class A (previously reserved for small number of WAN)
7 bits for network, 24 bits for host
2^7 – 2 different networks (1st bit is 0, 127… is reserved for loopback IP address)
Each network 2^24 -2 hosts (all zeros in host bits represent network ID, all 1s broadcast Id) (2^24 – 2 = 16,777,214)
Class B (previously reserved for medium sized campus networks)
14 bits for network, 16 bits for host
2^14 networks
Each network 2^ hosts ( = 65534)
Class C (previously reserved for large number of LANs)
21 bits for network, 8 bits for host (2^8 -2) = (256-2=254)
But convention not followed anymore – Classless addresses are used now
MAC address – 6 bytes 48 bits long

4. Scaling Issues in Routing
Class C address
21 bits for network and 8 bits for host
Each network 2^8 -2 hosts? What happens if we need to sub-divide the network and manage groups of computers individually ?
Should each group be given a different class A address?
Exhaust IP addresses faster, inefficient use of IP addresses
Group 1 – 2 hosts
2/2^8-2 = 2/ ~ .7% efficiency
CSS 432: Subnetting, CIDR, and Global Internet

5. Scaling Issues in Routing
Inefficient use of IP Address Space
Class C with 2 hosts (2/254 = 0.78% efficient)
Class B with 256 hosts (256/65534 = 0.39% efficient)
IP address space gets consumed too quickly
Too Many Networks
Routing tables do not scale (more networks, more entries)
Route propagation protocols do not scale
Router gets slower to scan a big forwarding table
Hierarchy
CSS 432: Subnetting, CIDR, and Global Internet

6. CSS 432: Subnetting, CIDR, and Global Internet
Efficiently use IP addresses, especially for autonomous systems
The practice of dividing a network into two or more networks is called subnetting.
A subnet is a way of taking a single IP network address (A/B/C) and locally splitting it up
Given 1 IP address how can different subnets be identified?
Computers that belong to a subnet are addressed with a common, identical, most-significant bit-group in their IP address.
Depends on how you split the network and host part
CSS 432: Subnetting, CIDR, and Global Internet

7. SubNetted Networks
Define subnetwork by using some bits of host address to identify the subnetwork
Borrowing 1 or more bits from the host bit portion

8. Example: Subnet Network Part
Dividing a network into 2 subnets requires to borrow 1 bit
Class C address:
Network Portion/Subnet Mask (Class C address 24 bits network)
Define new subnet ,
Borrow 1 bit from host address (borrow from the right most bit)
No. of. Subnetworks = 2^ number of bits for sub network = 2^1 = 2 subnetworks
No.of hosts per subnetwork = 2^number of host bits -2 = 2^7 -2= = 126
All host bits are 1’s are reserved for broadcast ID
All host bits 0’s are reserved for network ID
2^7
Subnet
Network Part

9. Subnet Mask 2nd subnet: Class C address: 172.16.25.0
What is the network address of the subnets? (each subnet has hosts)
1st subnet:
0 to hosts
to hosts
is the subnetwork ID, is broadcast IP, hosts can be to
belongs to this subnet
2nd subnet:
128 to hosts
to hosts
is the subnetwork ID, is broadcast IP, hosts can be to

10. Subnet Mask
Determines the way an IP address is split into network and hosts portions Class A - 0nnnnnnn.hhhhhhhh.hhhhhhhh.hhhhhhhh Subnet Mask = Class B - 10nnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh Subnet Mask = Class C - 100nnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh Subnet Mask =

11. Class C Subnetting # of Subnets # of Hosts/Subnet NetMask 4th Octet
# of Subnets
# of Hosts/Subnet
NetMask
4th Octet
CIDR Notation 2
126
/25 4 62
/26 8 30
/27 16 14
/28 32 6
/29 64
/30

12. Subnet Mask IP address & subnet mask = subnet number
How to identify if a host is within a given subnet, given the subnet Id, subnet mask and IP address of host
IP address & subnet mask = subnet number
Example: destination IP= ; subnet mask= ;
Subnet ID =
& =

13. Routing with simple IP
[Note: NetworkNum values would typically be more like ]
CSS 432: Subnetting & CIDR

14. Routing with subnetting
IP address & subnet mask = subnet number
Example: &
& =
Forwarding Table for R1
CSS 432: Subnetting & CIDR

15. CSS 432: Subnetting, CIDR, and Global Internet
Forwarding Algorithm
D = destination IP address
for each entry (SubnetNum, SubnetMask, NextHop)
D1 = SubnetMask & D
if D1 = SubnetNum
if NextHop is an interface
deliver datagram directly to destination
else
deliver datagram to NextHop (a router)
Use a default router if nothing matches
Not necessary for all 1s in subnet mask to be contiguous
But should be avoided
Can put multiple subnets on one physical network
Ex. Two or more departments want to have their own subnet and to allocate IP addresses in it while sharing just one physical network
Subnets not visible from the rest of the Internet
CSS 432: Subnetting, CIDR, and Global Internet

16. Supernetting Supernetting Subnetting
Purpose: given a class C address -> helps to divide into sub network numbers → helps assign addresses efficiently
Problem: an AS with more than 254 hosts still needs class B (65535 hosts), e.g., 256 hosts class B address still inefficient
Supernetting
Solution: assign block of contiguous network numbers to an institution.
Ex. Assign two class C network numbers instead of one class B network.
Side effect: The information that routers store and exchange increases dramatically
Ex. If an AS has 16 class C network numbers, every Internet router needs 16 entries for this AS.
CIDR: Classless Inter-Domain Routing

17. The University of Adelaide, School of Computer Science
22 October 2017
SuperNetting
Class C address:
Block1 ( )
Block2 ( )
Use this Mask instead of
If we take 1 bit for subnetwork
Number of host bits = 9 ~ 2^9 hosts; each subnetwork can have more number of hosts
Chapter 2 — Instructions: Language of the Computer

18. The University of Adelaide, School of Computer Science
22 October 2017
Classless Addressing
Exhaustion of IP address space centers on exhaustion of the class B network numbers
Solution
Say “NO” to any Autonomous System (AS) that requests a class B address unless they can show a need for something close to 64K addresses
Instead give them an appropriate number of class C addresses
For any AS with at least 256 hosts, we can guarantee an address space utilization of at least 50%
What is the problem with this solution?
Chapter 2 — Instructions: Language of the Computer

19. The University of Adelaide, School of Computer Science
22 October 2017
Classless Addressing
Problem with this solution
Excessive storage requirement at the routers.
If a single AS has, say 16 class C network numbers assigned to it,
Every Internet backbone router needs 16 entries in its routing tables for that AS
This is true, even if the path to every one of these networks is the same
If we had assigned a class B address to the AS
The same routing information can be stored in one entry
Efficiency = 16 × 255 / 65, 536 = 6.2%
Chapter 2 — Instructions: Language of the Computer

20. The University of Adelaide, School of Computer Science
22 October 2017
CIDR
CIDR tries to balance the desire to minimize the number of routes that a router needs to know against the need to hand out addresses efficiently.
CIDR uses aggregate routes
Uses a single entry in the forwarding table to tell the router how to reach a lot of different networks
Breaks the rigid boundaries between address classes
Chapter 2 — Instructions: Language of the Computer

21. The University of Adelaide, School of Computer Science
22 October 2017
CIDR NOTATION
Instead of having multiple entries for each contiguous block of class C address in the routing table, just specify 1 entry
The contiguous blocks have the same prefix
Example /20
First 20 bits is network part and next 12 bits is for the host
Contiguous subnets need not have different entries in the routing table but just one entry due to CIDR
CIDR Blocks
Contiguous IP addresses
Block size in power of 2
Chapter 2 — Instructions: Language of the Computer

22. The University of Adelaide, School of Computer Science
22 October 2017
CIDR
Consider an AS with 16 class C network numbers.
Instead of handing out 16 addresses at random, hand out a block of contiguous class C addresses
Suppose we assign the class C network numbers from through
Observe that top 20 bits of all the addresses in this range are the same ( )
We have created a 20-bit network number (which is in between class B (14 bits) network number and class C number (21 bits) )
Requires to hand out blocks of class C addresses that share a common prefix
Chapter 2 — Instructions: Language of the Computer

23. The University of Adelaide, School of Computer Science
22 October 2017
Classless Addressing
Classless Inter-Domain Routing
A technique that addresses two scaling concerns in the Internet
The growth of backbone routing table as more and more network numbers need to be stored in them
Potential exhaustion of the 32-bit address space
Address assignment efficiency
Arises because of the IP address structure with class A, B, and C addresses
Forces us to hand out network address space in fixed-size chunks of three very different sizes
A network with two hosts needs a class C address
Address assignment efficiency = 2/255 = 0.78
A network with 256 hosts needs a class B address
Address assignment efficiency = 256/65535 = 0.39
Chapter 2 — Instructions: Language of the Computer

24. Classless Addressing Examples
Given this routing table with CIDR notation To which of those two should we forward a packet destined to ?
Prefix Next Hop
/18 R2
/20 R3
/18
18 network bits, mask =
& = (Matches!)
/20
20 network bits, mask =
& = (Matches!)
Which one should I choose
Principle of Longest Match  (Matches!) with 20 network bits
Next hop is R3 2
power 1 4 8 3 16 32 5 64 6
128 7
CSS 432: Subnetting, CIDR, and Global Internet

25. Trie (Prefix Tree) Data structure for longest prefix match
Tree with child and parent nodes
If this Trie represents a dictionary,
find the longest word with prefix “The”
Ans: There
We will be considering a binary trie
Every parent has only 2 children

26. Binary Trie Tree contains path to network address
Bits before * represents the network portion
Each leaf contains a possible address
Prefixes in the table are marked (dark)
Binary Trie
Search:
Traverse the tree according to destination address
Most recent shaded node is the current longest prefix
Search ends when a leaf node is reached

27. Binary Trie Update: Search for the new entry1
h 1010* h
Update:
Search for the new entry
Search ends when a leaf node is reached
If there is no branch to take, insert new node(s)

28. Constructing a Binary Trie for lookup7 6 5 4 3 2 1
128 64 32 16 8
2^7
2^6
2^5
2^4
2^3
2^2
2^1
2^0 /2 /2
10* /1 /1 0*
/20
/20 *
/18
/18
00* /2
interface0 /1
Router 2
/20
Interface1
/18
Router 3
CSS 432: Subnetting, CIDR, and Global Internet

29. Classless Lookup Binary Trie for IP Routing Compress 1-child branches1
10 more zeros
/20
/1 ; 10*
/1 ; 0* 1 1
3 more zeros 1
/18
10* /2
interface0 0* /1
Router 2 *
/20
Interface1 *
/18
Router 3
CSS 432: Subnetting, CIDR, and Global Internet

30. CSS 432: Subnetting, CIDR, and Global Internet
Classless Lookup
Patricia Tree for IP Routing
/20 1
Skip 1 one 1 1 1
Skip 10 zeros
Skip 3 zeros
/1 ; 10*
/18
/1 ; 0*
Practical Algorithm To Retrieve Information Coded in Alphanumeric
10* /2
interface0 0* /1
Router 2 *
/20
Interface1 *
/18
Router 3
CSS 432: Subnetting, CIDR, and Global Internet

31. CSS 432: Subnetting, CIDR, and Global Internet
Classless Lookup
Destination is /32 
/20 1
Skip 1 one 1 1 1
Skip 10 zeros
Skip 3 zeros
/18
/1 ; 10*
/1 ; 0*
Destination ip
Longest Matching prefix
/32
/18
/20
10* /2
interface0 0* /1
Router 2 *
/20
Interface1 *
/18
Router 3
CSS 432: Subnetting, CIDR, and Global Internet

32. CSS 432: Subnetting, CIDR, and Global Internet
Route Propagation
NSFNET backbone
Stanford
BARRNET
regional
Berkeley P
ARC
NCAR UA
UNM
Westnet
UNL KU
ISU
MidNet …
Know a smarter router
Hosts know local (default) routers
Local routers know site routers
Site routers know core router
Core routers know everything
Site routers are called border routers.
Autonomous System (AS)
Corresponds to an administrative domain
Examples: University, company, backbone network
Two-level route propagation hierarchy
Interior gateway protocol (each AS selects its own)
Exterior gateway protocol (Internet-wide standard)
AS1
AS2 R2 R1
Interior
Exterior
CSS 432: Subnetting, CIDR, and Global Internet

33. Popular Interior Gateway Protocols
RIP: Route Information Protocol
Distributed with Unix
Distance-vector algorithm
Based on hop-count
OSPF: Open Shortest Path First
Recent Internet standard
Uses link-state algorithm
Supports load balancing
Supports authentication
CSS 432: Subnetting, CIDR, and Global Internet

34. Well-known Exterior Gateway Protocol
Border Gateway Protocol – 4th Version (BGP-4)
Assumption: Internet as an arbitrarily interconnected set of ASs
Goal: Reachability than optimality
Backbone service provider
Peering
point
Large corporation
Small
corporation “
Consumer ”
ISP
Stub AS:
A single connection to another AS
Only carries local traffic.
Multihomed AS:
Connections to multi ASs
Refuses to carry transit traffic
Transit AS:
Connections to multi-ASs
Carries both transit and local traffic
Local Traffic: traffic within AS
Transit Traffic: traffic across AS

35. BGP-4 (Routing across AS)
Routing path is a vector of AS to reach a particular network
Each AS has:
1 or more border routers (through which packet enter and leave AS)
one BGP speaker that advertises (can be border router):
local networks
other reachable networks (transit AS only)
gives complete path information
Characteristics
Inter-BGP speaker communication based on P2P and TCP
Consistent maintenance on routing information among multiple BGP speakers
Reachability-based information
Policy Support to distinguish between intra- and inter-AS reachability information
Incremental updates that sends only reachability change
Route aggregation to send multiple routes in one message
Authentication to allow a receiver to authenticate messages

36. BGP Example Speaker for AS2 advertises reachability to P and Q
network , , , and , can be reached directly from AS2
Speaker for backbone advertises
networks , , , and can be reached along the path (AS1, AS2).
Speaker can cancel previously advertised paths
BGP prevents loops: Each received advertisement is a full path to reach a destination, AS in the path must be unique. Receiver discards it if it finds itself in the path.
Backbone network
(AS 1)
Regional provider A
(AS 2)
Regional provider B
(AS 3)
Customer P
(AS 4)
Customer Q
(AS 5)
Customer R
(AS 6)
Customer S
(AS 7)
128.96

37. CSS 432: Subnetting, CIDR, and Global Internet
Routing Areas
AS divided into areas
Area 0
Known as the backbone area and connected to the back bone
Routers (R1, R2, R3) called ABR (Area Border Router)
OSPF link states
do not leave the area in which they originated if they are not ABRs.
ABRs summarize routing information that they have learned from one area and make it available in their advertisements to other areas. R4 R5 R6 R2 R3 R1 R7 R8 R9
Area 0
Area 3
Area 2
Area 1
Virtual Link NO
CSS 432: Subnetting, CIDR, and Global Internet

38. CSS 432: Subnetting, CIDR, and Global Internet
IP Version 6
Features
128-bit addresses (classless/CIDR) [IPv4 – 32 bit address]
multicast
real-time service
authentication and security
autoconfiguration
end-to-end fragmentation
protocol extensions
Header
40-byte “base” header [IPv4 – 20 bytes w/o options]
extension headers (fixed order, mostly fixed length, use when necessary)
fragmentation
source routing
other options
CSS 432: Subnetting, CIDR, and Global Internet

39. CSS 432: Subnetting, CIDR, and Global Internet
Reviews
Subnetting: How to address and forwarding algorithm
Supernetting: CIDR, principle of longest match, and classless lookup
Exterior gateway protocol: BGP and routing areas
Exercises in Chapter 3
Ex. 55 (Subnetting)
Ex. 68 (CIDR)
Ex. 72 (CIDR)
Ex. 74 (CIDR)
CSS 432: Subnetting, CIDR, and Global Internet