1. … and doesn’t Chris Taylor
How the Internet Works
… and doesn’t
Chris Taylor

2. Agenda Compare snail communication vs. e-communication
Talk with a few servers
Trouble-shooting

3. Objectives Snail Communications E-Communications
Send letter to Joe Fisherman in Salmon Bay, BC
Browse the OPCUG web site

4. Address Lookup Snail Communications E-Communications Phone book
Start with name (Joe Fisherman)
Returns the address (1 Water Street, Salmon Bay, BC)
Province – BC
City – Salmon Bay
Street – Water Street
House number – 1
DNS (Domain Name System)
Lookup name (opcug.ca)
Returns the address ( )
Network –
Host – 30

5. Envelope Snail Communications E-Communications Destination address
Network –
Host – 30
1 Water Street
Salmon Bay BC

6. Return Address Snail Communications E-Communications
9 Main Street, Ottawa, ON
Province – Ontario
City – Ottawa
Street – Main Street
House – 9
My computer
( )
Network –
Host – 111

7. Envelope Snail Communications E-Communications Destination address
Network –
Host – 30
Source address
Network –
Host - 111
9 Main Street
Ottawa, ON
1 Water Street
Salmon Bay BC

8. Who is it from / to Snail Communications E-Communications
Add the person’s name to the “to” and “from” addresses
To – Joe Fisherman
From – Chris Taylor
Add the “port” number to the destination and source IP addresses
Source port randomly generated (23172)
Destination ports well-known
Web 80 / 443
SMTP 25
POP 110
DNS 53
SMB over IP 445
NetBIOS

9. Ports
OPCUG.CA

10. Ports OPCUG.CA Web server Mail server Pop 3 server Telnet server
FTP server
FTP server two

11. Ports Port 80 25 110 23 21 OPCUG.CA Web server Mail server
Pop 3 server
Telnet server
FTP server
FTP server two

12. Ports Port 80 25 110 23 21 OPCUG.CA Web server Mail server
Pop 3 server
Telnet server
FTP server
FTP server two
Web browser:
Connects to whatever is listening on port 80

13. Ports Port 80 25 110 23 21 OPCUG.CA Web server Mail server
Pop 3 server
Telnet server
FTP server
FTP server two
Telnet client:
Connects to whatever is listening on port 23

14. Ports Port 80 25 110 23 21 OPCUG.CA Web server Mail server
Pop 3 server
Telnet server
FTP server
FTP server two
FTP client:
Connects to whatever is listening on port 21

15. Ports Port 80 25 110 23 21 1121 OPCUG.CA Web server Mail server
Pop 3 server
Telnet server
FTP server
FTP server two
FTP client:
Connects to whatever is listening on port 21

16. Ports Port 80 25 110 23 21 1121 OPCUG.CA Web server Mail server
Pop 3 server
Telnet server
FTP server
FTP server two
FTP client:
Tell it to connects to whatever is listening on port 1121

17. Envelope Snail Communications E-Communications Destination address
:80
Network –
Host – 30
Port 80 (web server)
Source address
:23172
Network –
Host – 111
Port 23172
Chris Taylor
9 Main Street
Ottawa, ON
Joe Fisherman
1 Water Street
Salmon Bay BC

18. There … Destination address: 206.47.37.30 Destination port: 80
Source address:
Source port: 23172
Port 80
OPCUG.CA
Web server

19. … and back Destination address: 206.47.37.30 Destination port: 80
Source address:
Source port: 23172
Port 80
OPCUG.CA
Web server
Destination address:
Destination port: 23172
Source address:
Source port: 80

20. How it gets there
Snail communications

21. How it gets there
Snail communications
Mailman picks up envelope

22. How it gets there Does not know how to get it to BC
Snail communications
Mailman picks up envelope
Does not know how to get it to BC

23. How it gets there
Snail communications
Mailman picks up envelope
Does not know how to get it to BC
Knows to deliver it to the Alta Vista postal terminal

24. How it gets there
Snail communications
Mailman picks up envelope
Does not know how to get it to BC
Knows to deliver it to the Alta Vista postal terminal
Alta Vista terminal gets it on the plane to Vancouver

25. How it gets there
Snail communications
Mailman picks up envelope
Does not know how to get it to BC
Knows to deliver it to the Alta Vista postal terminal
Alta Vista terminal gets it on the plane to Vancouver
Vancouver postal station gets it on truck to Salmon Bay

26. How it gets there Salmon Bay puts it in bag for postal carrier “Jim”
Snail communications
Mailman picks up envelope
Does not know how to get it to BC
Knows to deliver it to the Alta Vista postal terminal
Alta Vista terminal gets it on the plane to Vancouver
Vancouver postal station gets it on truck to Salmon Bay
Salmon Bay puts it in bag for postal carrier “Jim”

27. How it gets there Postal carrier “Jim” delivers it to 1 Water Street
Snail communications
Mailman picks up envelope
Does not know how to get it to BC
Knows to deliver it to the Alta Vista postal terminal
Alta Vista terminal gets it on the plane to Vancouver
Vancouver postal station gets it on truck to Salmon Bay
Salmon Bay puts it in bag for postal carrier “Jim”
Postal carrier “Jim” delivers it to 1 Water Street

28. How it gets there Someone at 1 Water Street gives it to Joe Fisherman
Snail communications
Mailman picks up envelope
Does not know how to get it to BC
Knows to deliver it to the Alta Vista postal terminal
Alta Vista terminal gets it on the plane to Vancouver
Vancouver postal station gets it on truck to Salmon Bay
Salmon Bay puts it in bag for postal carrier “Jim”
Postal carrier “Jim” delivers it to 1 Water Street
Someone at 1 Water Street gives it to Joe Fisherman

29. How it gets there
E- communications

30. How it gets there
E- communications
My computer looks at the destination IP address and sees that it is not on the same network

31. How it gets there Delivers the packet to the “default gateway”
E- communications
My computer looks at the destination IP address and sees that it is not on the same network
Delivers the packet to the “default gateway”

32. How it gets there Default gateway delivers it to an upstream address
E- communications
My computer looks at the destination IP address and sees that it is not on the same network
Delivers the packet to the “default gateway”
Default gateway delivers it to an upstream address

33. How it gets there Upstream address may deliver higher upstream
E- communications
My computer looks at the destination IP address and sees that it is not on the same network
Delivers the packet to the “default gateway”
Default gateway delivers it to an upstream address
Upstream address may deliver higher upstream

34. How it gets there
E- communications
My computer looks at the destination IP address and sees that it is not on the same network
Delivers the packet to the “default gateway”
Default gateway delivers it to an upstream address
Upstream address may deliver higher upstream
Eventually delivered to the destination network ( )

35. How it gets there Delivered to final host (206.47.37.30)
E- communications
My computer looks at the destination IP address and sees that it is not on the same network
Delivers the packet to the “default gateway”
Default gateway delivers it to an upstream address
Upstream address may deliver higher upstream
Eventually delivered to the destination network ( )
Delivered to final host ( )

36. How it gets there
E- communications
My computer looks at the destination IP address and sees that it is not on the same network
Delivers the packet to the “default gateway”
Default gateway delivers it to an upstream address
Upstream address may deliver higher upstream
Eventually delivered to the destination network ( )
Delivered to final host ( )
Host delivers it to application listening on destination port (80 – web server)

37. How a reply gets back Snail Communications E-Communications
Original return address contains all needed
Name
House number
Street
City
Province
Same steps take place in reverse
Original source information contains all needed
IP address
Source port
Same steps take place in reverse

38. Reply Destination address: 206.47.37.30 Destination port: 80
Source address:
Source port: 23172
Port 80
OPCUG.CA
Web server
Destination address:
Destination port: 23172
Source address:
Source port: 80

39. Putting it all together

40. Putting it all together
DNS lookup opcug.ca ( )

41. Putting it all together
DNS lookup opcug.ca ( )
is not local

42. Putting it all together
DNS lookup opcug.ca ( )
is not local
Packet goes to default gateway

43. Putting it all together
DNS lookup opcug.ca ( )
is not local
Packet goes to default gateway
Gateway sends upstream

44. Putting it all together
DNS lookup opcug.ca ( )
is not local
Packet goes to default gateway
Gateway sends upstream
Continue until someone knows route to network

45. Putting it all together
DNS lookup opcug.ca ( )
is not local
Packet goes to default gateway
Gateway sends upstream
Continue until someone knows route to network
Route to host on network

46. Putting it all together
DNS lookup opcug.ca ( )
is not local
Packet goes to default gateway
Gateway sends upstream
Continue until someone knows route to network
Route to host on network
Web server (port 80) gets packet

47. Talking to a server Often conversations are very simple Or complex
SMTP ( )
POP 3 ( )
FTP
Or complex
Web
Flash animation
Many binary format conversations

48. Talking to a mail server
HELO opcug.ca
MAIL FROM:
RCPT TO:
DATA
Subject: Just saying “hi”
Hello Bob
Yours truly,
Chris .

49. Talking to a mail server

50. Talking to a mail server

51. Talking to a mail server

52. Talking to a mail server

53. Talking to a mail server

54. Talking to a mail server

55. Talking to a mail server

56. Talking to a mail server

57. Talking to a mail server

58. Talking to a mail server

59. Talking to a mail server

60. Talking to a mail server

61. Talking to a mail server

62. Talking to a mail server

63. Talking to a mail server

64. Talking to a mail server

65. Talking to a mail server

66. Talking to a mail server

67. Talking to a mail server

68. Talking to a mail server

69. Talking to a web server
GET /index.html HTTP/1.1

70. Talking to a web server GET /index.html HTTP/1.1
HTTP/ Unauthorized - user not logged in
Expires: Mon, 01 Jan :00:00 GMT
Last-Modified: Sat, 25 Apr :16:56 GMT
Content-Length: 150
Server: Wildcat/v
X-Elapsed-Time: 14750
X-Powered-By: Wildcat.Net v
Content-Type: text/html
Date: Sat, 25 Apr :16:56 GMT
Set-Cookie: sessionid=6D3764DB; path=/
Set-Cookie: wcbbs=The PUB; path=/
WWW-Authenticate: basic realm="The PUB"
<HTML><HEAD><TITLE>Unauthorized</TITLE></HEAD>
<BODY><H1>Unauthorized</H1>
Proper authorization is required to access this web site.
</BODY></HTML>

71. Trouble-shooting

72. What to do when it doesn’t work!

73. Try another destination
What this tells you
If it works, your computer is fine
What you can do to fix
Probably nothing. The problem is at the other site

74. Try another destination

75. Try your ISP’s site What this tells you What you can do to fix
If it works but other sites don’t, your ISPs route to the Internet may be down
What you can do to fix
Probably not a whole lot

76. Try another computer on your network
What this tells you
If it works from another computer, the problem is local to your computer
What you can do to fix
Complete shutdown (not reboot) and restart your computer
Compare IP config to working computer
Check / swap cables
Change port on router

77. Try by IP address What this tells you What you can do to fix
If it works but domain name doesn’t, the problem is likely with DNS
What you can do to fix
Check the properties of IP config
Restart your router
Try manually with NSLOOKUP to see if your DNS server is down
Try another DNS (

81. Ping the gateway What this tells you What you can do to fix
If you have local networking
What you can do to fix
Restart your router
Ping other computers on your network

83. Check your ip address What this tells you What you can do to fix
Start | Run | CMD
ipconfig /all
If address is x.x (link-local or automatic private IP addressing)
What you can do to fix
Check if cable is disconnected
Check if your router is working

85. Remove your router What this tells you What you can do to fix
If it works without the router, the router may be faulty or config messed up
What you can do to fix
Restart router
Reconfigure router
Replace router
Don’t run longer than you have to without router

86. Prepare Know your current configuration
Know some ip addresses of well known servers
Know ip addresses for OpenDNS
Know what you can ping
When restarting, always begin with device furthest away (modem, then router, then computer)
Know your ISP’s instructions

87. Happy computing