Chabot College
ELEC 99.08
Ports (Layer 4)
CISCO NETWORKING ACADEMY
TCP/IP Protocol Suite (simplified)
Layer 7
Layer 4
FTP
HTTP
DNS
TFTP
:21
:80
:53
:69
TCP
UDP
(connection-oriented)
(connectionless)
ICMP
Layer 3
Layer 1 & 2
CISCO NETWORKING ACADEMY
ARP
Token Ring
IP
Ethernet
(includes Ping & Traceroute)
FDDI
Layer 3 - Network Layer
• In Chapters 10 & 11, we covered Layer
3 protocols:
– Internet Protocol (IP)
• Used for addressing
– Address Resolution Protocol (ARP)
• Used to relate IP and MAC addresses
– Internet Control Message Protocol
(ICMP)
• Used for ping and traceroute commands
CISCO NETWORKING ACADEMY
Layer 3 - IP, ARP, ICMP
Layer 7
Layer 4
FTP
HTTP
DNS
TFTP
:21
:80
:53
:69
TCP
UDP
(connection-oriented)
(connectionless)
ICMP
Layer 3
Layer 1 & 2
CISCO NETWORKING ACADEMY
ARP
Token Ring
IP
Ethernet
(includes Ping & Traceroute)
FDDI
A Key Limitation of Layer 3
• No way to keep track of multiple
conversations between between pairs
of hosts running multiple applications.
(e.g. mail & web)
CISCO NETWORKING ACADEMY
Layer 4 - Transport Layer
• Layer 4 uses port numbers to address
this limitation.
4
CISCO NETWORKING ACADEMY
Port Numbers
• Layer 4 uses port numbers to direct data
to/from correct upper layer application.
CISCO NETWORKING ACADEMY
Port Numbers
• Used to identify the application that Layer
4 should “hand off to.”
• Like a “hole in the ceiling” that Layer 4
“throws” the data up into…
• Applications “listen” on a particular port
number.
• IP address + port number = socket
Each socket must be unique, and is
another type of address.
CISCO NETWORKING ACADEMY
TCP & UDP port numbers
• TCP/UDP use 16 bits to represent a port
number (65,536 possibilities).
1 - 255 Used for standard public applications
256 - 1023 Used by companies for specific apps
1024 & up Unregulated, but many have been
“registered” by companies for specific
uses.
Also, this range is used as dynamically
assigned “source port” numbers by TCP.
CISCO NETWORKING ACADEMY
Well-Known Port Numbers
• Well-known port numbers are defined
in RFC 1700.
Port 80 is web/
www (TCP).
CISCO NETWORKING ACADEMY
Non-Standard Port Numbers
• A host can run its services on any port,
not just on the well-known ones.
• To direct traffic to a non-standard port,
specify the port number after the IP
address:
http://banner-web.clpccd.cc.ca.us:7000
CISCO NETWORKING ACADEMY
Port Numbers in Action
Client
port Service (listening)
LAYER 4
80
Web server
21
File server (FTP)
25
E-mail (SMTP)
I want 207.35.52.4,
but which service
do I want?
LAYER 3 207.35.52.4
LAYER 2 00-0c-4f-32-ab-41
CISCO NETWORKING ACADEMY
Port Numbers in Action
Client
TCP port 80
207.35.52.4
00-0c-4f-32-ab-41
The client addresses
the message with the
port # of the service it
wants.
CISCO NETWORKING ACADEMY
Destination Port 80 - HTTP
FTP
HTTP
DNS
TFTP
:21
:80
:53
:69
TCP
UDP
(connection-oriented)
(connectionless)
IP
Ethernet
CISCO NETWORKING ACADEMY
Port Numbers in Action
Client
TCP port 80
207.35.52.4
00-0c-4f-32-ab-41
TCP port 25
207.35.52.4
00-0c-4f-32-ab-41
TCP port 21
The client can maintain
multiple conversations with
the same host, but using
different applications.
CISCO NETWORKING ACADEMY
207.35.52.4
00-0c-4f-32-ab-41
Port Numbers in Action
Client
TCP port 1250
TCP port 80
24.1.103.109
207.35.52.4
00-4b-08-57-c6-9f
00-0c-4f-32-ab-41
DESTINATION
SOURCE
Note that client applications, like
web browsers, dynamically “grab”
a source port number above 1023
when needed.
CISCO NETWORKING ACADEMY
Web Server
Port Numbers in Action
Web Server
Client
TCP port 80
TCP port 1250
207.35.52.4
24.1.103.109
00-0c-4f-32-ab-41
SOURCE
CISCO NETWORKING ACADEMY
00-4b-08-57-c6-9f
DESTINATION
An Example TCP Header :
Sync Request
TCP - Transport Control Protocol
Source Port:
2352
Destination Port: 80 World Wide Web HTTP
Sequence Number: 102274726
Ack Number:
0
Offset:
7
Reserved:
%000000
Code:
%000010
Sync Sequence
Window:
8192
Checksum:
0x6711
Urgent Pointer:
0
TCP Options:
CISCO NETWORKING ACADEMY
An Example TCP Header:
Acknowlegement
TCP - Transport Control Protocol
Source Port:
80 World Wide Web HTTP
Destination Port: 2352
Sequence Number: 2913145779
Ack Number:
102274727
Offset:
6
Reserved:
%000000
Code:
%010010
Ack is valid
Sync Sequence
Window:
17520
Checksum:
0x9640
Urgent Pointer:
0
TCP Options:
CISCO NETWORKING ACADEMY