1. How data travels through a network The Internet
Packet routing: Journey of a packet in internet

2. Processing packet at source machine

3. Processing packet at source machine (Cont.)
Application generates a packet to be sent on the network and send it to layer below.
transport layer which manages end to end communication between two machines
it is sent to network layer which adds source and destination IP in the packet.
The packet reaches data link or MAC layer
where source and destination MAC address of machines are added
Data link layer push this packet to physical layer
it is sent as stream of “0” and “1” on physical medium available.

4. Processing a packet at router
Router takes the packet and does three basic operations
Routing
forwarding
encapsulation

5. routing When router receives packet
first of all it strips down the MAC layer header and looks into the IP header which contains destination IP address.
Once destination IP is known, router looks into it database in order to find where should this packet be forwarded to make it reach to destination.
This databases is known as routing table.

7. forwarding
the packet is switched from the ingress interface to egress interface
known as forwarding

8. Encapsulation
Router caches the MAC address of next hops it needs to send packet to, it replaces the source and destination MAC address in it and send it to physical layer.

9. Processing packet at destination host

10. Processing packet at destination host (cont.)
Packet is received at network card, physical layer, which generates an interrupt to CPU and CPU reads packet in,
At data link layer, destination MAC address is checked to see if packet is destined to this machine, If yes, packet is sent up to network layer.
At IP layer, packet validation like checksum verification etc is done and then passed on to relevant transport layer.
Transport later then passes it on to the appropriate port so that it reaches correct application.

11. Layer
Number
Function
Example
Application 7
Services that are used with end-user application
SMTP, HTTP,POP3, IMAP
Presentation 6
Formats the data so that it can be viewed by the user
Encrypt and Decrypt
JPEG, GIF, TIFF, HTTPS, SSL, TLS
Session 5
Establishes/ends connections between two hosts
RPC, SQL, NetBIOS, PPTP
Transport 4
Responsible for the transport protocol and error handling
TCP, UDP
Network 3
Reads the IP address from the packet
IP, ICMP, Routers, Layer 3 switches
Data Link 2
Reads the MAC address from the data packet
PPP, SLIP, switches
Physical 1
Sends data on the physical wire
Physical connections, hubs, NICs, cable

12. WELL-KNOWN PORTS Port Use 20-21 FTP 22 SSH 23 TELNET 25 SMTP 42 WINS53
DNS
80, 8080
HTTP 88
KERBEROS
110
POP3
111
PORTMAPPER-LINUX
123
NTP
135
RPC-DCOM
143
IMAP
161, 162
SNMP
389
LDAP
445
CIFS
514
SYSLOG
636
SECURE LDAP
WELL-KNOWN PORTS

13. REGISTERED PORTS OF INTEREST
USE
1080
SOCKS5
1241
NESSUS SERVER
1433, 1434
SQL SERVER
1494, 2598
CITRIX APPLICATIONS
1521
ORACLE LISTENER
2512, 2513
CIRTIX MANAGEMENT
3389
RDP
IRC