This document discusses internet architecture and security best practices. It covers topics like internet services to offer and not offer, developing communications architectures using single or multiple lines, designing demilitarized zones and appropriate architectures, understanding network address translation, and designing partner networks.

1. Lesson 18-Internet
Architecture

2. Overview
Internet services.
Develop a communications architecture.
Design a demilitarized zone.
Understand network address translation.
Design partner networks.

3. Internet Services
Services to offer.
Services not to offer.
Mail.
NetBIOS, Unix RPC, NFS,
Encrypted e-mail.
Web. “r” services, TFTP,
Internal access to Remote Control
Internet.
Protocols, and SNMP.
External access to
internal systems.
Control services.

4. Mail
Mail service is generally offered to internal employees to
send and receive messages.
It requires that at least one server be established to receive
inbound mail.
Outbound mail can move through the same server or
directly through desktop systems.
Organization may choose to establish relays for public mail
to be sent to discussion groups.

5. Encrypted E-mail
It is better to encrypt the contents of the e-mail to protect
any sensitive information.
Systems like desktop software and network appliances
placed in mail stream provide encrypted e-mail.

6. Web
To publish information via Web, the organization needs to
establish a Web server.
Web servers can provide static content or dynamic content.
HTTPS is used for Web pages that contain sensitive
information or require authentication.
File Transfer Protocol (FTP) server allows external
individuals to get or send files.

7. Internal Access to Internet
Most common services that employees are allowed to
access are:
HTTP (port 80) and HTTPS (port 443)
FTP (ports 21 and 20)
Telnet (port 23) and SSH (port 22)
POP-3 (port 110) and IMAP (port 143)
NNTP (port 119)

8. External Access to Internal
Systems
External access to sensitive internal systems is a delicate
matter.
The two forms of external access are employee access or
non-employee access.
External access may be accomplished through VPNs, dial-
up lines, leased lines, or unencrypted access over the
Internet.

9. Control Services
These services are required for smooth function of network and
Internet connection.
DNS - Domain Name Service is used to resolve system names into
IP addresses.
ICMP - Internet Control Message Protocol provides services such
as ping and messages that help the network function efficiently.
NTP - Network Time Protocol is used to synchronize time between
various systems.

10. Develop a Communications
Architecture
Primary issues for establishing an organization’s Internet
connection are throughput requirements and availability.
Availability requirements of the connection should be set by
the organization.

11. Develop a Communications
Architecture
Single-line access
Multiple-line access to a single ISP
Multiple-line access to multiple ISPs

12. Single-Line Access
Standard single-line access architecture

13. Single-Line Access
The following potential failures make single-line access
suitable for non-business-critical Internet connections:
Router failure.
CSU failure.
Cut local loop.
Damage to the telephone company’s CO (central office).
POP failure at the ISP.

14. Multiple-Line Access to a
Single ISP
They are used to overcome the single point of failure issues
with the single ISP architecture.
Shadow link or redundant circuit services offered by
different ISPs provide a second communication link in case
of failure.
Multiple-line access to a single ISP has Single-POP access
or Multiple-POP access.

15. Multiple-Line Access to a
Single ISP
Single-POP access:
An ISP can provide fail-over access by setting up a redundant
circuit to the same POP.
It addresses failures in router, CSU, phone company circuit to
CO, and ISP equipment.
Benefit to this architecture is the low cost of the redundant
circuit.

16. Multiple-Line Access to a
Single ISP
Multiple-POP access:
Running second connection to a second POP additional
availability and reliability can be obtained.
Border Gateway Protocol (BGP) protocol, run by ISP, specifies
routes between entities with such dual connections.
Single point failures of local loop and CO can be overcome if
the organization’s facility has two local loop connections.

17. Multiple-line Access to
Multiple ISPs
If architected correctly, use of multiple ISPs can reduce the
risk of loss of service dramatically.
Issues that occur in choosing ISPs are complexity of using
different ISPs, thorough knowledge in ISPs, and physical
routing of connections.
Working with multiple ISPs also involve routing and IP
address space issues that must be resolved.

18. Design a Demilitarized Zone
Defining the DMZ.
Systems to place in DMZ.
Appropriate DMZ architectures.

19. Defining the DMZ
A DMZ is created by providing a semi-protected network
zone.
The DMZ is delineated with network access controls, such
as firewalls or heavily filtered routers.
Any system that can be directly contacted by an external
user should be placed in a DMZ since they can be attacked.
External system’s access to sensitive systems must be
avoided.

20. Systems to Place in DMZ
Layout of systems between the DMZ and the
internal network

21. Systems to Place in DMZ
DMZ can have either both internal and external mail
servers or a single firewall mail server.
Using Web server for receiving user’s input and application
server for processing it provides protection to the database
server.
All externally accessible systems should be placed in the
DMZ.
The organization’s ISP can provide alternate DNS services.

22. Appropriate DMZ Architectures
The three common architectures are router and firewall,
single firewall, and dual firewall.
These architectures have their own advantages and
disadvantages; hence organizations must choose the
appropriate one.

23. Appropriate DMZ Architectures
Router and firewall architecture:
Router and firewall architecture
involves risk to systems on the
Internet. The risk can be
reduced using filters on the
router.
Risk to systems can also be
reduced by locking them so that
only services offered by DMZ run
on them.

24. Appropriate DMZ Architectures
Single firewall architecture:
A single firewall can be used to
create a DMZ using a third
interface.
The single firewall becomes a
single point of failure and a
potential bottleneck for traffic,
unless in fail-over configuration.
Single firewall architecture is
simple compared to the router
and firewall architecture.

25. Appropriate DMZ Architectures
Dual firewall architecture:
Dual firewall architecture uses
two firewalls to separate DMZ
from external and internal
networks.
Dual firewalls increase cost of
architecture and require
additional management and
configuration.

26. Understand Network Address
Translation
Any organization that plans to install a firewall will have to
deal with addressing issues.
In most networks, the firewall performs the NAT function of
translating one or more addresses into other addresses.
NAT can also provide a security function as hidden
addresses of internal systems are not visible to the Internet.

27. Understand Network Address
Translation
Private class addresses are used on internal networks
behind a firewall that performs NAT.
These addresses provide an organization with flexibility in
designing its internal addressing scheme.
Static NAT is a one- to-one configuration that allows
accessing internal network addresses from the Internet.
Static NAT maps a single real address from the
organization’s external network to a system on the DMZ.

29. Understand Network Address
Translation
Dynamic NAT maps many
internal addresses to a single
real address.
Dynamic NAT creates a
practical limit of about 64,000
simultaneous connections.
Dynamic NAT is useful for
desktop clients who use the
Dynamic Host Configuration
Protocol (DHCP).

30. Design Partner Networks
Partner networks are generally established to exchange
certain files or pieces of data between organizations.
Architectures and methodologies of Internet connection can
be used for partner networks as their requirements do not
differ much.
Rules must be added to firewall to allow systems at the
partner organization and internal systems to access partner
DMZ systems.
NAT should be used when connecting to partner networks.

32. Summary
Organizations can offer services like mail, encrypted e-mail,
Web, internal access to Internet, external access to internal
systems, and control services.
Control services include DNS, ICMP, and NTP.
To reduce security risks, services that are not required
should not be offered.
Types of Internet architectures are single-line access,
multiple-line access to a single ISP, and multiple-line access
to multiple ISPs.

33. Summary
Establishing a not truly trusted, semi-secure zone outside
of the trusted network creates a DMZ.
Router and firewall, single firewall, and dual firewall are the
three DMZ architectures.
Firewall performs the NAT function of translating one or
more addresses into other addresses.
Partner networks are generally established to exchange
data between organizations.