CCNA Guide to Cisco Networking Fundamentals, Fourth Edition 9-1
Chapter 9
Network Services
At a Glance
Instructor’s Manual Table of Contents
• Overview
• Objectives
• Teaching Tips
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Lecture Notes
Overview
This chapter explains the concepts of network services, such as the various flavors of NAT,
DHCP, and DNS. The chapter also provides instructions on how to configure these services
using Cisco’s SDM.
Chapter Objectives
• Understand the purpose and operation of network address translation (NAT)
• Understand and configure static NAT, dynamic NAT, and PAT
• Understand and configure Dynamic Host Configuration Protocol (DHCP)
• Understand and configure Domain Name Services (DNS)
• Configure network services using Cisco’s Security Device Manager (SDM)
Teaching Tips
Network Address Translation
1. Mention that network address translation (NAT) is defined in RFC 3022, which
describes methods for connecting private (internal) IP addresses to the Internet.
2. Explain that NAT uses a one-to-one mapping or one-to-many mapping method to allow
3. Describe the following advantages of NAT:
a. It conserves public IP addresses
4. Describe the following disadvantages of NAT:
a. It introduces a small amount of delay into your network because the NAT router
5. Mention that NAT is available in three forms:
a. Static NAT
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Teaching
Tip
Cisco developed NAT, and today the technology is used by routers, firewalls,
and even individual computers with multiple network connections.
Static NAT
2. Explain that the NAT router must maintain a table in memory that maps internal IP
3. Mention that the network configuration for NAT is quite simple in a small network. The
NAT router will be the default gateway for all clients.
4. Explain that in a larger network, the NAT router might be one of many routers. Routers
would have to be configured to use the NAT router for Internet communications. NAT
should be configured on the border router of a large network.
Teaching
Tip
As shown in Figure 9-1, NAT is needed on the router that connects directly to
the ISP.
Dynamic NAT
2. Mention that the network administrator is not concerned about which IP address the
internal clients use.
3. Explain that any private IP address will automatically be translated to one of the
available Internet IP addresses by the NAT router. Addresses for dynamic NAT are
pulled out of a predefined pool of public addresses.
Teaching
Tip
The administrator must define the pool of public addresses and then state which
internal private addresses can use the pool.
Port Address Translation
1. Define port address translation (PAT), also known as overloading, as a special form of
2. Explain that to differentiate between the connections, PAT uses multiple public TCP
4. Explain that overlapping occurs when the internal network has been incorrectly
5. Mention that overlapping can be solved using NAT because NAT hides the incorrectly
configured internal IP scheme.
Teaching
Tip
Most companies do not run into the overlapping problem because their network
engineers and designers know to use one of the private IP address ranges
(10.x.x.x, 172.16.x.x.–172.31.x.x, and 192.168.x.x.) when configuring a private
internal TCP/IP network.
Configuring Network Address Translation
Configuring Static NAT
1. Explain that configuring static NAT is a two-step process:
Configuring Dynamic NAT
1. Describe the following steps for configuring dynamic NAT:
a. Configure a standard access control list to define what internal traffic will be
2. Explain that to define the standard access list, you must use the following syntax:
3. Mention the syntax for defining the NAT pool: ip nat pool [pool name]
5. Use Figure 9-2 to describe the following steps for configuring port address translation:
a. Configure a standard access list to define what internal traffic will be translated
Quick Quiz 1
1. ____ is the simplest form of NAT, in which a single private IP address is mapped to a
single public IP address.
2. ____ means that the NAT router automatically maps a group of valid local IP addresses
to a group of Internet IP addresses, as needed.
3. Port address translation (PAT), also known as ____, is a special form of dynamic NAT.
4. ____ occurs when the internal network has been incorrectly configured for an IP range
that actually exists on the Internet (registered to another entity) or when two companies
merge and each company was using the same private IP address range.
Domain Name Service
1. Define Domain Name Service (DNS) as a popular and important naming service. Based
on the client/server model, DNS translates names into IP addresses.
3. Explain that by default, a Cisco router will try several times to find an IP address for a
4. Explain that the command to configure a DNS lookup on a Cisco router is ip name–
Dynamic Host Configuration Protocol
1. Explain that Dynamic Host Configuration Protocol (DHCP) provides IP configuration
2. Explain that DHCP manages addressing by leasing the IP information to the hosts. This
3. Use Figure 9-5 to describe the DHCP process.
4. Mention that you can configure your Cisco router to be a DHCP server. The router can
5. Describe the following steps to configure the router to be a DHCP server:
a. Enable the service using the service dhcp command at the global
6. Describe the following steps for monitoring DHCP:
a. The best way to check the bindings is to execute the show ip dhcp
Security Device Manager
1. Define the Cisco’s new Security Device Manager (SDM) as a Web-based tool for
3. Use Figure 9-13 to explain how to configure DNS using SDM.
Quick Quiz 2
1. Based on the client/server model, ____ (DNS) translates names into IP addresses.
2. By default, a Cisco router will try several times to find an IP address for a name if you
enter one. This automatic translation is called a(n) ____.
3. The ____ provides IP configuration information to hosts on bootup.
4. ____ is a Web-based tool for advanced configuration on Cisco routers.
Class Discussion Topics
1. What are the main advantages and disadvantages of using static NAT, dynamic NAT,
and PAT?
2. How can DHCP be compared with RARP and BOOTP?
Additional Projects
1. Ask your students to read more about DHCP and write a report explaining some of the
most well-known DHCP server security threats and their countermeasures.
2. Ask your students to read more about DNS Security Extensions (DNSSEC) and write a
report explaining how DNSSEC adds security to DNS. Use the following link as a
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Additional Resources
2. How Network Address Translation Works
4. DHCP FAQ
5. Domain Name System
6. How Domain Name Servers Work
Key Terms
➢ bindings IP to MAC address mappings that are found in the DHCP database.
➢ DHCP ACK A unicast acknowledgement sent by the DHCP server to the client.
RFC 3022. Internal hosts with private or unregistered IP addresses can effectively use
one or more public registered IP addresses to communicate with external systems.
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➢ overlapping When an organization wants to connect to the Internet, but its internal