1. CSS 432: Subnetting, CIDR, and Global Internet
CSS432 Subnetting and CIDR Textbook Ch Global Internet Textbook Ch4.1
Professor: Munehiro Fukuda
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. Scaling Issues in Routing
Inefficient use of IP Address Space
Class C with 2 hosts (2/255 = 0.78% efficient)
Class B with 256 hosts (256/65535 = 0.39% efficient)
IP address space gets consumed too quickly
Too Many Networks
Routing tables do not scale
Route propagation protocols do not scale
Router gets slower to scan a big forwarding table
Hierarchy
CSS 432: Subnetting, CIDR, and Global Internet

4. CSS 432: Subnetting, CIDR, and Global Internet
Subnetting - Concept
Flat networks
A collection of subnets
40 nodes: Class C
200 nodes: Class C
256 nodes: Class B
Internet
IAS
BBUS
EDU
CSS –
30 nodes: Class C
40 nodes
Subnet:
200 nodes
Subnet:
256 nodes
Subnet:
Internet
IAS
BBUS
EDU
CSS –
30 nodes
Subnet:
Class B:
Problem: Internet identifies only classes
Four networks must receive an independent class of network number, (which exhausts IP addresses and floods network #s.)
Subnet: collects networks belonging to the same AS and give a single class of network number, which is then divided into subnet numbers internally.
CSS 432: Subnetting, CIDR, and Global Internet

5. Subnetting – How to Address
Subnet masks define variable partition of host part
Subnets visible only within site
Network number
Host number
Class B address
Subnet mask ( )
Subnetted address
Host ID
Subnet ID –
# of bits in subnet mask
/24
Subnet ID
CSS 432: Subnetting, CIDR, and Global Internet

6. Subnetting – How to Address
#bits
Subnetwork Mask
#subnets in Class B
#subnets in Class C
# of hosts 16 1 -
65534 17
32766 18 2
16382 19 6
8190 20 14
4094 21 30
2046 22 62
1022 23
126
510 24
254 25 26 27 28 29 31 32
Note: subnet all 0’s and all 1’s are not recommended
CSS 432: Subnetting, CIDR, and Global Internet

7. CSS 432: Subnetting, CIDR, and Global Internet
Subnet Example
Subnet mask:
Subnet number: /25 H1 R1
Subnet number: /25 R2 H2
Subnet mask:
Subnet number: /24 H3
IP address & subnet mast = subnet number
Example: & =
Forwarding table at router R1
Subnet Number Subnet Mask Next Hop
interface 0
interface 1 R2
CSS 432: Subnetting, CIDR, and Global Internet

8. 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

9. CSS 432: Subnetting, CIDR, and Global Internet
Supernetting
Subnetting
Purpose: divide a large class of network number into sub network numbers → help assign address carefully.
Problem: an AS with more than 255 hosts still needs class B.
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 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
CSS 432: Subnetting, CIDR, and Global Internet

10. CSS 432: Subnetting, CIDR, and Global Internet
Basic concept of supernetting using class C:
Represent blocks with a single pair
(first_class_C_network_address, count)
Example ( , 3)
Points to a group of blocks such as , , and
In practice
No restriction to class C nor use of count
Restrict block sizes to powers of 2
Use a bit mask (CIDR mask) to identify block size
Ex. An AS assigned a block of 2048 contiguous addresses starting at , (i.e., a collection of 8 class C networks)
Lowest
Highest
CIDR mask (21bits)
Address Notation: /21
CSS 432: Subnetting, CIDR, and Global Internet

11. Classless Addressing Examples
CIDR allows to aggregate routes repeatedly
Then, what if there is a router capable of forwarding packets both to the regional network and to the cooperation Z?
Prefix Next Hop
/18 the regional network
/20 corporation Z
To which of those two should we forward a packet destined to ?
Use Principle of Longest Match
Regional network
Corporation Y
/20
Corporation X
/20
Internet
backbone
Border gateway
/18
Corporation Z
/20
CSS 432: Subnetting, CIDR, and Global Internet

12. CSS 432: Subnetting, CIDR, and Global Internet
Classless Lookup
Binary Tree for IP Routing
Patricia Tree for IP Routing 1 1 / / 1 / 1 1 / 1 / 1 1 1 1
Skip 10 /
Skip 4 / /
CSS 432: Subnetting, CIDR, and Global Internet

13. 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

14. 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

15. 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 connecitoin 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.
CSS 432: Subnetting, CIDR, and Global Internet

16. CSS 432: Subnetting, CIDR, and Global Internet
BGP-4
Each AS has:
one or more border routers
one BGP speaker that advertises:
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
CSS 432: Subnetting, CIDR, and Global Internet

17. CSS 432: Subnetting, CIDR, and Global Internet
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
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
CSS 432: Subnetting, CIDR, and Global Internet

18. CSS 432: Subnetting, CIDR, and Global Internet
BGP Messages
BGP Speaker (sender)
BGP Speaker (receiver)
OPEN( myAS#, timeRequiredToReceiveTheRestOfMessage, myIP, options)
KEEPALIVE( )
UPDATE( #Withdrawn, #MaskBits, IP, …., #NewPath, #MaskBits, IP, …., PathAttributes )
TCP connection
KEEPALIVE( )
Next UPDATE( ) or KEEPALIVE( )
Next UPDATE( ) or KEEPALIVE( )
Timer Interrupt
NOTIFICATION(ErrorCode, ErroSubCode, Data)
Timeline
CSS 432: Subnetting, CIDR, and Global Internet

19. CSS 432: Subnetting, CIDR, and Global Internet
BGP Configuration for Cisco Routers
AS 102
AS 777
(friend)
R_A
R_f
AS 101
(myself)
R_me
R_B
AS 103 NO
! Our AS number is 101, provide A’s is 102, provide B’s 103, and our firend’s is 777
router bgp 101
network
neighbor remote-as ! Provider A
neighbor filter-list 81 weight 100 ! Traffic to our friend 777 uses provider A
neighbor filter-list 82 out
neighbor remote-as ! Provider B
neighbor filter-list 83 out !
! Define an AS path access-list that selects our friend’s routes
ip as-path access-list 81 permit _777$ ! There is a path through A to our friend 777
! Define an AS path access-list that blocks provider B’s routes
ip as-path access-list 82 deny ^102_ ! Don’t let provider A use my network for transit
ip as-path access-list 82 permit .*
! Define an AS path access-list that blocks provider A’s routes
ip as-path access-list 83 deny ^103_ ! Don’t let provider B use my network for transit
Ip as-path access-list 83 permit .*
at the end
at the beginning
CSS 432: Subnetting, CIDR, and Global Internet

20. 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

21. CSS 432: Subnetting, CIDR, and Global Internet
OSPF Configuration for Cisco Routers
Router 1
Router 2
Router 3
Router 4
Router 5
/24
/24
/24
/24
! Router 1 starts an OSPF process and place all interfaces in area 0
router ospf 1
network area 0 ! Mask here means wild cards 0 or 1.
! Router 2 starts an OSPF process and place all interfaces in area 1
network area 0
! Router 3 starts an OSPF process and place in area1 the interface that is part
! of /22. Place all the other interfaces in area 0
network area 1 ! The last 10 bits may be 0 or 1
netwrok area 0
CSS 432: Subnetting, CIDR, and Global Internet

22. CSS 432: Subnetting, CIDR, and Global Internet
IP Version 6
Features
128-bit addresses (classless)
multicast
real-time service
authentication and security
autoconfiguration
end-to-end fragmentation
protocol extensions
Header
40-byte “base” header
extension headers (fixed order, mostly fixed length)
fragmentation
source routing
other options
CSS 432: Subnetting, CIDR, and Global Internet

23. 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