The document discusses how computers communicate over the internet using the Internet Protocol (IP). IP breaks data into packets that contain source and destination addresses and sends them over conduits. Each conduit can carry up to 65535 channels called ports. Firewalls were created to regulate this traffic and protect networks by allowing or denying packets based on complex rulesets that examine source/destination addresses and ports. Firewalls act like efficient mail sorting facilities that apply rules to determine if packets should be accepted or dropped.

1. Knowledge Base
How computers
communicate,
Internet Protocol
(IP) and Firewalls

2.  Most computer to computer communications use
the Internet Protocol (IP).
How computers communicate

3.  Most computer to computer communications use
the Internet Protocol (IP).
 This basically sends data information cut up in
chunks. We call these Packets.
How computers communicate

4.  Most computer to computer communications use
the Internet Protocol (IP).
 This basically sends data information cut up in
chunks. We call these Packets.
 Each Packet is identified with both the source and
the destination addresses. You can therefore think
of them as a shipping box containing a message.
How computers communicate

5.  In the IP protocol, each computer has it’s own
address.
How computers communicate

6.  But the Internet is a network… of networks!
 Billions of addresses are in play, making for a
complex environment.
How computers communicate

7.  There are tons of mechanisms in place to manage
addresses, but ultimately we can make abstraction
of that and concentrate on the fact that a
computer can communicate with another when its
address is known.
How computers communicate

8.  But the address is just that, an address.
What’s extremely important in IP is the conduit.
How computers communicate

9.  But the address is just that, an address.
What’s extremely important in IP is the conduit.
 The conduit can carry a lot of information to an
address, and we mean a lot!
How computers communicate

10.  But the address is just that, an address.
What’s extremely important in IP is the conduit.
 The conduit can carry a lot of information to an
address, and we mean a lot!
 A good analogy can be made with Cable
Television. A single cable (conduit) can carry many
channels to your house.
How computers communicate

11.  But the address is just that, an address.
What’s extremely important in IP is the conduit.
 The conduit can carry a lot of information to an
address, and we mean a lot!
 A good analogy can be made with Cable
Television. A single cable (conduit) can carry many
channels to your house.
 In fact, in the IP protocol there are 65535 channels!
We call them Ports.
How computers communicate

12.  So we have packets of information, being sent from
one computer to another.
How computers communicate

13.  So we have packets of information, being sent from
one computer to another.
 Travelling on a conduit allowing for 65535
“channels”.
How computers communicate

14.  This is one channel, receiving a flow of packets…
How computers communicate

15.  This is one channel, receiving a flow of packets…
 Now just imagine 65535 channels…
How computers communicate

16.  Going back to our previous diagram, we see two
networks connected to the Internet.
Malicious users

17.  But the Internet is not just composed of nice people,
hackers are either trying to infect your computer
with viruses, or to get access to your data
Malicious users

18.  This is why firewalls were created, a mechanism was
needed to allow only authorized traffic into your
company’s network.
Firewalls

19.  This is why firewalls were created, a mechanism was
needed to allow only authorized traffic into your
company’s network.
 They are often depicted by a nice brick wall, but
the analogy is misleading.
Firewalls

20.  We must allow some traffic through…
Firewalls

21.  …while rejecting most of the rest
Firewalls

22.  Really not how we see a brick wall, is it?
Firewalls

23.  Its much better to visualize the firewall as a mail
sorting facility, but with a highly regulated flow.
Firewalls

24.  Its much better to visualize the firewall as a mail
sorting facility, but with a highly regulated flow.
 You can create rules for any of the 65535 ports.
Firewalls

25.  Its much better to visualize the firewall as a mail
sorting facility, but with a highly regulated flow.
 You can create rules for any of the 65535 ports.
 There rules can restrict source AND destination
addresses.
Firewalls

26.  Let’s say that you do not want any packet destined
to port 21 to go through the firewall, you would
have a rule like:
Firewalls
Port From To Action
21 * * DENY

27.  Let’s say that you do not want any packet destined
to port 21 to go through the firewall, you would
have a rule like:
 The stars indicate that any address will match this
rule.
Firewalls
Port From To Action
21 * * DENY

28.  Let’s say that you want to allow your users to
browse the internet (incidentally, this uses port 80),
you would have a rule like
Firewalls
Port From To Action
80 LOCAL * ALLOW

29.  Let’s say that you want to allow your users to
browse the internet (incidentally, this uses port 80),
you would have a rule like
 In this case, the LOCAL keyword indicates that all
of your local addresses are allowed. Meaning the
addresses of your company’s computers.
Firewalls
Port From To Action
80 LOCAL * ALLOW

30.  So imagine a huge
table of rules…
Firewalls
Port From To Action
21 * * DENY
80 LOCAL * ALLOW
115 * * DENY
3389 LOCAL * ALLOW
22 LOCAL * ALLOW
443 LOCAL * ALLOW
8181 EXTERNAL 192.168.10.10 ALLOW
* 192.168.10.1 * DENY
34534 EXTERNAL 192.168.10.234 ALLOW
80 LOCAL facebook.com DENY
443 LOCAL facebook.com DENY
80 LOCAL pinterest.com DENY
443 LOCAL pinterest.com DENY
80 LOCAL ad.doubliclick.net DENY
443 LOCAL ad.doubliclick.net DENY
54322 EXTERNAL 192.189.10.199 ALLOW
22 LOCAL 165,10,23,33 ALLOW
1433 LOCAL 165,10.23.33 ALLOW
20222 EXTERNAL 192.168.10.143 ALLOW

31.  …and imagine a really efficient mail sorter
Firewalls

32.  …and imagine a really efficient mail sorter
 Looking at each packet
 Source address…
 Destination address…
 Port…
Firewalls

33.  …and imagine a really efficient mail sorter
 Looking at each packet
 Source address…
 Destination address…
 Port…
 Searching for rules for that port in the rule table
Firewalls

34.  …and imagine a really efficient mail sorter
 Looking at each packet
 Source address…
 Destination address…
 Port…
 Searching for rules for that port in the rule table
 Then ruthlessly applying the decision, for the Deny
rules, it just drops the packet
Firewalls

35.  So firewalls are that big mail sorting facility
Firewalls

36.  So firewalls are that big mail sorting facility
 Allowing some traffic to go through
Firewalls

37.  So firewalls are that big mail sorting facility
 Allowing some traffic to go through
 But protecting you by dropping packets that may
have been sent by malicious users
Firewalls

38.  So firewalls are that big mail sorting facility
 Allowing some traffic to go through
 But protecting you by dropping packets that may
have been sent by malicious users
 If you need to go through a firewall to perform some
work, you may have to use a Virtual Private Network
(VPN), please consult our presentation on that
subject.
Firewalls

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