1. Introduction to Networks
CSEN 404
Introduction to Networks
Amr El Mougy
Ali Saudi
** Slides are attributed to J. F. Kurose

2. Network layer transport segment from sending to receiving host
application
transport
network
data link
physical
network
data link
physical
transport segment from sending to receiving host
on sending side encapsulates segments into datagrams
on receiving side, delivers segments to transport layer
network layer protocols in every host, router
router examines header fields in all IP datagrams passing through it
application
transport
network
data link
physical
Network Layer

3. 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

4. Two Key Network-Layer Functions
forwarding: move packets from router’s input to appropriate router output
routing: determine route taken by packets from source to dest.
routing algorithms
analogy:
routing: process of planning trip from source to dest
forwarding: process of getting through single interchange
Network Layer

5. Forwarding in Datagram Networks
Forwarding is based on a table of destination addresses at the routers
IP addresses are 32-bits

6. 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

7. 32 bit destination IP address
IP datagram format
ver
length
32 bits
data
(variable length,
typically a TCP
or UDP segment)
16-bit identifier
header
checksum
time to
live
32 bit source IP address
IP protocol version
number
header length
(bytes)
max number
remaining hops
(decremented at
each router)
for
fragmentation/
reassembly
total datagram
length (bytes)
upper layer protocol
to deliver payload to
head.
len
type of
service
“type” of data
flgs
fragment
offset
upper
layer
32 bit destination IP address
Options (if any)
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
how much overhead with TCP?
20 bytes of TCP
20 bytes of IP
= 40 bytes + app layer overhead
Network Layer

8. IP Addressing: introduction
IP address: 32-bit identifier for host, router interface
interface: connection between host/router and physical link
router’s typically have multiple interfaces
host typically has one interface
IP addresses associated with each interface =
223 1 1 1
Network Layer

9. Subnets IP address: What’s a subnet ? 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
subnet
network consisting of 3 subnets
Network Layer

10. Subnets
/24
/24
/24
Recipe
To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.
Subnet mask: /24
Network Layer

11. Subnets
How many?
Network Layer

12. IP addressing: CIDR 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

13. Original IPv4 Address Classes
32 bits (4 bytes) unique value for each host
Address composed of 2 parts (2-level hierarchy):
Prefix(network ID): identifies network to which host attaches
Suffix(host ID): identifies host on that network

14. Special IP Addresses All 1’s: All 0’s:
Broadcast address for same network as source
All 0’s:
Used for IP address assignment

15. Subnetting A class B address is divided into two parts:
network part and
local part
Local part is further divided locally into:
subnet and
host parts
Splitting is done internally, yet looks like a single network to the outside world Network

16. Subnet Mask
Subnet mask needed to differentiate between different subnets
Allows hosts to determine if another IP address is on the same subnet or the same network
1’s represent network part, 0’s represent host part

17. Classful addressing: Good or Bad?
Pros:
simple, easy to understand
self-identifying
Can determine boundary between prefix and suffix from the address itself
No additional state needed to store boundary information
Cons:
limited address space
2^32 = 4G addresses, not enough?
limited network size choices (3)
Ex.: what if a class C net needs to grow beyond 255 hosts?
moving to a new network requires changing IP address

18. Example
An organization has 7 LANs as shown in the figure. The organization has a network address x. Determine:
The network address for each LAN.
The IP addresses of the hosts (H1H6) and the routers shown in the figure. Note that LAN 7 has no hosts but requires IP addresses.
The maximum number of hosts that can be connected to each of the first 6 LANs (LAN1  LAN 6).

19. Q6: Solution
7 networks  3 subnet bits and 5 host bits. Network address LAN 1: LAN 2: LAN 3: LAN 4: LAN 5: LAN 6: LAN 7: Host address LAN 1: , LAN 2: , LAN 3: , LAN 4: , LAN 5: , LAN 6: , LAN 7: , Number of hosts 2^5 = = 30
Transport Layer

20. IP addresses: how to get one?
Q: How does a host get IP address?
hard-coded by system admin in a file
Windows: 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”

21. Dynamic Host Configuration Protocol
Host sends DHCP discovery using as src IP and as dest IP
The DHCP server port number is 67
Server responds with an address offer yiaddr =
If the client accepts, then a Request/Ack transaction will follow