1. 2012 session 1 TELE3118: Network Technologies Week 4: Network Layer Basics, Addressing
Some slides have been taken from:
Computer Networking: A Top Down Approach Featuring the Internet, 4th edition. Jim Kurose, Keith Ross. Addison-Wesley, All material copyright.
J.F Kurose and K.W. Ross, All Rights Reserved.
Computer Networks, 4th edition. Andrew S. Tanenbaum. Prentice-Hall, 2003.
Network Layer

2. Why an internet layer? Why not one big flat LAN? IP provides:
Different LAN protocols
Flat address space not scalable
IP provides:
Global addressing
Scaling to WANs
Virtualization of network isolates end-to-end protocols from network details/changes
Why a single IP?
Maximise interoperability
Minimise service interfaces
Why a narrow IP?
Least common network functionality
“hourglass model”
(Steve Deering)
Network Layer

3. IP functions Addressing Transport packet from address A to address B
network
data link
physical
application
transport
Addressing
Transport packet from address A to address B
Where does IP run?
every host
every router
Mechanisms for:
Addressing: assigning addresses to hosts/routers
Route determination: algorithms to compute route to get packet to destination
Forwarding: move packets from source to destination
Network Layer

4. Interplay between routing and forwarding1 2 3
0111
value in arriving
packet’s header
routing algorithm
local forwarding table
header value
output link
0100
0101
1001
Network Layer

5. Network service model Connection-oriented network service:
Q: What service model for “channel” transporting datagrams from sender to rcvr?
Connection-oriented network service:
Virtual Circuits
Connection-less network service:
Datagrams
Network Layer

6. Virtual circuits
“source-to-dest path behaves much like telephone circuit”
performance-wise
network actions along source-to-dest path
call setup, teardown for each call before data can flow
each packet carries VC identifier (not destination host address)
every router on source-dest path maintains “state” for each passing connection
link, router resources (bandwidth, buffers) may be allocated to VC
Network Layer

7. Datagram networks no call setup at network layer
routers: no state about end-to-end connections
no network-level concept of “connection”
packets forwarded using destination host address
packets between same source-dest pair may take different paths
application
transport
network
data link
physical
application
transport
network
data link
physical
1. Send data
2. Receive data
Network Layer

8. Datagram or VC network: why?
Internet
data exchange among computers
“elastic” service, no strict timing req.
“smart” end systems (computers)
can adapt, perform control, error recovery
simple inside network, complexity at “edge”
many link types
different characteristics
uniform service difficult
Asynchronous Transfer Mode (ATM)
evolved from telephony
human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems
telephones
complexity inside network
Network Layer

9. IP: Mid-life crisis
Network Layer

10. The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
Routing protocols
path selection
RIP, OSPF, BGP
Network
layer
forwarding
table
ICMP protocol
error reporting
router “signaling”
Link layer
physical layer
Network Layer

11. Addressing IP address: IP versus Ethernet 4-bytes = 32-bits
Dot-notation
Globally unique
Hierarchical:
network + host
IP versus Ethernet
Hierarchical vs. flat
Portability: “location” vs. “identification” =
223 1 1 1
Network Layer

12. Subnets IP address: What’s a subnet ? subnet part (high order bits)
host part (low order bits)
What’s a subnet ?
device interfaces with same subnet part of IP address
can physically reach each other without intervening router
LAN
network consisting of 3 subnets
Network Layer

13. Subnets
How many?
Network Layer

14. IP addressing: “class-full”to
network
host B to 10
network
host to C
110
network
host to D
1110
multicast address
32 bits
Classful addressing:
inefficient use of address space, address space exhaustion
e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network
Network Layer

15. IP addressing: “class-less”
CIDR: Classless InterDomain Routing
subnet portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in subnet portion of address
subnet
part
host
/23
Network Layer

16. IP addresses: how to get one?
Q: How does host get IP address?
hard-coded by system admin in a file
Wintel: control-panel->network->configuration->tcp/ip->properties
UNIX: /etc/rc.config
DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server
“plug-and-play”
(more in next chapter)
Network Layer

17. IP addresses: how to get one?
Q: How does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23
Network Layer

18. Hierarchical addressing: route aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
/23
Organization 1
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning
/16”
ISPs-R-Us
Network Layer

19. Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning /16
or /23”
ISPs-R-Us
Organization 1
/23
Network Layer

20. IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
allocates addresses
manages DNS
assigns domain names, resolves disputes
Network Layer

21. NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
All datagrams leaving local
network have same single source NAT IP address: ,
different source port numbers
Datagrams with source or
destination in this network
have /24 address for
source, destination (as usual)
Network Layer

22. NAT: Network Address Translation
Motivation: local network uses just one IP address as far as outside word is concerned:
no need to be allocated range of addresses from ISP: - just one IP address is used for all devices
can change addresses of devices in local network without notifying outside world
can change ISP without changing addresses of devices in local network
devices inside local net not explicitly addressable, visible by outside world (a security plus).
Network Layer

23. NAT: Network Address Translation
Implementation: NAT router must:
outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)
. . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr.
remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair
incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table
Network Layer

24. NAT: Network Address Translation
NAT translation table
WAN side addr LAN side addr
1: host
sends datagram to
, 80
2: NAT router
changes datagram
source addr from
, 3345 to
, 5001,
updates table
, , 3345
…… ……
S: , 3345
D: , 80 1
S: , 80
D: , 3345 4
S: , 5001
D: , 80 2
S: , 80
D: , 5001 3
4: NAT router
changes datagram
dest addr from
, 5001 to , 3345
3: Reply arrives
dest. address:
, 5001
Network Layer

25. NAT: Network Address Translation
16-bit port-number field:
60,000 simultaneous connections with a single LAN-side address!
NAT is controversial:
routers should only process up to layer 3
violates end-to-end argument
NAT possibility must be taken into account by app designers, eg, P2P applications
address shortage should instead be solved by IPv6
Network Layer