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Сети и Телекоммуникации

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292 Chapter 8: Advanced TCP/IP Topics

Foundation Summary

The “Foundation Summary” section lists the most important facts from the chapter. Although this section does not list everything that will be on the exam, a well-prepared CCNA candidate should at a minimum know all the details in each Foundation Summary before taking the exam.

Table 8-11 shows the private address space deﬁned by RFC 1918.

Table 8-11 RFC 1918 Private Address Space

Range of IP Addresses	Class of Networks	Number of Networks

10.0.0.0 to 10.255.255.255	A	1

172.16.0.0 to 172.31.255.255	B	16

192.168.0.0 to 192.168.255.255	C	256

Figure 8-20 and Table 8-12 outline some of the terminology used with NAT.

Figure 8-20 Static NAT Terminology

SA 10.1.1.1

SA 200.1.1.1

Server

10.1.1.1

Internet

NAT

170.1.1.1

10.1.1.2

Inside

Outside

DA 10.1.1.1

DA 200.1.1.1

Inside Local

Inside Global

10.1.1.1

200.1.1.1

10.1.1.2

200.1.1.2

		Foundation Summary 293
Table 8-12 NAT Addressing Terms

	Term	Description

	Inside local	In a typical NAT design, the term “inside” refers to an address used for a
		host inside an enterprise. An inside local is the actual IP address assigned to
		a host in the private enterprise network. A more descriptive term might be
		“inside private,” because when using RFC 1918 addresses in an enterprise,
		the inside local represents the host inside the enterprise, and it is a private
		RFC 1918 address.

	Inside global	In a typical NAT design, the term “inside” refers to an address used for a
		host inside an enterprise. NAT uses an inside global address to represent the
		inside host as the packet is sent through the outside network, typically the
		Internet. A NAT router changes the source IP address of a packet sent by an
		inside host from an inside local address to an inside global address as the
		packet goes from the inside to the outside network.
		A more descriptive term might be “inside public,” because when using RFC
		1918 addresses in an enterprise, the inside global represents the inside host
		with a public IP address that can be used for routing in the public Internet.

	Outside global	In a typical NAT design, the term “outside” refers to an address used for a
		host outside an enterprise—in other words, in the Internet. An outside
		global is the actual IP address assigned to a host that resides in the outside
		network, typically the Internet. A more descriptive term might be “outside
		public,” because the outside global represents the outside host with a public
		IP address that can be used for routing in the public Internet.

	Outside local	In a typical NAT design, the term “outside” refers to an address used for a
		host outside an enterprise—in other words, in the Internet. NAT uses an
		outside local address to represent the outside host as the packet is sent
		through the private enterprise network (inside network). A NAT router
		changes a packet’s destination IP address, sent from an inside host to the
		outside global address, as the packet goes from the inside to the outside
		network. A more descriptive term might be “outside private,” because when
		using RFC 1918 addresses in an enterprise, the outside local represents the
		outside host with a private IP address from RFC 1918.

Example 8-8 shows a typical NAT conﬁguration, as well as some show commands.

Example 8-8 Dynamic NAT Conﬁguration

NAT# show running-config

! Lines omitted for Brevity

interface Ethernet0/0

ip address 10.1.1.3 255.255.255.0

ip nat inside

continues

294 Chapter 8: Advanced TCP/IP Topics

Example 8-8 Dynamic NAT Conﬁguration (Continued)

interface Serial0/0

ip address 200.1.1.251 255.255.255.0 ip nat outside

ip nat pool fred 200.1.1.1 200.1.1.2 netmask 255.255.255.252 ip nat inside source list 1 pool fred

access-list 1 permit 10.1.1.2 access-list 1 permit 10.1.1.1

NAT# show ip nat translations

NAT# show ip nat statistics

Total active translations: 0 (0 static, 0 dynamic; 0 extended)

Outside interfaces:

Serial0/0

Inside interfaces:

Ethernet0/0

Hits: 0 Misses: 0

Expired translations: 0

Dynamic mappings:

-- Inside Source

access-list 1 pool fred refcount 0

pool fred: netmask 255.255.255.252

start 200.1.1.1 end 200.1.1.2

type generic, total addresses 2, allocated 0 (0%), misses 0

! Telnet from 10.1.1.1 to 170.1.1.1 happened next; not shown

NAT# show ip nat statistics

Total active translations: 1 (0 static, 1 dynamic; 0 extended)

Outside interfaces:

Serial0/0

Inside interfaces:

Ethernet0/0

Hits: 69 Misses: 1

Expired translations: 0

Dynamic mappings:

-- Inside Source

access-list 1 pool fred refcount 1

pool fred: netmask 255.255.255.252

	start 200.1.1.1 end	200.1.1.2
	type generic, total	addresses 2, allocated 1 (50%), misses 0
NAT# show ip nat translations
Pro	Inside global	Inside local	Outside local	Outside global
---	200.1.1.1	10.1.1.1	---	---

Foundation Summary 295

The Destination Unreachable, Time Exceeded, Redirect, and Echo messages are described in Table 8-13.

Table 8-13	ICMP Message Types

	Message	Purpose

	Destination Unreachable	Tells the source host that there is a problem delivering a packet.

	Time Exceeded	The time it takes a packet to be delivered has expired, so the
		packet has been discarded.

	Redirect	The router sending this message has received a packet for which
		another router has a better route. The message tells the sender to
		use the better route.

	Echo	Used by the ping command to verify connectivity.

Table 8-14 summarizes some features of TFTP and FTP.

Table 8-14 Comparison of FTP and TFTP

FTP	TFTP

Uses TCP	Uses UDP

Uses robust control commands	Uses simple control commands

Sends data over a TCP connection separate from	Uses no connections because of UDP
control commands

Requires more memory and programming effort	Requires less memory and programming effort

Example 8-9 shows the router conﬁguration required to support ISL encapsulation and forwarding between three VLANs.

Example 8-9 Router Conﬁguration for ISL Encapsulation

interface fastethernet 0.1

ip address 10.1.1.1 255.255.255.0 encapsulation isl 1

interface fastethernet 0.2

ip address 10.1.2.1 255.255.255.0 encapsulation isl 2

interface fastethernet 0.3

ip address 10.1.3.1 255.255.255.0 encapsulation isl 3

296 Chapter 8: Advanced TCP/IP Topics

Q&A

As mentioned in the Introduction, you have two choices for review questions. The following questions give you a bigger challenge than the exam because they are open-ended. By reviewing with this more-difﬁcult question format, you can exercise your memory better and prove your conceptual and factual knowledge of the topics covered in this chapter. The answers to these questions are found in Appendix A.

For more practice with exam-like question formats, including multiple-choice questions and those using a router simulator, use the exam engine on the CD.

1.Deﬁne private addressing as deﬁned in RFC 1918.

2.List the range of private networks deﬁned in RFC 1918.

3.Does CIDR affect the size of Internet routing tables? If so, what does it do to those routing tables?

4.Deﬁne NAT and explain the basics of its operation.

5.Deﬁne the term inside local address in relation to NAT. Use Figure 8-21 to describe the answer.

Figure 8-21 Network for Use in Answering NAT Questions

10.1.1.1	200.1.1.1	Server
10.1.1.1

Internet

NAT 170.1.1.1

10.1.1.2

6.Deﬁne the term inside global address in relation to NAT. Use Figure 8-21 to describe the answer.

7.Create a conﬁguration for NAT overload to a single IP address for the router shown in Figure 8-21.

8.Create a conﬁguration for static NAT, mapping host 10.1.1.1 to 200.1.1.11, for the router shown in Figure 8-21.

Q&A 297

9.Which requires more lines of source code, FTP or TFTP? Explain your answer.

10.Does FTP or TFTP perform error recovery? If so, describe the basics of how this occurs.

11.Describe the process used by IP routers to perform fragmentation and reassembly of packets.

12.How many Class B-style networks are reserved by RFC 1918 private addressing?

13.Describe why ARP requests use an Ethernet broadcast address instead of an Ethernet unicast address.

14.Imagine that host 10.1.1.1 in Figure 8-21 sends a packet to the server at 170.1.1.1 and that the NAT router also happens to fragment the packet. Inside the context of this network, explain how reassembly of the fragments into the original packet occurs.

15.Imagine that R1 has an interface, FastEthernet 0/0, that uses ISL trunking to a switch.

R1 needs to route between VLAN 1 and VLAN 2. Create a valid router conﬁguration.

16.Describe how NAT overload manages to support more than one inside local IP address using a single inside global address.

Cisco Published ICND Exam Topics*

Covered in This Part:

4 Design a simple internetwork using Cisco products

6 Choose WAN protocols to meet design requirements

14Implement simple WAN protocols

15Utilize the OSI model as a guide for systematic network troubleshooting

19Troubleshoot a device as part of a working network

21Perform simple WAN troubleshooting

26Evaluate key characteristics of HDLC, PPP, Frame Relay, DDR, and ISDN technologies * Always re-check www.cisco.com for the latest posted exam topics

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